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T'h4sX1t-l'4.
SCIENCEXEMESB.
l^
^m
ILLUSTBATED GIFT BOOKS.
lEEDICAL ANATOMT. ByPBANCi8SiB8ON,M.D.,F.R.0.P.,
F.B.S. Imperial folio. 21 Coloured Plates. Cloth, £2 28.;
balf-morocco, £2 lOs.
S1TB6ICAL ASATOMT. A series of Dissections, illastrating
the Principal Regions of the Human Body. By Joseph
Maclise, F.B.C.S. 8ec<md EdUum, Folio. 52 Plates.
Cloth, £3 128.; half-morocco, £4 48.
OB DISLOCATIOBS AND FBACTUEES. By Joseph
Macijse, F.B.C.S. This Work is uniform with
"Surgical Anatomy." Folio. 36 Plates. Cloth, £2 108. ;
half-morocco, £2 178.
PATHOLOOT OF THE HTTMAH ETE. By John
Dalbymplb, P.R.C.S., F.R.S. Imperial 4to. 36
Coloured Plates. Half -bound morocco, gilt tops, £9 158.
POBTEAITS OF DISEASES OF THE SKIN. By Es^sHrs
WiMON, P.R.C.S., P.R.S. Folio. 48 Coloured Plates,
life size. Half-morocco, £13.
THE THAH ATOPHIBIA OF INDIA ; being a Description
of the Venomous Snakes of the Indian Peninsula ; with
ar Account of the Influence of their Poison on Life, and
a Series of Experiments. By J. Faybeb, M.D., P.R.C.S.,
C.S.I. FoUo. 31 Plates (28 Coloured), £7 78.
J. & A. CHURCHILL, New Bublinoton Stbeet.
J
Tfi£ £LEIENTS
OF
NATUEAL PHILOSOPHY.
THE ELEMEBIS OF
NATURAL PHILOSOPHY;
STUDY OF THE PHYSICAL SCIENCES.
CHAMESJSOOKE, M.A., KHS., PR.M.S,
GOLDlllG BIED, M.A., M.D., E.B.S,, F.I.S.
LONDON:
JOHN onUBCHILL AND SONS, NEW BUBLINOTON STKBET.
Thv^^'^'^^n'^
PREFACE.
Love before the pnblicatioD of the last edition of tliis work
in 1860, the light of physical truth had dawned on the Author's
mind, but it was yet in the dim and hazy distance ; but seven
years of further study and thought have served only to raise that
light high above the horizon of uncertainty. A change of views
was indicated in the last edition, by a substitution of the titles of
" Physics of Matter" and " Physics of Motion" for the two sec-
tions of the work, in place of the physics oi ponderahU and of
in^pondercible. matter ; but as the above distinction of sections is
illogical, and purely arbitrary, it has been altogether omitted.
The numberless facts that have in the interval been observed
and recorded, have tended only to confirm the opinion that the
various physical agents are not forms of matter, but modes of
motion; and that the unnatural quality of imponderability, or
exemption from the universal law of gravitation, need no longer
he reckoned amongst the physical attributes of any kind of matter :
these views will be found more fully developed in the Introduction.
The ideas of the late Author were mainly those current in his
day, and they were embodied or implied in the language in which
the physical facts were expressed and explained. But as most of
these views have been completely changed, an entire change of
diction became necessary ; and of the original treatise, the bare
facts, common, more or less, to all elementary treatises on Physics,
alone remain : it would therefore have been unreasonable to retain
the much- respected name of Grolding Bird, as author of the present
treatise, which contains numerous facts and principles that were
imknown at the date of his decease. This edition is the third that
has passed through the present Author's hands, each of which has
received considerable modification and extension, but neither to
nearly so great an extent as the present : which somewhat resem-
VI PREJfACB.
blcs the fowling-piece in the old story, that had first a new lock,
then a new stock, and lastly, a new barrel ; after which process of
renovation, it is needless to say that not much of the original
prodaction remained.
Amongst many other important topics, the magnetic properties
of iron in relation to marine architecture, the determination of
electrical constants, submarine telegraphy, the relations existing
between electric energy and the fnnctaons of muscles and nerves,
spectrum analysis, and the nature and properties of heat, have each
received a lai^ge development since the last edition was published,
and it is sincerely hoped that neither in these, nor in any other
department of physical science, has any important branch of the
subject remained entirely unnoticed.
Again the Author earnestly requests of his readers a communi-
cation of any observed errors or deficiencies: several minor
additions and improvements, occurring in the present edition,
are due to the kindly-expressed views and wants of careful and
intelligent readers.
10, FmaoT Sqttabk,
Mag, 1807.
TABLE OF CONTENTS.
INTEODTJCTION, Page xxi.
CHAPTER I. ; Page 1.
KLBVENTASr LAWS AND PBOPBHTIBS OP UATTBR — IKTEBNAL OS
UOLKCT7LAR FOBCE8.
If.B, — The numhtrt re/tr to paragrapkt, not to pages.
Dnittbilitj of matter ... 1
PnmertiM of matter ts
lIolMiilar forces 9,10
Att6iK»7prt>pertin of matter 12—19
I^efinite aggregation of mole-
ealet
Forms of crrttalB
Primary ana secondary forms
Law of symmetry in crystals ...
Remarkable deriatkms
Twin crystals
... ao— 22 i Dimorphoos snbstances
23,24
2o
28
29
27
28
CHAPTEE 11. ; Page 18.
PIOPBBTIBS OF MASSES OF UATTBB — BXTBBHAL P0BCE8.
DtfAision of gases ... ... 47
Ansell's fire-damp indicator ... 48
Transpiration of gases 48
DilTUsion of liquids W
FricUon of sar&ees 62—65
Friction of cordage 56
Friction rollers 67
Gravitation 68-82
Weight 83,64
AUnettTe forces
... 30,31
Coheiion
82
Cohcsioa figmres of oils
33
Adhesion
... 84,35
Oi(ii»aiy attraction ...
... 86—40
capillary repulsion ...
41
Gaieoas adhesion
43
Apparent attraction and repulsion 43
E&<loiaiose and exosmose ...44—46
CHAPTER IIL; Page 38.
STATICS— OB THR MBCHAKICAL BELATIONB OF BODIES AT BEST.
8tstioi *nd dynamica distin-
gvidMd 65
BqnUibrium of pressures ... 68
Pnoores repreiented bj Unes
or noobcrs 67,68
Composition and resolution
Kesoltant of pressures...
Parallel pressures
Moment of a pressure ...
Equilibrium ot moments
69
70—74
75,76
77
78—90
▼Ill
TABLE OF C0KTEKT8.
Theory of couples 81—83
StiuUcal problems 84
Centreofgravitj 86,86
Centre of gravity of a line ... 87
Centre of gravity of a sorfkoe ... 88, 89
Centre of gravity found by sus-
pension ... ... ... 00
Centre of gravity in a body of
variable density 92
Equilibrium 93,94
Stability 96
Path of the centre of gravity 96—100
Equilibrium ol a cone 101
Construction of an arch . . . 102
EquUibrated arch ... 103—106
Equilibrium of the arch in prac-
tice 106—108
CHAPTEB IV. J Page 69.
THE MECHANICAL POWERS, OB 8IUFLE MACHINES.
Exchange of time for power ... 109
The lever 110-113
The balance 114
Balance of precision 116
Weighing with fklso bslances... 116
Method of double-weighmg ... 117
Power of the lever ... 118,119
The steelyard 120
Varieties of levers ... 121—125
Principle of virtual velocities ... 126
Principle applied to the lever... 127
Roberval's balance ... .. 128
Weighing machine 129
The gcnou; lever press ... 180
The wheel and axle lai
The pulley 132,133
Systems of pulleys ... 134—136
Conditions of equilibrium ... 137
Diflbrential pulleys 138
Chinese capstan 138
Rigidity or cordage 139
The inclined plane 140
Conditions of eqnilibrixmi ... 141
These determined by experiment 142
The screw 143
Differential screw 144
The wedge 146,146
The eccentric cam 147
Animal mechanism j examples
of levers 14S— 160
CHAPTEB v.; Page 63.
rBINCIPLES OF MECHANISM.
Ol^ect and definitions ... 163—169
Elementary combinations ... 160
Constant velocity-ratio 161
Velocity-ratio in link-work ... 162
Velocity-ratio in contact motions 163
Amount of sliding 164
Boiling contact 166—169
Form of surfaces ... 170,171
Frictional gearing 172
Varieties of toothed wheels 173—179
Forms of teeth of wheels 180—187
Teeth and staves 188
Necessaiy number of teeth 189—192
Addendum, clearing ... 193—196
Practical forms of teeth 197—201
Endless screw 202—206
Uultiple gearing ... 206,207
Wrapping connectors ... 208,209
Forms of pulleys ... 210—216
Parallel link-work ... 217, 218
Link-work in general ... 219—223
Combinations in trains. . . 226-231
Calculating machines 232
Difference engine 233
Principle of its action 234
Varyhig velocity-ratio hi wheel-
work 236—240
Varying velodty-raUo in contact
motion
• ■•
241—243
Parallel cones ...
• ••
244
Fusee
• • •
246
Expanding pull^
Hooke's joint ...
• ••
246
247—260
Hangle-wheels ...
• »•
261,262
Mangle-rack
• ••
263
Cams
«■ a
264
Swash-plate
• • •
266
Escapements ...
■ ••
266-268
Propelments
• ••
269
Reciprocating linkwork
260—264
Ratchet and click
• ••
266—267
Silent dick
• •■
268
TABLE OF OOMTXIITS*
IX
CHAFTEB YT. ; Page 143.
]>r«AiaC8 — THB BEULTIONS OF BODIES IN UOTIOV.
of motion
[asure of Telodtj
Beiative reloci^...
Aeodexstin^ force
Man<tfabodj ...
Yolome of a oody
Momentain
The eooeossion foxe ...
Pint law of motion
Sesood law of motion ...
ParalleloffTam of vdodtiea
Third law of motion ...
Ccdliai<» and impact ...
Elasticity
CoUiaion of indaatic bodies
Collision of elaetio bodies
Inetdence and reflection
SSbcta of impact
Gravitation
Morin'a apporatoa
Attwood'a machine
Yalne of the force of gravi^
Motion of prqjectiles ...
Parabolic patb
Tdodty of projection ...
Greatest eleration
Hmcofflirht
HorizontaTraoge
OUIqaeranffe
AUb in a resisting medlnm
Prindple of the rme ...
The Mini^ ball
Fli^bt of a rocket
dotation and translation
.. 270
271
272
273
274
.. 276
276, 277
278,281
280
282
283
284,285
286,287
289.290
291,282
283—296
296
297
298—300
301
303
304—306
307
I.. 908
I.. tnW
310
311
312
813
814
315
316
317
818,319
Centrifugal foroe ... 820—4124
Formation of Saturn's ring ... 826
Motion on inclined planes 828; 327
Motion on a corre S2S
Motion of a pendulam 829
Oscillation in a cycloid 830
Time of oscillation ... 831,332
Length of a pendalmn . . . 383, 834
Application to terrestrial phy-
sics 836—837
Conical pendulam • 388
D'Alembert's principle 839
Conservation of vis Tiva 840, 841
Unit of work 34S
Accumulated energy ... 818—846
Moment of inertia 846
Centre of oadllation ... 847—849
Kater's pendulum 350
Centre of pereussion ... 861—363
Compeniated pendulums 864—866
Botation of plane of oscillation 867, 368
Rotation of a rigid body 86&-868
Examples of rotation 864
Thegyraaeope 366—367
PrecMsiou and nutation ... 368
Vibratory motion ... 869, 370
Pro^TdSsive undulations ... 371
Stationary undulations ... 872
Nodal points 373
Vibrations of rods . . . 87^ 376
Transverse and longltucUnal vi-
brations 876
Longitudinal vibrations of arow
of particles 877
Vibrations isochronous 378
CHAPTEB TIL; Page 208.
BTDBOSTAT1C8 — THE FB0PEBTIE8 OF FLUIDS AT BEST.
Properties of floida 879
Their elastidty 880
Oorsted's piexometer 881
C<mipression of fluids 882
Johnson's pressure-gauge ... 383
Socikce of liquids horizontal 884, 885
Spirit level 386
Level sur&ce in communicating
vessels ... ... ••> ... 09/
Hydrostatic level 388
Artesian well 389
Level of liferent fluids ... 390
Level of the sea 891
Pressure on the base of a vessel 892
HydrosUtie'* paradox" ... 893
Bramah's ^ress 394
Hydranlicjack and punch ... 896
Vessel of greatest strength ... 396
Upward pressure 897
Lateral pressure ... 898—400
Centre of pressure 401
Besultant pressure ... 402,408
Equilibrium of floatiDg bodies 404,406
Metacentre 406
TABLE OF CONTENTS.
StabOiiy of flotation ... 407,406
Equilibriam of immenod body 409
Principle of Archimedes 410, 411
Specific ffravity 412
Means or determining it 413—417
HTdrometer 418
Kicholaon's hydrometer ... 419
Hare's hydrometer 420
8tereometer 421
Temperaturo-rarrection of spe^
dfic grsTitics 428
Specific gravity of gases ... 423
luiamples 424
Table of specific graTlties ... 426
CHAPTEE VIIL ; Page 232.
HTDBODYNAMICS— THE PROrSBTIES OF FLUIDS TN MOTION.
Laws of sponting fluids 426, 427
Vena ooutracta 428
M. Lacontoure's results ... 429
Barker's mill 430
Velocity in channels 431
Springs and fountains 432
Geysers hi Iceland 433 '
Friction of li<)uids 434
Action of conical tubes ... 435
Lifting pump 436
Forcing pump 437
Stomach pump 439
California pump 439
Fire engine 440
Hydraulioram 441
Centrifhgal pump 442
Appold'spump 443
Chain ana bucket pump ... 441
Bope pump 445
The syphon 44A
Tantalus' cup 447
Hiero's fountain 448
Persian wheel, &C. 449
Water wheels 450
Turbine 451
Paddle wheels 452
Screw-propeller 453
Windmill sail 464
Steam-engine 456
Watt's steam-engine 466
Non-condensing en^ne ... 457
Stationary, locomotive, and ma-
rine engines 458
Water-engines 469
Steam-hammer 400
Gyrometric governor 461
Undulation of fluids 462,463
Reflection of undulations . . .464, 466
Interference of undulations ... 466
Inflection of undulations ... 467
Lateral accumulation 468
Undulations of elastic fluids 469—471
Theory of Tides 472,473
Sprinv and neap tides 474
Establishment of a port ... 476
Double tides 476
CHAPTEB IX.; Page 268.
FNEUHATIC8 — THE PROPERTIES OF ELASTIC FLUIDS.
Composition of the atmosphere 477
Finite extent of the atmosphere 478
Elasticity of the atmosphere . . . 479
Weight and pressure of the
atmosphere 480—482
Water barometer 483
Mercurial barometer ... ... 484
Syphon barometer 485
Standard barometer 486
Correction for temperature ... 487
Correction for capillarity ... 488
Conical barometer 469
Horary variations 490
Mean diurnal height 491
Annual variation ... ... 492
Geographical variation ... 403
Height at difi'erent altitudes ... 404
Self-registering barometer ... 496
Vedy's aneroid barometer ... 496
Bourdon's aneroid barometer... 497
Bojrle'slaw 406—600
Weight of the atmosphere ... 601
Preesure-gaoge 602
Exhausting syringe 603
Air-pump 604
Smeotons air-pump 606
Cuthbert's and Grove's air-pump 606
Barometex^gauge 607
S}'phon-gauge 606
Condenser 600
Air-gun 610
Pneumatie experiments ... 611
TABLE OF 0OKTEHT8.
Xt
of tbe fttmofiphere. . .
Vareef ■ sppantoa
Hdgbt of aoifonn atraoc^here
^'^ of denritiw of dry air and
Presrare of mixed gasM
Prassore of airandvaponr .,
Fl^irieal propertieB of yapoor
Pmrare of fapoar
612
613
514
616
616
617
618
619
Premxre of steam 620
Lateral pressore of gaaee . . .621, 622
Pressure of wind 623
Anemometers 624
Self-registering anemometers 626
Forecasts of weather 626
Pneumatic lerer 627
Compressed air engine 628
Lenoir's gas engine 629
CHAPTER X. ; Page 204.
acoustics; THB PBODOCTIOIT, TRANSMISSIOIV, AITD PBBCBPTIOll
OF SOUND.
Natvie of soond
Isoehronooa Tibrations
Inftrior numerical limit
GoDdoctiDg medium essentia]
Inteosity of soand
690
631
632
633
634—536
...537,633
ofHelmhoItz ... 639
ion of sound ... 640
CoDcarrence of sounds ... 641
Velocity of sound in air 642-^644
Tdocsfcf of soond in varioos
txidiea 645>-648
InterfereDce of sound 649, 650
Pboaographfl 661,562
Opdcal aeoostie fisfures ... 663
Bxamidas of interference ...664^ 656
riiiisur of sound through hete-
Yogeaaoua media 666, 657
Ooatinnitj of direction 658, 669
Beflectioo of sound 660,662
EdkO ... ... ... ... 661
Be&adiOD of sound 563
Acoustic lens 664
Inflection of sound 666
Siren .•• ... ... ... 666
Timbre, or quality of tone . . . 667
Musical notes 668
Length of sound-waves 569, 670
Yariations of pitch 571
Normal diapason 672
Scheibler's tonometer 673
Musical intenrals 674
Harmonic sounds 676
Vibrations of chords 576, 677
Vibrations of rods 578
Vibrations of air in tubes 679—681
Modes of exciting them ... 682
Vibrations of plates 583, 684
Vibrations of membranes ... 586
Strehlke's experiments ... 586
Trevelyan's experiments ... 5S7
Vocal sounds 688
Vowel sounds 689
Forced Tibntions 690
CHAPTEB XL; Page 839.
XAORBTISIC.— DIAMAGNETISIC.
On^gin of magnetism 691
Magnetic field 692
P^iles of A magnet ... 698,594
Magnetic inductioii ... 695,596
Magnetism a molecular pro-
perty 697—600
Hie mariner's compaae 601, 602
Devlatioo of the compass ... 608
Soorees of deviation 604
Varieties of deviation 606
HorixontAl and rertical induo-
tkni ... ... 606
Hedii^ error •.• 007
Tonstrlal polaiity 609
Declination 609,610
Inclination— dipping needle ... 611
Aclinic lines
Kraflt'slawofthedip
Secular change of dip
Solar diurnal variation of decli-
nation 616,616
Lnnar-diumal change of deoU-
nation ... ... ... ...
Annual change of force
Horizontal and vertical compo-
nents of total force
lyisturbance variations
Periodicity of variations
612
613
614
617
618
619
620
621
zu
TABLE OF CX>HTBKTB.
HMDiofobMiTAtioii ... 622,623
Antomatio registration ... 624
Magnetie storms 626
Eanh-ourrents 626
Local changes of intensity ... 627
VagDetism a <(Jrec<»o« force ... 628
Consecutive poles 629
Manetio metals 680
Moaes of excitation 831
Single tooch 632
DoiU)letonch 633
Jaoobi's method 634
Gompoimd magnets 636
Magnetic capacity 638
Temperatore-change of force... 687
Magnetic metals 688
Law of magnetic attraction ... 639
Artificial magnets 640
Diamagnetism 641— -644
Diamagnetic bodies ... 646—648
Dinmagnetic fluids 649
Effects of msgneUsm ... 660,661
Influence of molecular aggre-
gation on magnetism 662, 668
CHAPTER XIL; Page 366.
FBAKKLnriC ELECTRICITT.
Excitation of electricity ... 664
Attraction and repulsion ... 666
Positive snd negative electri-
city 66B
Conduction of electricity 667, 658
Insulation of electricity ... 669
Concurrence of opposite forms 660
Electroscopes 661—662
Baddiffb's electroscope 663
Coulomb's electrometer ... 664
Peltier's electrometer 666
Thomson's electrometer ... 666
Excitation of various bodies 667—669
Pyro-electric minerals 670
ETolution of light ... ... 671
Superficial distribution ... 672
Electrostatic laws 673
Potential depends on surfhce 674^ 675
Electric induction ... 676—678
Kind of electricity tested ... 679
Spedflc inductive capadtr ... 680
IvadaVs theory of induction. . . 681
Examplos of induction ... 682—684
Electrophorus ... 686—688
Theory of points and knobs ... 689
Earliest electric machines ... 690
Cylinder machine 691
Plate machine ... 692
Mode of using a machine ^ 693
Formstion oioxone 694
Development of potential ... 695
Action of amalgam 696
Ebonite plate 697
Hydro-electrio machine 696—700
Holt^ induction machine 701, 792
Brush and star 703
Sparks in Interropted oondoctor 704
Lane's discharger 706
Induction in a racuum 706^ 707
Heat accompanies a spark 706, 709
Henley's electrometer 710
Electrical toys ^ 711
Convective currents of air ... 712
Dynamic energy manifested ... 718
Spark evolves coloured light ... 714
Forms of discharge 716
Potential constrained 716
Induction on glass plates 717—719
Lerdenjar 720,721
Jomted discharger ... ... 723
Charge by induction 723
Leyden battery 724^726
Residual charge 726
Velocity of electricity 727
Charge not in the coating ... 728
Universal discharger 729
Eflbcts of discharge ... 730^781
Identity of electricity 732
Heat evolved l^ discharge ... 783
Light evolved bv discharge ... 784
Fiffures of Leuchtenberg ... 786
DifiVision of potential 736
Unitjar 737
Condenser 788—740
Multiple inductor 741
Applications of condenser ... 743
Lateral induction ... 743, 744
Unipolar bodies 746
Gradations of oondnctiTity ... 746
Potential of the atmosphere 747—763
Potential of clouds ... 764—766
Lightning conductors . . . 767, 768
cuiguiiies ... ... ... /Ov
St Elmo's lights— Aoiota ... 760
TABLS OP OOHTKVTB.
•••
XIU
CHAPTEB XnL; Page 424.
YOLTAIO ELEGTSICITT.
Amrent flrrfUtion hj contact
ofmetila ... ... ••• 761
Sflbet doe to ebemieal aotlon. . . 762
Beetric and ehamifal aotioii re-
lated 763
ElaetrlcaeTiet of elements ... 761
Toltaie aetlon of zinc and copper 76 >
Comae of tbe current 766
Conent dneio diemieal action 767—770
Saiee^a battery 771,772
Two flnids emplojed 773
Oaoieirs cell 774
Bkctrotype ... .*« ... 776
G rove's cdl •.. «■* ... 776
Booaen'seell 777
Scbonbeln'e cell 778
MOTnoothoell 779
Sobcrts's batteij 780
Leeson'g battery 781
PlsHnnm^potaasiiim comUna-
tion ... ... ... ••• 782
ZfaM-csrbon battery excited by
anlpbate of mercury 783
Modes of prodadngcarrenti 784—787
YolU'spile 78d»789
Pahrermaehei's cbsin-piila ... 790
Blrin^feUow'B battery 791
Marie Dayy'i pile 792
Crailuhanrs Dattexy 798
Snlell's battevy ... 79^795
Grovels battecy 796
Obm's theory 797
SadoedoDsfttmit ... 798^799
b's bridge 800
Form fVeqaently naed 801
Unitofresistaooe 808
Resistanoe coils 803
Kneostat ... ..■ ... ... 80a
Condnctivity of metals 805
Kleotrio light 806—809
Serrin's electric lamp 810
Slratiaed discharnre 811
Inflaenoeofresistanoe... ... 812
Insolation by a vacunm ... 813
Inflaence of magnetism ... 814
Ignition of wire 816
Oalranio Icuife and canteiy ... 816
Cold produced by a current ... 817
Dry piles ... ... ... ... 818
Grove's gas battenr ... 819,820
Water decomposed ... 821,823
Ozone formed 828
Ozone generator 834
Definite electrolyiie ... 825—827
Voltameter 838
Secondary currents 828
Contrary energy...
of th<
830
CondacCiTity ot the fluid !.'! 831
Electrolysis by one element 832—834
Beoquerel's battery 835
Seduction of metals ... 836—839
Reduction of ammonium 840,841
Electrolysis by a fl^nklinio cur-
rent 842,843
Apparent anomalies ... 844—846
Bednctlon of salts ... 847,848
Franlilinio and voltaic currents
compared 840
CHAPTEB XIY.; Page 478.
ELECTRO-DTKAMICB.
ActhmofaenrrentoD a magnet 850^ 851 1
Ampere's law 862
Law of amount of action ... 853
jBvenor ... ... ... ... 864
Galvanometer 856
Astatic needle 856
SquUfbrlnm of needle 857!
Bensltfve galvanometer ... 858
Tlkomson's reflecting galvano-
meter ... ... ... '•• 8)9
nomaon's marine galvanometer 860
Gaogain's tangent galvanometer 861
MagaetiamolAeo&diiGtor 862,863
Mutual action of ourrenta ... 864
Roget's galvanic spiral ... 866
Rotation of a magnet round a
conductor 866
Rotation of a conductor round a
magnet ... ... 867,868
Vibrating conductor 869
Spur wheel 870
M agnetic effects of a current 871, 872
Dels Rive's ring • 873
Rotation ofDe la Rive's ring 874
Electro-d>namic cylinder 875, 876
Ktoctro-magnets ... 877* 87B
XI?
TABLB OF COKTBKTS.
Botation of magnets ... 879,880
Botation of one condootor
round another ... 881,883
Theory of Ampere ... 883,884
Electro-dynamio engines ... 886
Induced, or secondary cur-
rents ... ... ... 886 — 880
Self-indaction in a coil. . . 880, 891
Elongation of a bar by magne-
tisation 892
Shock from seoondazy coil ^ 893
Currents in a revolving disc ... 894
Electro-magnetic machines 896—807
Inductorium 896—908
Faraday's original experiment 904
Magneto-electrio machines 906 — 907
Siemens and Halske's magneto-
electric machines 906
Wilde's magneto-electrio ma-
chines 911
Induced rotation ... 912,913
Induction on varioiu metals ... 914
CHAFTES XY. ; Page 614
ELECTRO-TELEGBAPHY.
Ronalds' telegraph 916
Needle-telegraph 916
Communicator of needle-tele-
graph ... ... ... ... 917
Alarum-bell 918
Wheat8tone*s rotating disc tele-
graph ... ... ... ... 919
Wheatstone's magneto-telegn^>h 920
The first magneto-telegraph ... 921
Siemens' magneto-telegraph ... 922
Wheatstone's private tele-
graph 923—926
Morse key 926
Horse telegraph 927
If orse alphabet 928
Horse printing telegraph ... 929
Belay ... ... ••• ... 980
Siemens' polarised relay ... 931
Bain's eieotro-chemical tele-
graph ... ... ... ••• 932
Wheatstone's printing telegraph 933
Wheatstone's automatic tele-
graph ... ... ... 084
Buewell's copying telegraph... 936
Conditions of long circoits ... 936
Submarine tel^raphy ... 937
Siemens and Ualske's cable ... 938
Hooper's core 939
F. Jenkin's diiforential gearing 940
Electrical teste 941
Resistances of metals and alloys 942
Resistances of copper, ftc., at
various temperatures ... 943
Insulation teste 944
Induction teste 945
Teste for fkulte in a cable 946, 947
Teste for fkulte in land Unes ... 948
Tests of resistance of a battery 949
Electro-magnetic loom 950
Electric docks 951
Bain's electric clock 962
Shepherd's electric clock ... 963
Froment and Hardy's clock ... 966
Synchronous docks , 956
Wheatetone's chronbsoope ... 967
Chronofrraphs 968,060
Greenwich time-bell
Electric regcdator
060
961
CHAPTEB XVL; Page 646.
THEBKO-ELECTRICITT.
Origin of thermo-dectridty ... 962
iniermo-series of metals ... 968
Current from one metal un-
equally heated ... ... 964
Cmnent shown by attraction... 966
Thermo-dectric rotation 966,967
Thermopiles 968,969
Pdtier's hygrometer
Beoonerd's and Marcus* thermo-
Conversion of thermic and dy-
namic onergy
ThermoHsnrrente of low poten-
tial
w.— * ... ... ... ...
Thermo-dectric oombinatioDS
970 Eleotro-thennio eflBMta...
971
972
m
974
976
TABLE OP COMTEHTS.
XY
CKAPTEB XVIL; Page 662.
OBOANIO ELEGTBIdTT.
SItetrielbhes 976
ToTpeik) 977
GymDotot 978
Fvadaj'B experiments on gjvch
notofl 979
Direettooof emrrent in gymnotna MO
Silanis 961
Electric imectfl 982
GahraBii's discovery 983
Yolta't experiments 984
Aldlni'f experiments ... ... 985
LawofVaUi 968
Kenro-electric theory ... 987,988
Vncealsr currents 989
Matteofid's fVotr-bftttery ... 990
Mstteood'a ftog rheosoope ... 991
Pigeon-battery 992
Dlieetion of currents 993
Proper corrent in frogs ... 994
BeseanhesofDaBois-BeynKmd 996
BeaeaRhesordianveaa ... 996
Fh^ological effects of a onrrent 997
Evidence in favoor of the dy-
namic theory 996
PhvBiological effects not dne to
direction of current . . . 999, 1000
Muscular and nerre currents
subside with functional acti-
vity 1001
Eckbard's experimimts ... 1002
Rate of transmission in nerves 1003
Time of - transmitting impres-
sions 1004^1006
Electricity firom friction ... 1006
Electricity firom chemical
changes 1007—1009
Liebig's theory lOlO
Indications of potential 1011,1012
Piles of organic tissue 1013
Electricity in animal ftmo-
tions 1014—1016
Vegetable electricity 1017
CHAPTER XVIIL; Page 676.
LIGHT — CATOPTRICS AHD DIOPTBIC8.
Naftnre of light ... ... •••
Undnlatory hypothesis
Means of transmission
Natare of wave-motion
Analogies of sound and light ...
Telodty of light
Lizminoail7 1024^
Law of intensity
Fhotocnetry ... •*•
Cokored rays 1028,
Modifications of light
Direction of rays
Law of illumination
BeAeetaon of light ... 10S3—
Bcflection from a plane
Image defined ... ... ...
Baecwaion of images
Eefleetlon from a concave mir-
ror-focus 1039 — '.
BcAectlon firom a oonvez mirror
Caustie carves
Image in concave mirror
Image in convex mirror
SpheriMl aberration
Least circle ci aberration
ObUqiae reflection— focal lines
CSrele of least confiiaion
:oi8
019
.020
1021
022
.023
026
.026
.027
i020
.090
031
032
066
086
.087
038
.041
042
.043
044
.046
.046
.047
048
.049
Curve of sorfiMe of mirrors 1060, 1051
Fouoault's gloss speculum ... 1052
Method of silvering 1063
Refraction— law of sines ... 1054
Mutual direction of ravs ... 1065
Movable diagram of refraction 1066
Index of refraction 1057
Relative refraction 1058
Velocity varied in refraction ... 1069
Intemu reflection — limiting
angle of refiraction 1000
Irregular refraction 1061
Newton's experiment 1062
Direction of refracted rays ... 1063
Prism 1084
Refiraction at a spherical sur*
ilMiO 1065,1006
Caustic bvreflm^ion 1067
Forms of lenses 1068
Fochs of a sphere ... 1069, 1070
Refiraction through a lens ... 1071
Focus of lenses 1072
F06US of concave lenses 1073—1076
Focal length of combinations 1076
Images formed by lenses 1077, 1078
Magnifying power ... 1079^1080
Spherical i
ition in lenaes 1061
ZTi
TABLE OF CONTEVTB.
CHAPTEB XIX.; Page 611,
UQHT — CIIJU>MATIC8.
Prismttlc deeompoeitioii
Goloon in solar Bpectnun
Beflraottre indices of coloured rays
Kecompodtion of colourless
MM^Mmw ••■ ■■■ ••• ■••
Leiwth and Telocity of waves
of ooloared light
LaTender band of Herschd ...
Simple oolonrs
Primary ooloors
Superposed spectra ... ...
Ccnnposition of colours
Gornam's colour-top
Absorption by transmission ...
Absorpti<m by reflection
Mean rays ofspectrum
Prismatic dispersion
Dispersive powers
Irrationalitr of spectrum
Fraunhdfers lines
Absorption bands
Indices of fixed lines
Temperature-ebaDge of index ...
Lonjgmapofspec^m
Variation of light in upectrum
Tariation of heat in spectrum
Variation of chemical action 1106,
Spectrum-analysis
082
083
084
066
086
067
068
089
090
091
092
093
094
096
096
097
098
099
100
101
102
103
104
106
107
108
Cttsium, Rubidium, and Thal-
lium discoTered
Spectra of gases
Character of Ifaies in spectrum
Inferred nature of solar photo-
sphere ... ... ... ...
Spectrum of a temporary star
Fluorescence
Spectrum of invisible rays
Laws of fluorescence ...
Ingesta detected
Phosphoresceuce
1108
1109
1110
nil
1113
1118, 1114
1116
1116
1117
1118
Chromatic aberration of a lens 1 1 19
Mode of correction 1120
Interference of waves 1121
Analogy in sound 1122
Experiments on interference 1128,1124
Difflraction 1126
Explanation of diffhustion ... 1126
Experiments ou diffraction 1127—1131
Colours of thin plates ... 1132
Complementary colours ... 1133
Kewton's chromatic table ... 1134
Rings in.bomogeneous light 1136, 1136
ThinplHtesofair 1137
DiffWiction by small particles 1138
Barton's buttons— Moberfs lines 1118
Theory of the rainbow ... 1140
CHAPTEB XX. ; Page 648.
OPTICAL IKBTRUMEMT8.
The concave mirror ... ...
Newton's telescope
Chregory's telescope
Cassegr^u's telescope
Browning's equatorial telescope
Speculum and plane mirror of
tdescope ••• ...
Camera ebscora
Megascope
Prismatic lens
Solar microscope
Magic lantern
Camera ludda ... •.• ...
Mirror of Soemmering
Stanhope and Coddington lenses
Shnple microscopes
'Wollastou's doublet
Compound microscope
Achramatio microscope
Compound olyectives
Roar a<yustinigol4eetive
1141 Ne([ative eye-pieoe • 1160
1142 Positive eye-inece 1160
1143 Modes of obtaining Ugh power 1161
1144 Wenham's binocular microscope 1162
1146 Modem compound microsoope 1163
Magnifying powers 1164
1146 Adjustment of illumination
1147 necessary 1166
1148 Dark-ground illumination ... 1166
1149 Glllitrs condenser 1167
1160 Oblique illumination 1167
1161 Amid prism 1168
1162 Kelracting telescope 1169
1163 Erecting telescope 1170
1164 Galiho's telescope 1171
1164 Opera gUuses 1172
1166 ' Spectroscope 1173
1166 i Rigid spectroscope 1174
1167 I Straight spectroscope 1176
1168 I Speciro-Rucroacope 1176
1160 I Fahrenheit's heliostat 1177
TABLB OP 00HTBST8.
mi
flnbsrmann's helkMtei 1178
StnifCture of the eje 1179
BefravtioD of the hamoan ... 1180
Their action on light 1181
Xjei of the higher orders liniilar
iaatructure 1182
Fonaation of images 1183
The two imagee corohlned ... 1184
Sllbet of hlno<ailar Tiflioa ... 1185
Ailaptation of the ^e 1186
M jopie vt«ioo 1186
Pnabjopie Tiaion 1187
Astigmatiam 1188
Duration of ImpreMioDt on the
retina 1189
Spectral coloars ... 1190—1193
Coloor-biindnees 1194
Binocular Tiition 1196
Itefleeting stereofcope 1195
Refiractiug stereoeoope ... 1199
Pieudoaoope 1197
Ophthalmoscope 1198
Larrngoaoope 1198
Kodofloope 1199
GHAPTEB XXL; Page 688.
POLABIZED UGHT.
Dooble refraedon, ordinary and
extraord nary rays
Principal aection in oystali ...
Optic axia
Uniaxial ciyatala
Hnjgena' law of ▼dodties . . .
Lawofrefractioo
Biaxial crystals
Beriation of both rajs
ftrahi prodneea doable refraction
Form oT wave-ftront
Plane of polarization
Modes of polarixlog
jneol'8 prism ... ••• ...
DoaUe-image prism
Folariayon by agate
Polarisation by tonrmaline ...
Pdariation hy hcrapathite ...
Biof s polarisoope
latcsstty of polarised light
Identity of polarized Hsrht
Actum of a Dandle of plates
Piitlal poiarixation ...
Bktwater's law of tangents . . .
Polarisation by internal reflection
Polarixation of coloured rays ...
Polarisation of day-light
Wbeat«toae'a polar cloclc
Iaws of Arago and Fresnal ...
Ttau of vdenite plates
Oomplnncntary tints ..,
Biafs <^ intorterenoe ..
BiagB in calcite
1200
1-aOI
1202
1203
1204
1206
1206
1207
1206
12Q0
... 1210
... 1211
... 1212
... 1213
... 1214
... 1215
... 1216
1217, 1218
... 1219
... 1220
... 1221
1222,1223
... 1224
1226
1226
1227
1228
12z9
1230
1231—1234
1236,1237
Interferenoe of positiTe and
negstive crystals 1238
Rings with homogeneous light 1239
Rings in biaxial crystals 1240-1243
Rings in nitre. Ac. ... 1244^ 1246
lirewBter's mode of analysis ... 1246
Rings in aiiaunealed glass 1247^1260
Leconnt's polariscope 1261
Depolarization by organic sob*
stances 1262—1265
Circular polarization 1266
Resultant waves ... 1267, 1268
Fremel's rhomb 1269
Actiun of quarts 1260
TioU produced 1261
Rotation of rays ... 1262,1263
Tints always mixed 1264
Boution by msgnedo energy 1266, 1266
Blot's apparatus 1267
Rotating power of Tarious
bodies 1268—1271
1272
1272a
1273
1274
1275
1276
1277
1278
1279
128(k
Opposite rotating powers
Jellett's saccharometer
Influence of temperature
Influence of solution
Elliptic polarization
Change of rinrs in ealdte
Angle of polarization
Polarization by suocesai?e re-
flections
Table of elliptical constants ...
Dichroiam
CHAPTEB XXIL; Page 720.
CHBHICAL. AOnOH OF UOHT — PHOTOORAPHT.
Chamieal action of light ... 1281
CUoride ot siWer decomposed 1282
Xassaremeni of tolsr acwoa... 1283
Amount dependent on state of
atmosphere ... ... ... 1284
Action of colooredn^ ... 1285
zrili
TABLB OF OORTKim.
1286
Opposing inflaence of t»ts • . .
Influenoe of rays ttt mfferent
places 1287
Action of lupra-cpectnl njt . . . 128S
Photography 1289
CopvtnjT by raperposition ... 1280
Posltivea and negatirca ... 1291
Pint attempt! 1292
The camera 1293
Achromatic lens 1294
Orthoacopio lens 1295
Panoramic lens 1296
Camera frame 1297
Dagaerreolype 1298—1300
Processes on paper 1301
Selection of paper 1302
Application of chemicals ... 1303
Solution of silver 1304
Argentotype 1305
PotitiTe pictures 1306
Developing a^nts 1307
Calotype 1306
Modification of calotype ... 1309
Means of incressing sensibility 1310
Channing's process 1311
Paper for photographic registration 1312
Waxed paper process 1313
Albuminiied paper 1314
Perrotype* 1816
Cyanotype 1316,1317
Amphitype 1318
Chromotype 1319
Anthotype 1320
Action of heat 1321
Potiitive printing 1322
Collodion processes 1323
Preparation of plate 1324
Development 1325
Protection by varnish 1326
Collodion positives 1327
Black varnish for ooUodion
positives 1828
Dry collodion process 1329
Albuminized glass 1330
Reproduction of colours ... 1331
Image in microscope photo-
graphed 1332
Micro* photographs 1383
Stereoscopic slides 1334
Celestial photography 1835
Carbon-printing 1336
Photo-lithography 1837
Photo-glyphy 1338
Photo-galvaiiography 1839
Photo-sculpture 1840
Necessity of cleanliness ... 1841
CHAPTEB XXIIL; Page 766.
THBBIIIC8.
Theories of heat 1342
Proximate sources of heat . . . 18-13
Dynamic theory of heat ... 1344
Dynamic equivalent of heat ... 1345
Relations ot heat and work 1346,1347
point of absolute cold 1848
Sensations of heat and cold ... 1349
DilaUtion by heat 1850
Dilatation of glass, water, and
mercury 1351
Dilatation of gases 1352
Bxoeptions to general law ... 1358
Air-thermometer 1354
Air and mercurial oompared ... 1356
Differential thermometer ... 1856
Mercurial thermometer ... 1857
Scales of division of thermometer 1858
Compound bar thermometer ... 1359
JohxMon's deep-sea thermometer 1360
Maximnm and minimum
thermometer 1861
Merourial maximum thermometer 1862
Casella's minimum tiiermometer 1863
Actinometer 1864
Aoenrate graduation 1865
Index errors 1368
Thermograph 1367
Appold's regulator 1368
Wedgwood's pyrometer ... 1869
Dani ell's pyrometer 1370
Bvstrdm's hydro-pyrometer ... 1371
Thermic unit 1372
Conduction of heat 1373
Law of propagation 1874
Influence of stmcture on
conductivity 1876
Influence of temperature ... 1876
Influence of molecular aggre-
gation 1877
Liquids bad oonducton ... 1378
Gases still worse conductors ... 1379
Application to clothing ... 1380
Senvation of heat 1381
Convection of heat ... 1882,1883
Hot-wster apparatus 1384
Law of convection by gaaes ... 1886
Trade winds and Gulf-stream 1386
Isothermal lines 1387
Specific heat 1388
Calorimeters 1389
Calorimeter of Regnanlt ... 1380
Calorimeter of Prof. U. Kopp 1891
Specific heat of various bodies 1892
Influence of temperature on
specific heat 1308
Specific heat of gaaei 1894
TABLB OF COXTBMT8.
Z1X
Atomieheat 1906
Iaw of spedfie eapadty In gnaes 1396
Latent heat 1397, 130H
Amoants of latent heat ... 1399
Absorption of heat in liqneflus-
tkmnrsolida 1400
ErolutiooofheatonioUdlfleation 1401
Abaorption by vapours ... 1402
Distillation 1403
'*Dr7"ateam 1404
Latent heat of steam 1405
Latent heat of Tapoun ... 1406
Temperafcareoffaiiion 1407
Infliuncie of preuure on the
melting-point 1406
Influence of mixture of elements 1409
Kegelation 1410
Temperature of ebullition de-
pend* on prenure ... 1411,1412
BelatioD of boiling-point to
eompoeition 1413
Presence of matter necessary. . . 1414
Spheroidal state of liquids 1415, 1 416
Bonti(^y's experiments ... 1417
Vaporizition of fluids 1418
Pall of temperature in fluids ... 1419
I eo-inaking machines 1420
Cryophorus 1431
Aqueous vapour at common
temperatures 1421
Deir-point 1438
Daniell's hygrometer 1434
iveguault's hygrometer ... 1425
Mason's hygrometer 1426
Psychromeler 1126
Evaporation in animals ... 1427
IMrect transition fh>m solid to
gaseous state 1428
Condensation of gases 1429
Heat of combination 1430
Water decomposed by heat ... 1431
Terrestrial heat 1432
CHAPTEB XXIV. ; Page 801.
BADIAMT HEAT.
Pereeption of radiant heat ...
Heat reflected to a fucua 1434^
Theorj uf exchangee
Identity of njiture of light and
Bate of cooling by radiation ...
Bate of oooHnff by convection. ..
Proportiuns of heat reflected ...
Forbes' thermopile
Befraction uf heat
CooTen^iice by a rock-«alt lens
Beftaetion by prism of rock-
■BvW -•• *•• •■• ■••
Tramcolcncv and transparency
contras'eu
Hrat-spectmm of electric lamp
Calorei>cett>-e
BurfiMe-radiatlon
Bate of cooling depends partly
ooaarfluM «
Terrestrial ntdiation
Heaos uf comparing radiation
Xoaer's fig*ires
Thenp<igraphy
Badiatioii and absorption paral-
klpiopv-riies
Influence of pulverization
Influence of chemical constitu-
tion
Ba^acinir power little influ-
enced bvcommlnatiun
QHaiii/^urbeat
Means of cumpariaon
Abaorption oj transparent
BOnia «•• ••• •••
1433
1495
1436
1437
1433
1439
1410
1441
1442
14&3
1444
1445
1416
1447
1446
1440
14S0
1451
1462
1453
1454
1455
1456
1457
1458
1459
1460
Quality of transcalency ... 1461
Comparison of results 1462
Physical change of heat-beam 1443
Hypo^pectral rays nut luminous 1464
Bock-salt UoHt permeable to its
own radiations
lYansoalency of fluids
Internal absorption
Quality of radiaut heat
I ronscalency of gases
TranM^alency of gases at dUTe-
rent densities 1470, 1471
Radiation and absorption paral-
lel in gases
Absorption at different pressures
TrA'isparency and transcalency
not comparable
Absorption by vapours
A bsorptions compared
Absorption by aromatic vapours
Radiation from nsea
Internal nuiiaiion
Absorption by aqueous rapour
Absorption at various tempera-
•111 68 ••* •■• ..a aaa
Radiation flrom flames
I'olartsation of heat
Polariz.iiion by plates of mica...
Polarization by reftacliou
ttimugh rock-salt
Polarization of solar heat
Uepolarisation of heat
Circular polarizntioii of heat 1480,1400
CirouUr polarization by Fres-
nel's rhomb 1401
1465
1466
1467
1463
1160
1472
1473
1474
1475
1476
1477
1478
1479
14d0
1481
1432
14!»
1484
1496
1486
1488
62
INTRODUCTION,
OV TUB VATURS OF SNBMQT, AND TBS OOSRBLATlOir ABB
TBAB8MUTATJ0N8 OF ITS VARIOUS PBTSICAL FORXA
As the teniiB '' force** and ''energy,** with their qualifying adjuncts,
'* actual'* and "potential/* will, in the following treatise, be defi-
nitely employed, it may be as well, in liminet to define the terms
themselves. The term energy means simply the power of doing
work ; force means the power of prodncing energy. These terms
have been frequently confounded together ; thos we are accustomed
to speak indifferently of the force of the powder, and the " force*'^
of the shot. But this is one of those confusions of terms, that is
very likely to lead to a confusion of ideas: strictly speaking, the
powder has force, the shot only energy* Again, the force of the
powder is only potenHal^ or capable of being called into activity,
while it remains yet unignited ; but on the moment of ignition, its
force becomes aetttal. Again, while the raised steam-hammer
reposes tranquilly on its soft cushion of steam, the force of gravity
in the one is counteracted by elastic force in the other, and the
energy of the hammer is potential only ; but when the cushion is
withdrawn, that energy soon becomes destructively actual. The
term " actual" is not constantly employed, but may always be im-
plied in the abeence of the qualifying adjunct, " potential :*'^ and
moreover, the term, potential, is frequently employed elliptically
for potential energy, thus we speak of the potential of an electric
charge, or of a voltaic current.
while speaking of the relations of force and enei^, it may
be well to notice, in reference to the sequel of this work, a
radical and misleading error that has found its way into some
elementary treatises on Physics, that of including time as an
element of an unit of worlc, or d^amic unit, or foot-pound, as
It is commonly estimated in this country. The Author has
noticed the foot-pound defined as the force required to raise one
pound through the height of one foot tti one second. Now the
insertion of the last clause of the definition is worse than useless :
it is mischievous, because it is misleading ; — ^the definition shoola
XXll IHTSODUCTIOV.
involve the amount of work only, which is wholly irrespective of
the length of time occupied in doing it.
The inevitable tendency of the comparison of innumerable
carefully observed and recorded facts, revealed by modem phy-
sicists, is to satisfy and convince the reflecting mind of the unity
and universality of force, and thence to guide the mental vision
(unless it be obscured and distorted by the false pride of human
reason) to the unity of that Almighty power by whose arm uni-
versal force is wielded.
It is a difficult thing to dislodge -ideas from the deeply-worn
grooves, in which they have long and perhaps smoothly run ; hut
the writer cannot doubt that ere long all the physical phenomena,
that are amenable in all their endless variety to our senses, will
he acknowledged to be the results or effects of various but inter-
changeable modifications of energy. And there can be probably
as little doubt, that the universal medium of communication be-
tween mind and matter, the means by which the impressions of
external things are one and all rendered cognizable to the senses,
is wave-moivon^ excit«d in the molecules of palpable matter.
With regard to the functions of the eye and ear, the application
of this law 18 not less evident, than universally acknowledged, pro-
Tided electricity and magnetism be admitted (as they must be)
into the category of wave-motions.
The sense of touch or feeling is awakened only by actual con-
tact or impact, or by the closed-allied wave-motions of heat and
electricity, %.«., by dynamic, thermic, or electric energy ; and the
amount of sensuous impression is progressive from the lightest
touch to the actual teanng asunder of the nerve filaments by saw-
teeth (for the edge of the keenest razor is nothing more than a
saw), and the mode of excitation may, in this case, be roughly
■ymbolized by the action of a bow on a stretched chord. But in
physiological as in physical dynamics (279) time fr^uently enters
in as an important element, for relatively considerable time is
occupied by the transit of electric motion alon^ the nerves (1003),
this transference being probably in analogy with that of thermio
motion by conduction, whereas electric induction is probably more
in analogy with thermic radiation. Thus also in a rapid stroke of a
keen cutting instrument, or in the rapid motion of a bullet through
the living tissues, the impression on the nerves may be too sudden
for transmission : just as the damage to a sheet of glass by a rifle
shot is confined to the point of impact, while the glass is shivered
into a thousand pieces by the far less energetic impact of a stone,
or a brickbat.
Again, the sense of itching or tingling is excited by the frec^uent
repetition of very slight impulses either of dynamic, thermic, or
electric energy ; and the intensity of the sensation (when intense
more intolerable than even actual pain) is proportioned not to the
mnargy of each impresdon, but to their^/refueiMy. The excesaivo
IVTBODUCTTIOir. ZZlll
tingliiig of the 8o-caIled " foot asleep" at the moment of the resto-
ration of onimpeded fluid motion through the capillary vessels is
notorious.
llie dependence on wave-molion of the closely allied senses of
taste ana smell may be not less readily indicated. Very cold
sabstances are notoriously tasted and smelt with difficulty, and
the sensibility of the terminal loops of the olfactory and gusta-
tory neryes is considerably impaired by cold : for example, odours
are less perceptible on a very cold day, when the olfactory sur-
faces are chilled by respiration; and the sense of taste is noto-
riously impaired when the gustatory surfaces of the mouth are
chilled by ice. Thus it appears that the sensuous impressions of
smell and taste are influenced quantitatively by temperature, %.e,,
by the amount of thermic energy. But as to qualitative influence
nothing is known with certainty : the minute anatomy of the heat
spectrum is a much more ioscrutMble subject than that of the speo-
tnun of light ; but for all that is known to the contrary, the odours
of the rose and the violet may be due to relatively red and violet
rays of the heat-spectrum impingiug upon " resonant'* or recipro-
cating vapours, which are capable of taking up and imparting
to the sentient nerve-fibres their own peculiar periods of wave-
motion. It may perchance be, moreover, that to the naturally
acute and hiehly cultivated olfactory sensibility of a Rimmel, the
" Harmony oT Perfumes" is as much a reality, as the harmony of
ooloiirs to the eye of the painter, or the harmony of sweet sounds
to the musician's ear : these several faculties being alike acute
perceptions of vibratory cougruencies. It may then with much
probability be assumed, that the universal means of exciting
sensuous impressions is wave-motion.
The universality of wave-motion as the connecting link between
mind and matter, having been thus premised, the nature of the
motion resulting from each recognised form of energy becomes an
interesting subject of investigation. In the waves of sound the
direction of the disolacement of each vibrating particle is demon-
strabW lon^tudinal, or coincident with the direction of the wave
(Ch. X.). In the waves of light and heat that displacement is in-
ferentiaUy proved to be transverse by the phenomena of refraction,
diffraction, interference, and polarization (Ch. XIX. and XXI.)i
which are wholly inexplicable on any other hypothesis. It will
appear in the sequel tnat there exist valid grounds for assuming
etectricit)r, and conseouently magnetism, to be forms or modes of
wave-motion, and furtner that at all events the magnetic wave is
a spiral, the path of each disturbed particle being probably a
cin^e in a plane to which the direction of the wave is a normal.
The principle of the "conservation of energy" implies that
when once actual energy has been developed in matter it cannot
be annihilated, it can only be transferred in some form to other
matter. So imivenal is the troth and practical application of this
ZZIV iKTBODUCnOV.
principle of conserratioii that it may almost be taken as an axiom,
that it is no more within the narrowly bounded power of man to
create or annihilate /orce or energy, than it is to create or annihi-
late matter itself: energy mny be yariously transmuted and
directed, and matter may be variously combined and modified in
form and physical properties, bnt that is all. This principle has
been so ably advocated by Mr. Grove in his "Correlation of
Physical Forces," and by others elsewhere, and will be found so
repeatedly illustrated in the following pages, that it will snfBce
here to mention a few examples that nave more recently been
presented to notice. The writer has clearly shown the inter-
change of thermic and dynamic energy at the point of junction of
the bars of a thermoelectric element of antimony and bismuth
(972), and he has also pointed out (997) that the dynamic nature
of electric enei^gy is not less clearly indicated by tiie long known
fact that an ordinary voltaic current always commences with a
rush, as it were, the instant that the circuit is closed. The dyna-
mical cause of this is clearly pointed out by an experiment due to
the genius.of Prof Wheatstone. If a tuning-fork, tne tail of which
is inserted longitudinally into a wooden handle, like a file or chisel,
be made to vibrate, and the end of the handle rested obliquely on
a table, the resonance of the table will instantly be heard, but on
moving the diapason parallel to itself in any direction on the table,
the resonance ceases, from the perpetual interference of the suc-
cessive planes of vibration with each other. But now comes the
illustration : — On arresting the motion of translation the resonance
immediately recommences, but with a rush or momentary increase
of sound : this most unquestionably arise from the resistance offered
by the inertia of the molecules of wood to the commencement of
wave motion ; and the parallel phenomenon in electricity may un-
doubtedly be similarly accounted for. And the momentary reflex
current (the terminal extra current of Faraday), which is well
kno^n to take place at the instant of opening the circuit, is equally
susceptible of a dynamic interpretation ; it is the analogue of the
wave reflected from the fixed end of a stretched chord, after having
been imparted by the hand to the free end.
The dynamic nature of electric energy is clearly indicated by
the dynamo-electric* machine of Holtz (701), in which dynamic
is directly converted into electric energy, and by the cognate
machines of Wilde (911), Wheatstone.f Siemen8,t and Ladd,t in
* The Author hM elsewhere appUed (p. 660. «o<«) a definite and inteUiipble
meanrng to the oooBtraotion of these oompoond terms, which moat be
oonetantly employed in reUtioo to the conTeraioos of energy ; this may be
•ooompliahed by uking the llret section of the tenn to mean the acHno
eoMe, the aeoood, the ruuUin§ efeet} thns • dynsmo-eleolrio machine wiU
he O'te in whieh dynamic eoergy la emploved to produce an electric onrreot^
and an elfotro-dynamio engine, one in which a correat is employed to evolve
dynamic energy.
t Prooeedings of the Boyal Society, Feb. 1867. | lb. Msrah, 1867.
nmoDncnoy. xxv
ftU of whicli alike there is an interTening coiiTenion of dynaimo
into ma^etic energy. The enormoos amount of current-energy
erolTed in Mr. Wilde's machine, when the power of a steam-engpne
18 employed to rotate the armatures, may he jud^d of by the fact
that a long piece of platinum wire 0*2 inch in thickness was seen
to be disintegrated and partially fused. It is difficult to conceive
that in these instances dynamic energy can be converted into
magnetic " fluid,*' and that again into thermic energy : the con-
veniion of motion into matter, and the subsequent reconversion of
matter into motion, is obviously impossible.
Some further consideration of the effects of electric energy may
serve to indicate the probable nature of the wave-motion. The
facts of electric and magnetic polarity imply and necessitate
a polaritjT or directionality in the motion itself, which has no
analogue in the wares of sound, light, or heat. This requirement
is fully met by the hypothesis of a circular spiral wave, analogous
to that of a pencil of circularly polarized light, the motion of
which 18 direct or positive if viewed from one end, and retro-
grade or negative if from the other; and this suffices to explain
the well-known polarity of electric and magnetic induction.
Thns far the spiral hypothesis is merely inferential| but in ro«
gaid to magnetic wave-motion some strong presumptive evidence
may be aduuced. it appears from the experiments of Mr. Joule,
made more than twenty years ago, that if a suspended mass of
copper be, by twisting the suspension, made to rotate between the
poles of an unexcited electro-magnet, the rotation of the mass is
anestod the instant the magnet is excited ; and furthermore, if the
mass be forcibly rotated, heat is developed in it. And it has been
since ascertained that if two cylindrical magnets be so placed
that their axes lie in the same straight line, and their contrary
poles are opposed to each other, then if a cylinder of copper be
made to rotate on its own axis, coinciding with the common axis
of the ma^ets, no heat will be evolved by its rotation.
Now these phenomena must alike be the necessary conse-
quences of the assumed dynamical theory ; for if the copper mole-
cules be thrown into sniral-wave motion, then the motion of all the
distorbed particles wiA be one of revolution in planes to which the
lines of magnetic force are normals: and the inertia, or energy, of
rotation (as it has been variously termed), ».«., the resistance offered
hv each revolving particle to any change in the direction of its
plane of revolution (as exemplified by the gyrascope), will resist
the rotation of the mass in any direction perpendicular to that of
the axes of molecular revolution, and arrest its motion. And con-
versely^ if the mass be forcibly rotated in the above direction, or
in any other direction at right angles to the lines of roaenetio
force, heat will be freely developed, doubtless bv internal friction
arising from the ]>erpetnal displacement of the planes of molecular
isn^ution. But in the second case, the axis of rotation of the
XXTl TNTRODDOTIOH.
mass coincides in direction with those of the axes of molecular
revoluiioni hence there is no displacement of the molecular orbits,
and consequent!/ no internal friction, and ver/ little if any heat
is generated.
The rotatory character of the magnetic wave is further con-
firmed by the known fact that if a plane polarized beam pass
through a transparent solid in the direction of the lines of force of
a powerful electro-magnet, the plane of polarization will be
rotated the instant that the magnet is excited. The truth of a
theory can be established only by the Terification of its necessary
consequences; and it may not be too much to assume that in the
present case the evidence already adduced by the writer is, in the
entire absence of all contradictory evidence, strongly presumptive
of the reality of the hypothesis.
It has been authoritatively stated that ordinary electric and
magnetic waves cannot both be assumed to be spirals, because
each of these forms of energy notoriously evolves the other in a
direction perpendicular to its course ; and the question is not
without grave dynamical difficulties, but they may perhaps not be
insuperable. It may possibly be that from some unknown con-
straining condition or property inherent in magnetic bodies, a
spiral wave, on being constrained into a spiral course, may lose
its original spirality, and become a secondary spiral, having mole-
cular motion in a direction perpendicular to that of the primary
spiral.
The relation between the various modes of motion, their physical
results, and the sensuous perception of those results, having thus
been inferred, the question next arises as to the nature of the
media by which the several modes of motion are transmitted. It
is unquestionable that sound-waves are transmissible by all kinds
of matter, but can any valid reason be assigned in favour of the
still prevalent opinion that other modes of wave-motion are in-
capable of transmission by ordinary matter? — ^this incapacitpr being
implied in the adoption of the self-contradictory hypothesis of an
imaginary medium, not cogniasable by any known means of per-
ception. It is a remarkable fact that in all the superseded crude
notions of physical causation, each phase of physical energy has
been presented in the garb either of impalpable, imponderable (in
fact imm<Uerial) matter itself, or of the vibrations thereof; and
each of these hypotheses has been successively subjected to some
violent supplementary hypothesis, in order adequately to meet the
reonirements of advancing knowledge.
To begin with chemical action: — ^What are now universally
recognised as simple metals were once supposed to consist of some
earthy matter (their oxides) combined with "Phlogiston,'* — the
material principle of brilliancy. But, unfortunately for the theory,
it was soon found that the metals, on parting with their share of
DrrsoDUcnoir. xzvn
phlogiston (t.e., beoomine oxidated), not only did not lo»e anj,
Dot actually acxptired weij^ht ; tbereforo phlogiston was assumed
to be not only impondsrahlt^ but Ayper-imptinderable — t.e., en-
doiwed with the property of absolute leyity, or negative
weight !
In the next place, the Newtonian theory of light assumod light
to consist of molecules (of course imponderM^ emanating from
the source of light, and impinging on the perceptive organs of
Tiaion. But this hypothesiB would not fit the phenomena of dif-
fraction (1125) and interference (1121), and to suit these physical
facts, the molecules must either be thrown into periodical *' fits^*
of transmission or reflection, or the ray must be a row of eg^-shaped
molecaies perpetually making isoperiodic somersaults, and plun^>
ing into a medium if they come on their heads, or bounding off, if
they^ fall sideways against it. Then again, heat was supposed to
oonsiat of material pacticies emanating from the source of heat;
and aa a ball of ice placed in one focus of a concave mirror was
found to lower the temperature of a thermometer placed in the
conjugate focus, there were assumed to be particles of coU, aa
well as of htal : it is needless to add how completely the theory of
exchanges (1392) accounts for the latter fact. At length these wild
speculations were superseded, and light and heat were admitted
into the category of wave-motion ; but electricity and magnetism
were still supposed to be either single or dual forms of " fluid"
matter: and
"Svca etism molll dora tanmtur sqoi i"
these "fluids" are probably still running in the deep channels
they have wpm in some philosophic minds.
But the principle of admittiag imponderability into the cate-
gory of legitimate physical hypothestes had become tacitly ac-
oeptod ; and the conclusion was at once jumped at by the authors
of the undulatory theory that the wave-motions of light and heat
take place in an imperceptible, imponderable, highly elastic fluid
medium, pervading all space, andaUmcUtert denominated "ether :"
and this theory, with all its inconsistencies and inconsequences,
is still probably entertained by many physicists.
That some highly elastic and attenuated medium pervades infi-
nite space, as the means of transmission of the energies of light and
heat uom the centre of each solar system to its oependent satel-
lites, is a necessary consequence of the dynamic theory: its ex-
istence is, in fact, demonstrated by the periodic retardation of
Eocke s Comet. Bat the remainder of the hypothesis, namely,
that all palpable matter is pervaded by ether, tor the purpose of
transmitting light- and heat-waves is by no means equally neces-
sary, or even tenable ; for not a shadow of evidence of the inade-
qoAoy of aU matter to transmit these motions has e ver been produced,
xzriii nrrBODUcTiov.
and in default of sncli evidence, the contrary hypothesis is at least
eqnallj tenuMe : and this interstitial-ether theory (in common with
all preceding physical theories involving imponderability) is hnr-
dened with grave inconsistencies. In the first place the well-known
phenomena of single anddouble refraction and p<>lariasation, whether
of light or heat, necessitate the Romewhat viiJent hypothesis that
the elasticity of (he supposed transmitting mediam, etner, is not, as
it is in all cognizable fluids, a fixed and definite (jualitv capable
of numerical estimation, but an ever- varying quality, depending
quantitatively on the elasticity of adjacent matter, and in the
case of double refraction, actually varying in tioo or in three
directions, within the same svbttance : it would be not more re*
pugnant to reason to assume that the elasticity of a gas is one
thing in a glass bottle, and another in one of braes; or that the
specific gravity of silver is a function of the moon's age, or the
melting point of gold dependent on the- sun's zenith distance.
Secondly, the fundamental ideas of inertia, energy, and *'work''
are inseparably associated with gravitation, and a contradiction
of terms seems to be implied, in ascribing either inertia or energy,
t.e., the capability of doing work, to an imponderable particle,
which is consequently destitute of attraction for any other particle
in the universe.
The known enormous velocity, probably not less than 250,000
miles in a second, at which electricity travels through a cop-
per conductor is complete evidence that ordinary matter is
capable of transmitting aomeihing (whether matter or motion it
signifies nothing for the present argument) at a considerably
greater velocity than the waves of light and neat, why bhuuld not
appropriate kinds of matter be assumed cspable of transmitting
these also? and if so, the need of the interstitial presence of ether
ceases altogether ; and it may with great advantage be excluded
from the domains of ponderable palpable matter, by the very mild
hypothesis that it is not miedble with air, any more than oil or
ptupahU ether with water, but that it floata above the boundaiy
surface of our atmosphere: this hypothesis is not repugnant to
reason, nor adverse to physical experience. On this supposition it
is no longer needed to impute to ether imponderability, t.€., an
exemption from the otherwise universal law of gravitation ; it will
then be imperceptible, only because it exists beyond the reach of
observation : and thus imponderability will cease to he reckoned
amongst the physical attributes of matter. Moreover, as there
are no means of limiting the poeeihle amount of molecular dis-
placement in a medium so attenuated as ether must be, an amount
of energy is conceivable sufficient to impart effective motion to
indefinitely denser matter ; and thus, witnout doing any violence
to the fundamental principles of dynamics, this denizen of infinite
apace may be assumed competent to ito divine mission of impart*
nTBODDcnoir* zxix
ing to material worlds thow essentials to corporeal existence,—
the very mainsprings of organic lire, — light and heat.
The question then natuitttly aris'^s — what becomes of the waves
of heat and light, when they reach the confines of tiie atmo-
sphere?— and is ordinary matter sufficient and effectual for their
transmission ? Thin question can be answered only from analogy,
which appears to infer an affirmative.
That sound-waves are transmitted by air, and not by interstitial
ether, is unquestionable; and if air be capable of transmitting
S6,000 vibrations in one second, it will probably be difficult to
aasigu any valid reason why the same medium is incapable of
tnmsmitting the far more rapid waves of heat and light ; and if
capable, then where lies the necessity for assuming the presence
of another medium? Again, the refraction of sound, as demon-
strated by the experiments of Hujech and Sondhaus (563, 564), is
in exact accordance with the laws hitherto assigneil to the refrac-
tion of light and heat. And it appears that the velocity of sound
in solids and liquids is much greater than in air (545) ; in water
it is nearly 5000 feet, and in iron nearly 17,000 feet in one second :
is there, then, any known fact whatever that tends to assign a
fimit to the powihU velocity of transmission of wave-motion
through these and other material media? — ^if not, then the pre-
sence of ether, as generally assumed, cannot be deemed essential
to the transmission of light and heat ; and if not essential, why
should the interititiaXrWier hypothesis be any longer entertained ?
'* If eo Deat intenit, nisi dignot tindioe nodus
Indderit.'*
Moreover, Prof. Tyndall, to whom the progress of Dynamical
Physics is indebted for many laborious and important researches,
has observed that in various kinds of wood there is a remarkable
hannony between their respective condnctivitiesfor sound and heat
in three mutually perpendicular directions, namely, longitudinal,
transverse-radial, and trms verse-tangential (546) : now although
there is certainly no direct analogy between the conduction of
heat, and the radiation of light and heat, beyond that of their
coounon dynamic origin, a much closer analogy may nevertheless
be traced through the phenomena of pho.«phorescence, fluorescence,
and caloreseence. It appears to oe highly probable that the
"caloresoence'' of a plate of platinizeuplatinum, the phospho-
nscence by heat of the minerals Fluor and Apatite, and ordinary
incandescence are analogous phenomena; differing only in the
temperatare (t.e., the amonnt of thermic energy) at which heat-
notion, impressed on the molecules of different substances, is im-
parted as nght-rootion to the surrounding medium. And some
phenomena of phosphorescence present further evidence of the in-
tiointe relations existing between light, heat, and electricity : it
XZX IXTBODDOTXOV.
haB been observed that Flaor mtij be rendered phosphorescent
by a very moderate application of heat ; but that it will not again
Eoosnhoresce under similar circumHtances, until an electric spark
as Deen repeatedly passed over its surface. But perhaps the
closest analogy between the radiation of light and the conduction
of heat may be traced in the observations of M. De Scnarmont,
who has found that, in plates of ciystals cut in a direction coin-
ciding with that of the optic axis, the relative conduction of heat,
in directions parallel and perpendicular to the optic axis, is
governed by precisely the same laws as those of optical elasticity,
which determine the relations of the ordinary and extraordinary
polarized rays : and moreover that in plates cut perpendicularly
to the optic axis, thermic conductivity is, like optical elasticity,
equal in all directions. It may also be remarked that the con-
Terse permeabilities to light and heat of a crystal of alum, and
one of dark smoky quartz, or a smoked plate of rock-salt, present
striking examples of the existing yet unknown diflemnces of
physical constitution, which are met with in the variiius kinds of
matter, and which involve special capabilities of transmitting, or
of arresting and diffusing, particular kinds of energy.
The correlation of the various lornis of energy and their trans-
mutations have been so well collated by Mr. Grove, and so fre-
quently referred to in the preceding and subsequent pa^es, that
repetition here w*ou1d be wearisome ; it will suffice to refer to the
before-mentioned enormous magneto-electric engine dehigued for
the illumination of lighthouses, and constructed also by Mr.
Wilde of Manchester (91 1). Acting like those of Messrs. Wheat-
stone, Siemens, and Ladd, on a principle of reduplication, or self-
reaction, it is a grand instance uf the transmutation of energy ;
and it may here be cited as a good example of a series of succes-
sive conversions.
In long bygone ages the energies of solar light and heat were
occupied in the development of woody linsue, and this became gra-
dually converted into coal, perhaps without much gain or loss of
energy. The dynamic energy arising from the collision of the
molecules of carbcm with those of atmospheric oxygen, in the act
of combustion, f.e., combination, is yielded up as thermic energy
to the boiler of a st-eam-engine, and generates steam, the elastic
force of which, through the medium of the engine, drives round the
annatnres of the electro-magnetit. Dynamic becomes now con\*erted
into magnetic, and this again into electric energy, and an in-
terrupted current of intense power is produced. This being trans-
mitted between carbon electrodes, an immense amount of light
and heat is produced by molecular friction at the point of great
resistance to the passage of the current, and these are prodnced
at the expense of electric energy, as proved by the loss of current ;
here, theo, we have the final transmutation of electric into thennic
ISTBODUCTIOV. ZXZl
•Did pbotic* energy: the latter being so intense as to have thrown a
shaditw a:rro88 the brightest sanbeam, and to produce an amoant of
illomiDiition unattainable by any other known means. This entire
series of transmutations ma^ be viewed as a complicated process
of erolving stored up saudhine in an intensified form, for occa-
sionaJ use.
Having sufficiently considered the physical, it now remains to
Rview the moral aspect of the large questionH that have been
raised in the preceding pages. The Author cannot but feel that in
thus seeking to simplify by generalization the conception of the
powers of nature, and to unify their source, he may in some minds,
misguided by the pride of human reason, unwittingly encourage
the pantheistic tendencies of the present age ; than which nothing
can possibly be further removed from his desire, or design. Where,
it may be asked, is the faintest shade of difference between modern
pantheism and the nature-worship of the oldest times, save that
the mode of culture may have been humanized by civilization? in
both alike reason and the objects of sense are deified, the thing
created is set up on the Creator's throne, and the light of reve-
lation, with it« priceless coiiseqnenceM, being invisible lo the mere
carnal eye of sense, is practically extinguished. Ihe author
vottid rather remonstrate openly with those who, conscious it
maT be of great intellectual power, have unfortunately been led
to Ignore ail things divine, tnat are not cognizable by unaided
reason, and to aucn he would say, in the emphatic words of the
peat Tishbite of old, *' How loug halt ye between two opinions ?
if the Lord be God, follow Him ; but if Baal, then follow him :" and
he would earnestly entreat them to reflect that between the two
alternatives there can be no compromise.
The grand mysteries of Crention must of course be ignored by
all those who have closed their eyes against all things not visible
m the light of unaided reason ; and accordingly it has not long
siooe been asked before a large and intelligent audience, Whence
came ihe fir^t elephant? *'did he fall from the sky (i.e., from the
interplanetary space) ? did he rise moulded out of a mass of
amoruhoDS earth or rock ? did he appear out of the cleft of a
tree? ' — to which, responding in the same key, the writer answers
so : — ^nor yet from the endless transmutations either of a sign-post,
or a galinwM-tree ;-r>nor even from the adaptive efforts of countless
generations of huge boar-pigs, inheriting from their primseval
ancestor a weakness for the green tufts that grow on tall and leaf-
less tropical stems; — ^nol — that wonderful mechanism of inter-
Iscing niujvcular fibres, by which a boneless appendage can acquire
abno*«t the rigidity of an outstretched arm, and the prehensile
sensibi'ity of opposed digits, could only have come, and must have
coiue direct, from the hand of an all-beneficent Creator.
* ThA Greek roots are generallj adopted in all similar words.
zzzii iHTBODucnoir.
It may by some be thonght that tbeae remarks are ^xtra vires
in a treatise on Phjsics, but the Anthor cannot ignore the apostolic
injunction — " Whether ye eat or drink, or whatsoever ye dOj do all
to the gloiy of God ;*' and he feels that when such language has
been publicly proclaimed, he would fail in his Christian duty if he
failed to give equal publicity to his earnest and indignant protest.
ERRATA.
N.B. — It will be foand oonvenient to make these corrections
before reading the work.
Line,
Error.
OMTtoAon*
S3
i,note
log Pi -log Pj
p f «.tan^
w
n
log,I>i-log,P,
205
26
andiB
and in the first column is
SIS
126
casting
forging
268
23
manytinieB
much
289
16
M. Clement Desormes hai
MM. Clement and Desormes
hare
310
10
of the hypothetical me-
} dele
dium ether
312
86
%
313
8
in table
s
0»
346
1, note
6A.lf.
Sajc
887
1
potential electricity
electric potential
308
632
176
16
TnieBuued
(713)
unTmcanlaed
M
146
600
flOOO
WW
126
800
8000
Ss
Pig. 636
B» 10 coils
B, 11 coils
NATUEAL PHILOSOPHY.
1. Au. TBrietieB tnd drmi of nuttter ue nmiUrly composed,
bring mule ap of an iDdefinile Dumber of extremelT, and mdeed
inooDceiTtLblj, minute indcatmctible porticlei, irbich, from their
not admitting of Further mecluQical oinBion, &re lenned atotnt*
Soma philoaophen baie, bowaier, conceiied that no tma atom
czi»ts, and that all matter ia capable of undetgoine diiinon to
mfinitv. a atatement capable of being aatiibctori^ prored in
r^snC to space, by the ooiuideration of matbemarical lines and
HontB. Tbns, let ^ b, c d, -. .
be lines drawn parallel to ^' '
cBchotber^drawuieabliqtw I
line r o, aod ^m r on the I
indefinite right Lne, c n, I
take anv number o! equal I
paitt,aerabed,&c. Ymm I
B draw lines connecting ihis I
point to<i,i,e,(i,&c.,C''tUDg |
lbs oblique line r a ; ihen,
aa the munber of poinle a, h, c, &c., on the line t; i> ni» be infinite,
H bllows Uiat the line r o ma; be infinitel; dirided by lines cod-
Decting such points with ».
Acgnmenta of this kind ongbt, however, to be regarded as
qipficable onl; to mathematical lines and point*, wliich, the
former being witLont breadth, and the latter withont magnitode.f
* A, ud ■ri^rm, Hbido.
t Endid, Book L itb. 1, 1.
n bo ref^rded but ■« mental conceptions, md not plijsicnl
2. The nltimmte particles or alomi [I] of matter poawss the
three funUiaZ cbnrnoterB a( impenetnAtliiy, txtetuion, Hodjiirure.
Of thsMi properties, the first flows directly frotu the definition of
sn at^nn, as it is obrious that nothin;; cnn be to impenetrable aa
that which ia incapable of farther division. When anj solid 'bod;
is immersed in a fluid, some portion of ihe latter is displaced,
sndllinB, on a snperficial ticw, might ba supposed lobe penetrated
bj the immersed body; it nil], however, be found that no real
penetration occnrs, as a quantity of fluid becomes displaced, equal
in hoik to the solid immersed. (Ch. VII.) On forcing a nail or a,
ki.ifeinto a piece of wood, Iheultimate physical atoms of the latter
are not penetrated, the instromentbeinK merely
«*. »■ insinnaied into the interstices eiisti 05 between
the indivisible molecules. Again, air and all
them, are really as impenetrable as solids,
allhough their particles arc tapable of mucli
^ater condensation bj nieclianica) means. If
a glsBB receiver, *, be inverted over a lighted
taper Ried on a cork floating en the surface of
water, it can bo pushed to the bottom of the
containing vessel, and the taper will thus con-
, tinue to bnm under water so long ss sufficient
'b present to support combustion; a
3. The aeccnd chnraclcr, or erttniion, is also a necessary conse-
quence of the delinilion of nn atom alrcndj ^ven, as that which
possesses a pliysical existence must necessarily occupy a portion of
space, and puiiiicss sides nod rurfnccii in relation to other atonis.
"nie citpnsion iif bodies is oipTeased by the ibi-ee dimeneions of
leDgth. breadth, and thiitness.
4. The third character, yti/ure or ^rm, is nl<>o essential to tlic
existence of an atom, as nulhiug c;in bo conceiII^d aa phybicallv
existing, unless it possesiies some determinate aliajie, althoush this
]iroperly is tiot tnmcient of itself to prove the material eiislonce
ufan o^'cct ; for in sha^on-s and in spectral illusions, produced by
various optical means, wo have cxbRiples of ligure or form wiiliout
5. Of the actual form or sixe of atoms, nolhinf; p'wiiive is
knon-H, it is. however, probable that Ihov are sphorlcnl : hut In
their difnenalont Ecarcely an app'oiimaliun can be oblnincd by
MIKUTEXB98 OF ATOMV. 3
nnj meiuiB we are jet acquainted with. An ounce of gnid can b^
drawn into wire several miles in length (12), and jet no flaw, of
evidence of separation between its atoms can be diflcoyered by the
closest microscopic examination. Cbemistir affords ns eyidence
of the ezcessiye minuteness of atoms, for when seyeral metals, as
nickel, cobalt, or iron, are reduced from their oxides at the lowest
possible temperature bj means of a current of hydrogen gas, the
fitate of diyiMon of the reduced metal is almost inconcciyablt*.
Each particle of metal slowlj evolving its ox jgen, forms a powder
which maj be considered as composed of ultimate atoms. These
are in everj case less than the one-hundred-millionth of an inch in
diameter, so that bj a simple calculation it maj be proved that a
cubic inch of them would, if extended on a level surrace so that
they maj touch, but not overlap each other, cover an area of
218,166 square feet, or more than five acres of ground.
6. AnoUier illustration of the extreme minuteness of atoms is
met with in the thin films of soap bubbles. These present fine
iridescent coloured bands, and at the npper part ot each, it is
demonstrated that the thickness of the film, just before it burets,
cannot exceed the four-millionth of an inch ; and jet even this thin
lajer is not composed of a single stratum of atoms ; as it must
consist at least of the atom of soap and ono of water ; the former
compoeed of soda, stearic, or margaric, and oleic acids, in the
simplest view that cm be taken of its composition, and the
latter made up of at least a molecule of oxjgen and one of
hydrogen.
We may likewise appeal to organic life for evidence of the un-
limited divisibilitj of matter, in the extreme minuteness of definite
structures that have been revealed bj the microscope, exhibiting
the wonders of creation not less manifested in the most minute,
than in the most stupendous works of which our senses are cogni-
sant. Animalcules exist, so minute that mjriads can swim in a
drop of water, and jet everj individnal possesses organs of diges-
tion, circulation, and reproduction. The polishing-slate from Bilin
in Bohemia, composed almost entirelj of the siliceous shells of
infusoria, has been calculated to contain 41,000,000,000 in one
cubic inch, which weighs 220 grains; consequentlj each shell,
possessing nevertheless, the most exquisite beauty of structure,
would weigh little more than the two nundred-millionth part of a
grain.
7. The minute molecules composing masses of matter maj be, and
often are, chemicallj compound, althongfa phjsicallj simple ; thus
a piece of marble maj be divided into its ultimate molecules, each
CQouisting of carbonate of lime, and here physical analjsis stops ;
but bj chemical analysis we can separate each of these atoms
into carbonic acid and lime, the former being asrain chemicallj
divisible into carbon and oxygen, and the latter into calcium and
oxygen. In physics, therefore, a molecule is reganled as simpio
B 2
4 GBHEBAL PB0PEBTIE8 OF MATTER.
when it cannot be farther diyided without separating its chemical
elements.
8. The indestroctibility of matter must be regarded as one of
its inherent properties. It is no more within the limited scope of
human agency to destroy any of the ultimate material elements,
than it is to create^ or even to commute them, as the alchemists
of old vainly attempted. The tenns "destruction by fire,'*
" destructive distillation," must be understood in a limited sense,as
referring only to the previously existing form of matter, and not
to the matter itself. The stick of charcoal is consumed, and leaves
no vinbiU trace of its existence but a minute quantity of white
ash : it is not however destroyed, an invisible gas has been gene>
rated by the union of the carbon with the oxygen of the atmo-
sphera, which manifests its existence by its power of extinguishing
alike (and for the same reasons) die flame of a candle, and the
vital spark of organic life.
Many fluids, water for example, will readily evaporate ; but its
particles are merely suspended invisibly in the atmosphere. This
mav be rendered evident by their precipitation on any suitable
cola surface, as on that of a glass vessel filled with iced- water.
Tbe history of the earth's crust informs us that these wonderful
transformations of matter have been progressively in operation
during vast periods of time, of the extent of which the human
mind can form no conception. £ven the humble "earth-worm
that we tread on" plays an important part in the scheme of
creation in continually reclaiming to a hi^ner grade of organiza-
tion the organisable materials of the soil in which it lives and
moves and Las its being.
9. Atoms and molecules are held together by means of a force
denominated aUraction^ the finnness of their union being modified
bv the presence of an opposing force, termed reptdtion; and upon
the preponderance of one of these forces over the other, depends all
the physical properties of matter, known as hardneu^ Boftnen^
JUiiaity, &c. ^ Tne intensihr of this molecular attraction varies
considerably in different bodies, which thus acquire very varying
degrees of coherence.
If the mutual attraction of atoms be so considerable as to prevent
a rigid body beine readily inserted between them, the mass is
said to be hard; but if so feeble as to permit their easy separation,
the resulting mass is soft; and &fiuui or a acueous body results,
when the intensity of the mutual attraction between the atoms is
so far counterbalanced, as to allow any substance to be moved
between them without experiencing any considerable resistance.
The hardness of many bodies is mucn influenced by external con-
ditions ; thus most metals may be considerably hardened by ham-
mering or rolling ; and the hardness <^ steel, resulting from sudden
cooling, when at a red heat, is a familiar example. On the con-
traiy, an alloy of one part of tin and four of copper is said to be
-n
DBH8ITT OF MATTES.
5
ductile when cooled enddeDlf , and Inittle when cooled slowly.
Thne the varions states in which matter exists, as soUdj viteouSf
Uqvidt or oasetnu, merely dej^nd upon the yaiying intensity of
the moleciuar forces of attraction ana repnlsion. The property of
emitting a sonnd on percussion is ascribed to the hardness ofbodies ;
hat this property may be shown to be possessed hy hoth fluids and
gaseous ixxiies. The water-hammer, a glass tube containing water,
nt»n which the air has been nearly exhausted hy boiling the water
in the tube, and then hermetically sealing it, emits a loud sound,
prodnced hy the concussion of the water against the elass. The
crack of a whip, or the peal of thunder, auke show the* effect of
the concussion of the particles of air against each other, when
they ha^ been separated by .anything passing between them with
a -velocity greater than that with which air would rush into a
yacnum. These seyeral states of matter are readily conyertible
into each other by yarious mechanical means, and by alterations of
temperature : thus, water at 32** F., and mercury at — 40** F., or
72* lower, are solids, the one being transparent, the other opaaue ;
and at about — 90* F. carbonic acid may be obtained in the form
of snow. At ordinaty temperatures the former are liquids, and
the latter, gaseous : whilst at 212* F. water, and 670* F. mercury,
become yapours or gases, both being transparent ; these seyeral
changes depending merely on the neater separation of their atoms
efiected by the repulsiye power of heat. The original yolume of
&e fluid becomes amazingly increased by this separation of the
oonsfiitnent molecules. The following table shows at a glance their
enormous increase of yolume by yaporization, under ordinary
atmoapherio pressure.
1 cubic foot of water expands into 1689*0 cubic feet of yapour.
alcohol . . . 493'5
ether .... 21218
turpentine . . . 192*15
10. The most elastic gases can, h^ the application of sufficient
pressure, be compelled to asmme a yisible form ; becoming liquids
if the pressure be great enough to bring their constituent atoms
sufficiently near to each other.
it
n
n
n
It
It
ti
ti
it
tt
GSMt.
Tamperatore.
Sulphurous acid
Chlorine . . .
Carbonic acid .
Nitrous acid . .
2
4
86
60
45* Fahr.
60 „
32 „
45 „
11. The density of matter in any of its three states is measured
6 GENERAL PROPEETIES OF MATT&B.
hy the quantity contained in a given bulk, and is expressed hy its
specific gravity or relative weight, as compared with some body,
taken as a standard ; thus, if a given bulk of water consists of
1,000 atoms of matter, an equal bulk of platinum Avill contain
about 23,000, if each atom has the same weight ; of copper nearly
9,000, of iron 8,000, and of glass about 3,000 ; these several num-
bers being proportional to the specific weight or gravity (Ch. Yll.)
of the respective substances.
Masses of matter moreover possess several properties which may
be considered as accessorv, all depending upon the different de-
grees of intensity with which the physical atoms are mutually tie- 1
together. Among the more important of these may be ranked
DivmbiUti/, I^lexibilitVf TetiacUyi BritUeness, £lcwticUyf &c.
12. Divisibility or iJxtenaion of Maasea, — This character may
be considered as well illustrating the extreme, and almost incon-
ceivable, minuteness of phvsical atoms ; depending upon the im-
mense, although finite number of parts into which a mass may be
divided. Thus, an imperceptibly small portion of strychnia will
render a whole pint of water bitter, and a single grain of the
ammoniacal hyposulphite of silver will render intensely sweet
32,000 grains of water. One grain of iodide of potassium dis-
solved in 480,000 of water, when mixed with a little starch, will
tint every drop of the fluid blue on the addition of a solution of
chlorine. In all these cases, we have at once evidence of the
extreme minuteness of atoms furnished by the divisibility of the
masses of strychnia, silver, and iodine by means of solution.
Excellent illustrations of the same property are met with in many
])rocesses of art ; a sinde pound oi wool will furnish a piece of
yarn 100 miles in length. Gold ander the hammer is reduced to
such state of tenuity, that 360,000 of the leaves produced would,
if piled on each other, only equal the thickness of an inch. Even
this is far exceeded in the art of the wire-drawer, who, in the most
economical mode of preparing gilded silver wiiy, extends two
ounces of gold over a length of 1,351,900 feet, or rather more than
768 miles. The exquisitelv delicate wires of platinum made by
the ingenious process of Dr. Wollnston, afford a remarkable instance
of the extension of matter, no less than of the almost inconceivable
minuteness of the component atcms. The finest of these wires is but
the three-millionth of an inch in diameter, and 140 of them placed
together would just equal in thickness a single fibre of silk. This
extreme degree of tenuitv was attained by enclosing a platinum
'wire in a silver tube, tnen drawing both together, and lastly,
dissolving awav the silver coaling by an acid.
13. Flexihility. — When any substance is capable of being bent
in anv given manner within moderate limits, hj the application of
sufficient force, it is said to he flexible. For a body to possess this
property it is necessary that the distance between its contiguous
])articles should be capable of being slightly augmented, without
removing them beyond the sphere of their mutual attraction. The
TLEXIIUUTT.
property of flexilnlitj maj be illnstrated by tbe followiDg simple
aj^paratas : — ^Let ▲ b be a piece of whalebone, haviiig a number of
viies CD, d vff &c., passed
througb equidistant bolea : let ^« >•
two series of balls c, c', &c.,
i>, ]/, &c., be fixed to tbe ends
of tbe wires, and let a third
series, b, e', &c., loose on the
wires, rest on tbe piece of
whalebone. If tbe rod be bent,
as p o, the row of particles rs-
presented by the balls c will
oaTe receded from each other,
and the particles d have
become more closely approxi-
mated, while the distance be-
tween the particles b is not
perceptibly altered. That
such a change in the relative
distance of the atoms really occurs, is rendered evident by merely
inspecting the fignre of a thick wooden plank which has been
allowed to become curved by _, .
its own weight. Let a b, c d, ^*
represent the section of such a
p!ank supported at ita extre-
mities c, D, it win be seen at
QQce that the surfaces a b and
CD represent two concentric
cm^es, of which a b is the smaller, consequently the atoms nearest
the Borface ab must be more closely approximated than those
seaieat c d. The atoms lying in some line intermediate between
A B and c D, undergo no change, the line b p, therefore, in which
these lie, constitutes what is called the neutral €ucia of the body,
and this portion mij^ht be excavated and removed ■without mate-
rially diminishing the strength of the plank, provided a sufficient
amoant of substance be left, to prevent the collapse of the surfaces
ABandcD. ^
14. On this principle, hollow cylinders of different materials are
employed instead of solid ones, when used as mechanical supports.
Inoeed, if all opposing causes in the shape of flaws, bad workman-
ship, &c., are absent, such hollow cylinders not only have the
advantage of lightness and economy of material, but are found in
practioe to be actuaUy stronger than solid ones of equal weight.
Tredgold found that when the inner semi-diameter of the hollow
c^inder is to the outer as 7 to 10, it will possess double the
strength of a solid cylinder of the same weight
Similariy in the construction of ca6t>iron girders, for supporting
the floors of buildings, it is found that the greatest strength is
obta^ied by making the transverse action lu some degree to
8 MOLECULAR OB IMTBRHAL FORCES.
resemble the itiverted letter Xi ^^® reBistance to oompressfon and
extension being equally sustained by the upi>er and lower lamins
respectively, the areas of the sections of which are as 1 : 6, while
the yertical lamina serves to maintain the equidistant position of
the former. Wroufht-iron is, however, now almost universally
used for girders, ana as in this form of the metal, its powers of
resisting extension and compression are much more nearly e^ual,
the upper and lower laminn are usually made of equal dimensions^
the giraer, if small, being rolled between grooved rollers, like the
railway rails ; or if larfi:e, being formed of strips of boiler plate
and angle-pieces riveted together.
15. Tenacity. — This character is dependent u^n the intensity
of attractive force existing between atoms bemg sufficient to
oppose their ready separation, to such an extent as to cause the
rupture or fracture of the whole mass. Consequently, all flexible,
ductilOi and malleable bodies are tenacious ; althougn many sub-
stances possess the latter property without the former. The
tenacity of matter is well shown in the remarkable malleability of
copper ; for from a flat plate of this metal the skilful workman
forms a hollow vessel without any joint or seam by the use of his
hammer alone ; and by well-directed and repeated blows, the vessel
he has formed, however much differing in ngure from tne original
plate, is everywhere of nearly the same thickness. Tenacity varies
extremely in different substiuices : metals afford the best examples
of it ; dius, a piece of steel wire of g^ven diameter is capable of
suj^porting witnout fracture 39,000 feet, or seven miles and a half,
of its own length.^ Wires of different metals of the same diameter
require different weights to overcome the mutual attraction of
their component atoms, as shown in the following table; the
figures representing the number of pounds avoirdupois required to
break wires of the metals enumerated, each being ono-tenth of an
inch in diameter: —
M«tals.
POQDdfl.
Metals.
Pooiidi
Bismuth .
. 20-1
Silver . .
18713
Lead . .
. 27-7
Platinum .
274-31
Tin . .
. 34-7
Copper . .
302-26
Zino . .
. 109-8
Iron . . .
54925
Gold . .
. 16007
Cables constructed of fine iron wires of from tt ^^ rir ^^^^ in
diameter, are stated to possess the enormous tenacity of 60 tons
in each souare inch. It is this wonderful tenacity which renders
wires of this metal so applicable to the construction of light sus-
pension bridges. The followinp^ table shows the tenacity possessed
by different bodies calculated m tons weight.*
* MoMley'i *< lUnstntiona of MediMiifl%" p. 886.
TEVACJTT.
9
Teaaoiif in tona^
p«r iqaireiiiolu
Wrongbt iitm, in vire ^ to^ inch in diameter 60 — 91
„ in wire ^ inch diameter . . 36 — 43
in Iwn (English) 25i
„ in bars nammered 30
„ in chains of six-inch links . . . 21 ( — 25
Cast iron 6 — 9J
Steel, cast 44
„ Damascus 31 ■ 44
Ccnpper, cast 8^
wire 274
Silver, cast 8
„ wire 17
Gold, cast 9
„ wife 14
Flatinom 17
The tenacity of the fibres used in the mannfactnre of different
&brica, has been found bj M. Labillaidi6re to be very different ;
he has ascertained the tensile stoeugth of fibres of equal sectional
area of silk, New Zealand flax, hemp, and common flaz^ to be nearly
proportional to the numbers 17, 12, 8, and 6, respectively.
16. Tredgold has shown that many solids witt bear an enormous
amount of pressure before they yield sufficiently to allow any ^r-
maaent alteration in their shape. The fieures in the following
table represent the weight in pounds required to effect a change in
the figiue of a one-inch cube of the solids submitted to experiment.
Malleable inm. . 17,8001bs.
Cast iron . . . 15,300 „
Brass 6,700 „
Zinc 5,700 „
Tin 3,880 „
Lead 1,500 „
KedFir . . . . 4,2901be.
Oak 3,960,,
White Fir . . . 3,630,,
Ash 3,540,,
Ehn 3,240,,
From a comparison of these tables it will be observed that the
relative powers o£ resisting compression and extension differ consi-
derably m different substances : thus while the tenacity of iron is
to that of sine as 5 : 1, the resistance to compression is nearly as
3:1; hence the position of the neotral axis will differ in different
mat^ials, and titerefbre likewise the sectional form of a beam of
greatest strength in proportion to its weifcht, a point of great im-
portance in the arts of constructioo. This may be further illus-
tnted by the preceding ^paratus (13), in which the mutual
recessioD of the particles o may be in any required ratio to the
sppraximation or the particles d, by varying their relative distances
Ami; *
10 UOLECULAJl OK 2KTBRXAL FORCES.
Connt Rumford found that a cylindrical roll of paper, with the
folds glued together, and presenting a sectional area of one square
inch, would support a weight of 30,000 pounds.
The tenacity of metals is greatly influenced bj their tempera-
ture. At a certain elevation of temperature the readily fusible
metals entirely lose this property, and assume the consistence of
putty. This peculiar st^te is made use of in the arts, as in the
formation of a plumber*s joint ; and lead pipe is mafle by the metal
in this condition being forced through a round hole in an iron
plate, with a concentric plug placed in the aperture.
17. £rUtleness,^TLh\H is obviously the converse of the last pro-
perty of matter ; it points out that condition of a substance, in
wliich the attraction between its molecules, althoughperhaps very
intense, is mnch limited in its sphere of action. Hardness and
brittlencHs are not incompatible qualities, but, on the contrary,
Irequcutly coexist ; thus a piece of glass, notwithstanding its pro-
verbial brittleness, will scratch a surface of polished steel. If,
however, glass be spun into fine threads, or blown into thin laminie,
it exhibits a high aegrec of flexibility as well as elasticity. The
opposite properties of hanluess and brittleness, or ductility and
tenacity, are frequently manifested by the same substance under
different conditions of molecular arrangement ; thus cast-iron and
hardened steel are brittle, while bar-iron and soil steel are amongst
the toughest substltnces in nature.
18. JSlasticity. — A body is said to be elastic when, after being
bent in anv direction, it spontaneoualv tends to recover its former
shape on the force which had altered its figure being removed ; all
elastic bodies must be so constituted as to allow a certain number
of their atoms to be brought, at least momentarily, nearer each
other than they previously were. If the body be a metallic rod,
then, on being bent in the curved form a b D o. Fig. 4, it will have
a tendency to resume its primitive rectilinear form on the removal
of the coercing force, in consequence of the exertion of two forces,
\iz., attraction between the partially-separated atoms on the out-
side, and repulsion between the closely-approximated atoms on the
inside of the curve. In this case, the cnange of form which brought
into action the elasticity of the body is very obviouB, from the
ourve produced by its flexure ; sometimes this change of figure,
even in the most highly elastic bodies, is not evident to the
eye, from its short duration ; still such change does demonstrably
take place. Thus a ball of ivory is elastic, and this property causes
it to rebound from the floor when forcibly thrown upon it, its figure,
on impact, becoming altered and compressed ; as may be shown
by causing onn ivory ball to impinge forcibly on another, smeared
with some dark unctuous matter, as printing ink; the surface
marked by the impact will be much larger thau if the balls were
merely brought gently into contact with each other, thus showing
that a mutual compression of the substanco of the balls hati taken
MOLECULAR AGOBEOATION. 11
place (Inring the impact. Two balls of caoutchouc, which poMtesa
a mnch greater degree of compreaaibility, will produce this effect
io a mai« marked degree.
19. A body is said to be perfectly elastic, when the force with
which it tenda to recover its original form, or as it is called, the
force of rtHitviion^ w exactly equal to the compressing force. No
kind of solid matter is perfectly elastic, bat many are elastic io a
high deeree, the forces of compression and restitution being nearly
^qual. Different elastic bodies vary extremely in the extent to
wiiich lliey will yield without rupture ; thus caoutchouc, and espe-
cially the vulcemized variety, may be stretched to five or six times
its original length, and will afterwards very nearly regain its
ftinuer shape, nnleas the tenuion has been maintained for some
time. Threads, and thin laminss of glass, and tempered steel
springs are highly ehuitic ; unannealed iron, brass, and copper, are
also elastic, but in a less degree than the former. Fluids, and
especially gases, are the only forms of matter that exhibit the
property of perfect elasticity : the latter, on account of their phy-
sical constitution, will permit their atoms to be very considerably
appcoximatod, by the application of sufficient force ; again sepa-
rating instant jineousiy, and even with violence, on the removal of
pressnre : the air-g^n, and condensed air-fountain are examples of
this property in atmospheric air.
20. AIthriu|^h the scope of an elementary treatise forbids a de-
tailed discussion of many important branches of phjrsical research,
the sulject of molecular attraction would be incomplete without
some notice of those remarkable conditions of pohmty in molecular
aj!gregation, that ^ve rise to the formation of crystals. The term
cry9tal originally implied transparency, bnt, in its more extended
sense, it is applied to any portion of matter that has spontaneously
assumed a definite geometrical form, bounded by four or more plane
surfaces ; four planes being the least number that can enclose a
fipace. Crystals that have been ibrmed by the agency of natural
causes are tenned natural crystals, while tho^e that result arti-
ficially from fnsion, solution, or any kind of chemical action, are
called arf(^ScuzZ crystals.
21. The varieties of crrstalh'ne form depend on the different
directions in which the forces of molecular aggregation act most
pnweriiilly in different kinds of matter, and also on the relative
mtensity of those forces. Some direct evidence of definite direc-
tions of greatest molecular attraction may be derived from the fact,
that many crystals will yield to any applied foix;c and break or
hph't only in plane surfaces having a constant direction ; this pro-
perty is callea cleavage. If a natiual crystal of Calcite, commonly
Known as Iceland tpar^ or of the lead-ore called Galena, be broken
into fragments, the surfaces of each fragment will have the same
relative position as those of the entire crystal, or portion of a crys-
tal, from which they were derived ; and if these migments be tri-
13
tanited in > morUr to an impalpable powder, and examined bj a
micToacope, it will be foaiid tnateach particle is boundsd by plaiMi
having the same mutual iDclinationB.
22. The lUreotiona of greatait molecnlar action CMDot in all
cijitali be di«coTered hj cleavage ; bnt in all caaei, three lines of
direction ma; be taken, pasting throngh the Bame point, bnt not
lyinff in the (ame plane, which will bear some known relation to
the direotioni of the molecnlar forces, or to the obterred inrfacei of
Uie ctjita] ; ^eee three lines are called the Axe* of the crplal.
28. All the obeerved hnai ofetTstaU maj be referred te oite of
nx AMentiall; diffenet ■jatems c^ crTstallixation, each of which
may be defined by die relative podtion andmsffnitudeof the axes.
The relative lengths of the axes, which define the formof a crystal,
•re called voroiMlsrs.
S4. In tnree sjatenu the ciystallographic axes ore all perpen.
dicolar to each other.
I. If the axes are all equal, the crjstal formed beloDKS to the
QtUe system, of which common Salt, Alum, and Floor Spar are
examples. Ctystals belonging to the cubic ^item are chiirac-
terised by an appearance of symmetiy in whatever direction they
tig. i. rsf. t.
Silicate ofNickel are examples. Ciyslak of this class ftw^aentiy
it tin mppearmnce of > iqiiare pyramid. When Tiawed in the
.... ion at tbe mieqnal aiia, ttieu oatline U Brmmetrioal with
regaid to the foitr ndei or eoniera of > aqnAre, bat when riewed In
tbe directioii of either of the two equal axes, the ontline ii ijmme-
trioLimlj with regard lo the opporite lidei of a rectansle : this
taaj be imdentood bj a reference to the flgntet, in wbiob tbe nn-
aqnal axes are TertkkL
IIL If th« axes are nnequal'in ail three directiona, Ihe renilting
fcrmatioa lepreaenta the iVitniatie B.vatem, of which tbe Snlpbatea
of Folaab aitd Zinc, and Rochelie Salt are eiamplee: the latter
salt, LoweTBT, fracjiKiitlj crjstalliEeB in half prianiB, ahowinE one
of the irrwdariliea that occssionally occur in tbe (onoation of
CTjstala. The fonns belonging to this ajslein freonently present
a ijnunetrical bz«ng&«baped ontline, when view^ in the direc-
tion of tbe «xi* of tbe priam, which is Tertioal in tbe entire ci7>taL
FIg.t.
[isles to each other in a
_, .__ , but in a vertica! plana,
ptiing tbrongh either of the other two, tbe OUtjue s^tem is
eumpTified. Proto-snlpbate of Iron, Carbonate of Soda, and Tar-
taric Acid, with nameroos other natural and artificial ciratals
beknig to thia sjBtem. Ciystals of thia claee can be divided into
two mnmetrical halves only by tbe plane in whioii the obliqae
axis bee : thia paaaea through the edge at which tbe planes Mand
n' meet, and tbrongh a diagonal of the plane p.
T. If tbe three axea are eqoal and equally inclined to each other,
Prttt-tM2fiat§ i^Itoi.
14
nil. 13 but not at right anglM, the B\omhoktdral
iyilera reiulta. Tha crjutala nf thJH tjpo
ubiibUj pntMnt a trianeular or bexaguiial
STmmetrical ouclino, irben viewed in tho
direction of the niia of the Rhombohsdran,
wliich IB UBiMlly drawn verticallj, u in ibc
following figure; but it muBt be obaeireJ
ihat thiH niia in Hat One nr tlio crjstallo-
graphic aiee, Calcite, and Quartz ar Rock
Crriitiil, aa [t ia commnnly turmed, are fa-
miliar examples of ihis clau.
of the forcf^iag conditionx Hfe fulfilled, Uie
O the AnoTthic Bj-Btem ; which although com-
pnsing an alrncul endlesB lariet; of fnuiblo
relntialia, hw verv few knnwn reproaeiitalivBH
ID nature ; in which fact we rccogtiiae the
universal tendency to STmmetrical and har-
moiiiauB arrangnnient, thai Ih met with in all
llic wonderful wurka of creation.
biulpbnt« of Cop^r, and Aiiuite (Fig. 13),
are ffiod iliualr«iionB of ihiii gjateni ; Ihe
cry bUIb belonging tuwIiichSirBcliiirBcteriioil
Aiifiit. \jy a total want nf syiumolry.
Bft. Tlie position of the rariouB planes or faca at which the
surface of a cryBtal !-> compoaed, is aometiiues determined lijtbeir
relation to the facea of a paralleb^pipcd of the limpluBt form IliHt
exbibils the characterigLicH of the eyBti^m to which it belongs.
Thin ia called the prinurtf form ; and the planea by which its
edges and angles an modified nro culled ttamdary planes. In tho
pivreding figures the primary planes are marked by capital letters,
and the secondary by italics.
Jr. 14 26. 'He law of aymmelry, which prevBita
to a remarkable eiteut amonff cryatala, re-
quires that all simiUr edges and angles of the
primary form should be similarly modilied ;
hence in a Urge number of inatunccs, tbo ayni-
nietrical arrangement of the aecundary pluneii
will point out the Byslemto which thecrystiil
ahoukl be referred.
27. In some crystals a deviation from the
law ofs.vmmeti^ is obiicrved in the exiatencu
nf only batf tlie comphto number of planeii,
ticher the altemata or tho oppojiCe plnnen
beinp Duiitted, aa in Fig. 14 : lUse are callcl
ItejaihedTol fmma. In some of these tbere ii
a correapondenco between the unajmmetricHl
fiinn. and other phTeical ibnractora; as in
aitKiQ nbich cithertbe right or Ihu kit huDil
CTALS. 15
plines are omitted, as in tlie itnneied Heart, In tlieas an effect is
pmdoced on n ray of l!f;ht trBnsmitt«d Inmugb > horizoutal slice
flf the crjitsJ, which will ba Bubsequentlj explained aa righv or
Wa-banded circular polsritation. (Cb. XXUL)
In Ihe cubic sjstem two ilintinct cliasea nf bemihednl fnnns
ara freoDentlj met witb. One oT tbeae, the heniiheitral ictlA
paraBei face*, reiults from the alternate deficKnc; of o:
■hich the complete form 120 (that of wbich the indicia are 1, 3. 0,1
ia represented in a (Fig. 15) while the corresponding hemihedral
form is b; in which of tbe pnirg of similar ptaneii a, b, in a, the
plaaci a odIj are developed.
The bemihedml form mth obliqut fane* ariwa from the defi-
ciency of one of each pair of immiu pUnes, of Ibis the simplciit
fxaaipk ii the tetTohedrori, which resuiis from the depoailioD of
hucceMiTe lajera of molecules oa tbe alternate faces of the octa-
hedron (Fig. 61 ; this is represented in a (fi^. IC); The tetmhe-
dron ii unall/ dnwD in Ihe poaition b, in which position its
retatioD to the oclAhednm is less apparent.
anorthic si hemihednl formi of the obliqiH ; but no utiiiwtoi^
nnlt appear* U foUaw from this att«mptad genenluatioD.
In PfTO^lectrie cr;»t»l», or thaw wbioh ezhilnt electrical pola-
rity wben beated^ aa the TourmaliDe, a want of ajmmetrj between
the two eitremlties ot the cryital ia nsuallj obaerred,
38. In many crjatiUine subttancei a remarkable, bat not un-
■ymmetrica], deviatioa Irom the nnialfbrtn ii oocanonallj obeerred.
An; crjatal may be dirided into two equal and nmilar portione bj
a plane passing throagh ita centre, sad parallel to one of ita
faces, aa in Fig. IT, or (ai it i< in aome cases man ccmTtnienllj
considered) perpendionlar to one of ita edges ; but the two halves
are in a rSTenodjiaaliaQ u regarda each other. If, however, the
aucoearire depoaition of particlea shoold take plaoe on opposite
aides of thia median plane in Ihe tame dirMtion, the crrstal npre-
sented in the seoond figure woold result, io place of tbe fonner;
the letters 01^,0, Ac, showing the felatiooi of tbe planes in tha
two fignrea. Crystsb of this kind are ealled twin oijttals or
macles. The/ have also been called Henutroptt, beoanse the
same resolt would be obtained if an ordinair ciystat were cut in
half, and one portion tamed half ronnd on the other, a* maj bo
easil; shown bj a model representbg these figures.
«».iT.
'odiilinct
forms not referable (0 the same class: tbns Calcite, which is
rhombobedral, is chemically identical with Arragonite, which is
Eriamatic; and natural crTatabofSulphar,as well as those obtaioed
T anbiimation, and bv the slow araporatioD of a solution of Sal-
pilar in bisulphide of Carboo, are pnraatic ; bnl those formed in
the cooling of ^nlphor heated considerably ebave the point of
tonon. HM occasionally, though rarely, those fonnedio the above-
named sehition, bebng to tbe oblique system. Substances poa-
sessiog tiiis property of aaanming two different fbrms are called
dunorphout. Some substances also are trimoT^hovt, or oocorring
in three different forma; thus Titanic acid in the mineral spedea
Butile and Analase is pyramidal, bat the inclinations of tbe
laces are very different, while in Brookile it is prismatic
By moat recent aathors, the podtioD of any face of a crystal is
h
POemON OF A FACE DEFIITED. 17
detennined, without reference to a hypothetical primaiy form, by
the relative distances from the origin (the point at which all the
axes intersect each other) of the points at which the given plane
intersects the axes: these distances are generally either in&nite,
in which case the plane is parallel to an axis, or very simple sub-
mnltiplea of the parameters (23) of the crystal : these submul-
tiples are called tne indices of the face in question.
In the language of analytical geometry a face of a crystal is
defined by the equation
A + B + c ^'
in which A, B, C, are the parameters of the species, ajidp^ q, r, the
indices of the plane.*
* For ftirther i2if<»ination on tlda sabjeot the reader is referred to the Art.
CrfBtallofraphy, in the " Encjdopsdi* Meiropolitanm," and to a reprodac-
tianof Pfaiilipars "Mineralogy," Vj the late Mr. H. J. Brooke and Prof.
W. H. Miller.
18
CHAPTER II.
PHOPESTIES OF MAflSEB OF If ATTEB : — EXTERNAL FOBOSS.
30. ATTSAcnYE forces are capable of acting not only between
atoms but also between masses, and form a very important sabject
of consideration. The Molecular attraction of aggregation, which
ties atom to atom, has been already alluded to. iVe have next to
examine those forces which act between masses of matter ; these
may be divided into two sections, the first comprehending attrac-
tions at insensible distances, including cohesion and ecmjUarUy;
the second, attractions at sensible, and eyen at unlimited distances,
including gratfitation.
31. iAJl attractive forces, whether exerted between atoms or
masses, diminish in intensity as the centres of attraction of the
attracting molecules or masses recede from each other, snd gene-
rally obey one law of the attractive force being invertdy as the
equares of the distances between the attracting bodies. Attraction
is always mutual, and exerted by one body on another, cceteris
paribuSf in the ratio of their masses. As an example of the
general law of attraction, let us suppose that two bodies, A and b,
mutually attract each other when at a certain distance with a
force equal to 1, at double that distance this force will be ^ instead
of k of that when at a distance of 1, because the square of 2 is 4 ;
at four times the distance the force will be diminished to ^, and
BO on.
ATTRACTION AT INSENSIBLE DISTANCES.
Cohesion and Adhesion.
32. Wheneyer two smooth and clean flat surfaces of ax\j sub-
stance are pressed together, a considerable resistance is expenenced
in attempting to separate them ; this is owing to an attractive force
called cohesion^ so termed from its causing bodies to cohere, or
stick together. As an example of this force, if two clean surfaces
of iron, at a white heat, be forcibly pressed, or hammered together,
the cohesion is so perfect that they are subsequently inseparable :
— this is the ordinary process of welding ; and some other metals
are capable of being similarly united more or less perfectly. The
mode of preparing black-lead for pencils, devised oy the lat« Mr.
Brockedon, is another good example of the utilization of the force
of cohesion. The plumbago is ground and finely levigated, all
nit being csrcfall; ramOTftd ; Iha powdsr ii than labjected to
njdnuilic premira, by which it i« to e(Hnp1el«lj oonnlidated a* (o
be eqiul, or erea laperior, in hanlDflaa to the nataral runnaltoa.
Two freshlT-cat Bomce* of lead or caontchono will, on being
preaKd together, cohere w tightlj that it ii acanxlj poarible to
aepaiste them : uid availing luinaelf of thia fact, ths chemist pi«-
pam tabes of the latter valaable anbatance, applicable to numeiuiu
important parposei id bia maDipalaCioDa.
33. Cohaion fyara. — Soma caiioDB obMrration* ham been
made bj Hr. Tomhnsati* oo the Tariet; of figurea produced, when
dropa 01 diSerent kinds of oil are drepprid on the surface of water.
To tbeaa the name of cohenon-fignr^a has been applied, aa their
difirencei most depend on the different dogreee of coheaion exiit-
ing between the reapedive particles. These figures appear to bi
very constant for the same kind of ail, bnt to dISer connderablj ii
connderablj in
taken as a teat of the purity of the oi
diKient kinds ; so mnch ao indeed, that the figure may
■ ■ "leoil. Oli'
M. Attraction takes place not nnlj between two portions of the
aame tind of matter, but also between the a4)acent surface! of
4iifereDt substances, as between those of s soliil and a liquid ; this
Tanety of attractJTe force has tieen termed adJiaiim.
It fnnn one arm of a balance, a plate of copper, c, be anspendecl,
and carefully counterpoised by weigbta in the scale auspen^ed from
the oppoeite end of the beam, a very alight additional weight will
cause either the plate or the acale Co prepondenite ; place a basin
full of water, B, nnder the plate, c, in mch a manner that the latter
may jnat touch the surface nf the water in b ; on placing weights
in a, a very considerable reaistsnce is experienced to the aeparation
• "Jaon^DTtlHSacMrrof Aru," 1M4, p.HS.
20
EZTEfiHAL F0BCE8.
of c from the fluid surface, owing to this adhesive attraction. With
a circular plate of smooth copper, presenting an area of 6*75 inches,
Fig. 19,
4f^
l£ —
~
<? n
\ 1 111
1!
ii ill
ii 1
the weight required to overcome the attraction of the metallic
surface for the water exceeded 1000 grains.
35. The intensity of this force, although constant, cceteris pari-
bus, for the same solids and liquids, varies considerably in dinerent
kinds of solids or liquids ; the following table represents the com-
Sarative intensity of the adhesive attraction exercised between
ifferent metallic surfaces and mercury, according to the researches
of Guyton and Quetelet»
HetaJ didn
Force of adheaion
in grain*.*
Biakof
metal.
ComparatiTe force
of adbeaion.t
Gold . . .
Silver . . .
Tin. . . .
Lead . . .
Bismuth . .
Zino . . .
Copper . .
Antmiony
Iron . . .
Cobalt . . .
446
429
418
817
872
204
140
126
115
8
Gold . .
Silver .
Tin . .
Lead . .
Bismuth .
Platina .
Zinc . .
Copper .
Iron . .
23-63
22-74
2215
2104
19-71
14-98
10-81
7-52
610
Gay-Lussac suspended a circular disk of glass, 4*6 inches in dia-
meter, over surfaces of water, alcohol, and oil of turpentine. He
* Onyton-Marreau, in Kaatner*! ''Ezperimentalpfaysik." Heidelberg, 1810.
t Quetelet, ** Foaitiona de Vhjuque," p. 104. BnxzeUca, 1834.
CAPILLAB7 ATTSACnOH.
21
fiMiiid die force required to separate the diak from the floids to Tarj
isoasiderabl J, as shown in the following table : —
AdhMire force
Flnid.
Water
Alcohol
If
Spaelfio gr»Ti^. ^ ^^^^
. 1000 414-7
. 0-8196 477-4
. 0-8695 6061
. 0-9415 669-8
0-8695 623-6
.B
Oil of turpentine ,
The force which causes the disks in these experiments to adhere
to the fluid is identical with that which causes fluids to ascend in
capillary tubes (37). The disk attracts an infinitely thin layer of
the fluid on which it rests, and it is the molecular attraction of the
mass of fluid for this thin layer adhering to the plate, which causes
the resistance opposed to raising it from the surface of the liquid
submitted to experiment
CapOiarity,*
36. If a plate or rod of any substance be plunged into a fluid
capable of moistening it, as a plate of glass in water ; the surface
of the fluid, ab, fig. 20, instead
of remaining perfectly horizontal, ^' ^*
will rise to a nigher level at the
eodes of the plate, as shown by the
dotted lines, as if the water were
attracted bj the glass. If the glass
plate be slightly greased prior to
immersion, or be plunged into a
fluid incapable of moistening it,
as mercunr, then a depression
instead of elevation will take
flace on either side of the plate,
f a plate of glass, e, Fig. 21, be
plunged into mercury, c d, this ap-
parent repulsion will take place;
and appears to be owing less to
any peculiar property of the fluid
metai, than to the presence of a
minute film of moisture adhering
to tiie immersed solid, and pre-
venting the actual contact of the
mercury with the glass.
37. These phenomena are best witnessed by immersing glass
tubes of small diameter in water tinted with archil or ioi ; the
fluid will rapidly rise, attaining the greatest elevation in the
• For an saalytiettl iiiTMtigation of this Bubieot the reader is referred to
J.Da«Milz,"11i(eori0Metb£ii»tiqaedelaCapflUri^^^ Paris, 1886.
Fig, tl.
O-
^
7"
>D
miMt RkpillHrf InbM. Thna it will rise mDch
higher in Jl tbnn ia B, in > than in o, &c. Tbis mnde of ftttroo-
tion, eTiclently * modificBtion of ibo last-described pheaomena, ia
^, ji_ termed capillority tnm its beint most ob-
vious in tabes of capillsrj orhaiiOike bores.
He. height attaitjed by fluids ia tfaei^e
tnbflH is constaot, and incT^aeea ioTerselj
as tbe disnieten of the tuhea ; it bears do
evident ratio to the density or speciBo gra-
I vitj of the fluid emploji-d in the oxpvri-
' ment : for Moschenbriick* found that, in
Inhes of equal diameter, fluids rose to the
comparative heights shown in lh« IblJowing table : —
Nun* of flaid. ElrrMioiu
SulphDric acid ■ ■ ; I '30
Sulphuric ether, eoDtaining alcohol . . I'40
AnhydroQ* aloohol 1-80
H^rochloric acid ....
Niti
207
2-53
3'40
Uil of turpentine ■ .
Distilled water . . .
SolutiDn of ammonia .
Solution of carbonate of .
H. Gay-Luasac hu ascertained tliat water, altobol, and m\ of
turpentine, ascend in tubes of tiie diameter of -05 inch to the fol-
luwiDg elevations : —
PtuM. Speclfla gnvftf. SlvratiBo.
Water 1*000
. 081 98
Atcuhol . . .
Oil of turpentini
0-9415
0-37
0-33
0^9
comea ipto play equally between two
fluids, aa in the case of lubes. If two
platea of glass, i, B, touching at
o, and aeparated at b at a very
small angle, be plnneed into a
trough D, filled with coloured
the fiuid will, aHer a i^hort
. rise between the platea,
attaining the greatest elevation
where the edges of the g!
scribing II
ectangular hyperbola. The utmoe
• "DiM.P)>7Sla.Bip«lmaBt,L.B.."in«.
CAFtLLABT ATTlUOnOV.
23
Fig.U.
aittmed by the fluid in this arrangement is one-half of that which
vodld haTO taken place in tubes having their diameters equal to
the distance between the plates, and is always inversely as t lis
distance. And the distance between the plates at any given point
is, by similar triangles, proportional to toe distance of the point
from the vertical edge, c ; hence the elevation of the fluid at any
point multiplied by the distance of the point fi^)m the ver(ic<vl
edge, c, is a constant quantity, which is a property of the rectan-
gular hyperbola.
39. Ita drop of water be placed in the wide end of a conical glass
tube, as at B, it will rapidly move towards the smaller end, a. The
drop on being placed in the tube,
becomes bounded by two concave
sni&ees, of which that nearest
the apex of the tube is the most
curved ; the drop, therefore, moves
towaids the apex in consequence
of the attraction of the sides of
tbe cone for th^ water ; bein^, according to Laplace, inversely as
the radius of the curve termmating the fluid coramn.
Let A B D, Fig. 25, be a compound tube, consisting of a flue tube,
bavmg a capillary bore, inserted into a wider one. ^ Let the latter
be immersea in water, the fluid will rise to a certain elevation, l.
Then let the whole tube be filled with water, and again immerse
it, the fluid will fall to a certain point in the finer tube, as to m,
sod there remain suspended as perfectly as if the whole tube had
been of the same diameter as the part a b. On
ndfiing the tube gradually until the point u reaches
B, the fluid will again sink rapidly until it again
attains its former elevation, L. In tnis experiment a
Iai)|;e quantity of fluid is supported partly b v capillary
attraction in the small tube, and partly by tne co-
hesion of its particles in the large one ; and as the
elevation at m is found to be the same, whether the
lower part of the tube be larae or small, it follows
that the portion of fluid which, oy its gravity, opposes
JTirther elevation at m, is a column of which the height
n that of M above the surface, and its area the section
of the tube at u. The convene of this will subse-
quently (see Hydrostatics) be found to be true with
regard to the pressure of a fluid on the base of a vessel contain-
ingit.
It is a remarkable fact, that capillary attraction is capable of
eppoeiog the evaporation of fluids under its influence. Fine tubes
01 glass, containing as much water as they could under the influ-
ence of this force retain, have actually been suspended for months
together in the summer's sun, without losing by evaporation any
appreciable portion of their contents. It is, however, questionable
^.26.
24 EXTEBKAL FORCES.
whether this result is not partly due to the extreme minatenees
of the evaporatiog surface.
40. By means of capillary attraction, oil is raised in the wicks
of lamps, water ir. bibulous paper, cotton threads, or any porous
substance immersed therein; m fact, all phenomena, in which
fluids insinuate themselves into the pores of solids, are referable
to this force.
41. If, instead of using water in the experiments just detailed,
a fluid incapable of moistening the surfaces of the solids immersed
be employed, the converse of the phenomena is observed, repulsion
taking place instead of attraction. Thus tubes or plates of glass
immersed in mercurv in their ordinary state, cause a depression
instead of elevation (36) ; or, if water be used, and the tubes are
greased or rubbed over with resin, or still better, lycopodium, the
same thing occurs. In tubes thus circumstanced, the depressed
surface of the fluid always presents a convex, instead of concave
surface. This repulsion at some distance is well observed by
nibbing the hana over with lycopodium, and immersine it in
water ; on withdrawing it, it will be found to be perfectly dry, not
a drop of water adhenng to it.
The following table shows the amount of this capillary repul-
sion observed when glass tubes are immersed in mercury, luler
care has been taken by boiling the li(^uid metal in the tubes to
expel all air and moisture adhering to their surfaces, which, accord-
ing to Baniell, diminishes the capillary depression one-half. The
amount of the depression of the mercury is always in the inverse
ratio of the diameter of the tube.
Diameter. I>epre8Bion.
0-60 in. . . 0002 in.
0'60 „ . . 0*003 „
0-46 „ . . 0 005 „
0-40 „ . . 0-007 „
Diamet«r. Depreadon.
0-30 in. . . 0-014 in.
0-25 „ . . 0-020 „
0-20 „ . . 0029 „
0-15 „ . . 0044 „
0-10 „ . . 0070 „
0-35 „ . . 0-010 „
42. Adhesive attraction (34) is exerted not only between liquids
and solids, but is equally active between the latter and invisible
gases. Thin films of air adhere by virtue of their attractive force
to the surfaces of most solids, and become very obvious in glass
tubes when mercury is poured into them ; the fluid metal, instead
of closely and equally adhering to the inner surface of the tube,
will be separated from it in several places by interposed bubbles of
air, which adhere with the utmost obstinacy to the glass. This
curious form of attraction is well shown in porous bodies, as cork,
pumice-stone, charcoal, &c. When a fragment of either of these
18 immersed in water, and placed under the receiver of an air*
pump, the escape of torrents of bubbles of air on exhausting
the receiver is very evident. The term tibsorption is generally
applied to this power of porous bodies in attracting gaseSj and is
EHDOfiUOSS AKD EZ0BM06E.
25
remarluMT intense in the cftfie of fresbly-bomt charooaL Thus
one cubic mch of this subatance will reaoily absorb^
90 cab, in. of ammonia.
86 „ hydrochloric acid.
55 „ hydrosulpburic acid.
35 ,« carbonic acid.
n
9*2 cub. in. of oxygen.
7*5 „ nitrogen.
17 „ hydrogen.
1}
All hodies in the state of powder possess this property of absorbing
air, which becomes obvious when they are immersed in water.
Toleiably coarse iron-filings will thus actually float in water, if
carefully sifted on its surface, being buoyed up by the adhering
air, which appears like little gbbules of polished silTor in the
water.
43. A class of phenomena referable to eaptUariiy is the appa-
rent attraction and repulsion of small bodies floating on water,
when placed at small distances from each other. If one of the
bodies only be composed of a substance capable of being moistened
by water, mutual repulsion will occur. But if both are incapable
of being moistened, as two balls of wax, mutual attraction ensues.
If the balls a, b. Fig. 26, be of wax, or cork, rubbed over with lyoo-
podium or resin, the water is repelled, and two depressions in
which the balls lie are producea. If thev are then placed suf-
ficiently near each other, the repulsion of the opposed surfaces of
the balls exerted on the water at c will ^^ 26.
lender its surface concave, and the
balls^ by the lateral pressure of the
water beyond, will be pushed together,
and appear to attract each other. In
the second case, if the ball d be of
clean moistened cork and e of wax, the
reverse takes place, the water being
raised by attractive force on all sides
of the first, and repelled by e. There-
ibre, on the balls being placed in con-
tact, they appear to repel each other in
consequence of d attracting the fluid,
which is repelled by e, the Tatter being
incapable of being moistened by the water. If both bodies are
wetted by the fluid in which they float, as two clean cork balls,
r, G, in water, they will be drawn together by the united effects of
the cohesion of the particles of fluid, and their adhesion to the
Rufaces of the balls ; and when in contact, the fluid will rise higher
between the balls, than at any other part of their surfaces.
44. Closely allied to capillarity are the phenomena of endomioae
and exosfnosCf discovered by Dutrochet. Whenever two liquids of
di&ivnt densities, capable of being mixed with each other, are
separated by a ii!iembranous or porous partition, two currents be-
come esUblirheil, ona & currant or fluid proceeding
from witbia to without (exoBinaBe, JE Bid iiafiit,
impulse), uid anolLer in the contrar; direction (cndoB-
moM, Iviov and uir/iic). If a gUu tube cloied at one
end witb a piece of bladder, ±, be paitl; filled with a
I tolution of sugar, aalt, &c., and immened In a teasel
partly filled with pure water, the fluid will rapidl*
rise ID the tuba b, the water having entered tbronK"
jl the bladder b/ endosmoae, and, adding to tbe
I contents of the tnbe, cause tbe fluid to be elevated
much above its former level. If. now, the conditions
be revereed, Bvrup being placed in c, and water in B,
exosmoee will occur, b; which the tobe b will become
nearlj emptied. Ab a general rale, liable, howerer, to
Mveiil eicoptioos, it appears that fluids of tea tpe-
dfic gravity have a tendency (o pass througb membranes and
B bodies, to mix with those of greater density (provided thef
.-iscibla), and consequently to dilute them.*
Tbe general rule may probably be thus more correctly ex-
pressed, that the fluid, between which and the porous paitilian
the greatest amount of capillary attraction eiiBts, will usually
pass through and mix with the other fluid.
i6. These phenomena admit of a very simple explanation,
(banded on tbe capillary attraction or repulsion- exerted by the
porous diaphragm npon the fluide expoeed to ita influvnce. In the
case of a piece of bladder, this is readily moislened by water, but
not bjr alcohol. Let the tube b be partly filled with alcohol and
then immenied in water, Tbe first action in this coae ia the
~ ID of the membrane to the water, whilst it repels the alco-
hol. A portion of water permeates the bladder,
ia immediately mixed with the alcohol, and is do
longer attracted by tbe bladder. A fresh portion
then enters, and this continucB mitil the al(»hal is
considerably diliit«d.
Ths moBt conTcnient form of apparatus for
demonatrating endoamosa or eioamoae is a
amall rnnnel-sbsped flaaa vessel, tbe month of
which ia about two inchcB in diameter, and fur-
niahed with a rim, over which a piece of thin
bladder may be aecorely tied. Int« the neck
of this a piece nf barometer tube about 18 inches
long, and O'l in internal diameter ia ground.
By this means tbe veaael may be readi^ filled
and emptied, and the area of tbe acting sarface
Ib bo large compared witb tbe section of the
tuba, that the fluid will nae two or three inches in the tube in s
quarter of an hour.
Tbe endoamose or influx of fluid is always attended by an
> "IIaaT.l(MtMnih.iBrrKiido«ao«*,"lU,pwH.Dilncbat. Fuda, 1898.
mdfttinn of > ceitki'n porticni of the liquid confinod
by the poron» diaphragm, Thii ihiit bo illmrtrslBd bypUcing in
the tube ■ aolation ^ sulphate of iron, unci immening it in
water. In * short time the lolution will ri«e in the tube from Ihe
FDtnoce of water ; and if then a few dropi of tincture of galls
be added to the water is the external lessel, the purple colour
which ia produced will saliafactorilj prove that a portion of
Ihe solutioD of iron has really ainded through the membrane.
The onuotic foireof dillerant saline solutions difiers considerablv;
tbns a ■olulion of chloride of almniDUEn is, according to Mr.
QTshain, 45 times more eneigetic than one of chloride of sodiam.
46. If the capillary action of the two ffnids on the diaphragm ia
nearly equal, the endoBmose and eioamoee will be rery feeble ; bat
will take place with considsrablo actirity in the direction of a gal-
Tank current transmitted throuzh the flaids. This fact maj be
ludjly ahown thua : take four pieces of glass tabe (hat will suc-
ceniTcly pass within each olher, close the oaler one with a piece of
cvtk, through which the poaitiTe electrode of a Tollaic battery may
be paaaed, and tie a piece of Chin mem-
hnne oTer the ends of the olher three. 'V- ^
If a satmsted solution of gallic acid be
placed in the Srst and fourth tube, and a
weak solntionorprotoaulphate of iron* in
the aBcond and third, and the tabes be |
placed one within (hs other, and a current
pissed throDgh the whole, by dipping
the iiegatiTB electrode in the flnid con-
tained in the inner tube, the formation
oTgallate of iron in the second and fourth
lobM will immediately indicate the pas-
sage of gallic acid in one case, and of
prcto-enlphale of iron in the other case,
aoooTding to the direction of the current. "*
This fact ia important in Physiology in +
indicating the probable manner in which
the nerToos syitem influences the tarioos animal Beoratians.
47. Analogous phenomena are also eihiblled hj gases or seri-
(bnn fiuida. If, fur instance, a glass Teasel full of airliaTs a piece
of thin bladder tied firmly OTer its mouth, and he then placed in
ajar of bydroEen, the gas will permeate the membrane and enter
l>w Teasel. The contents ofthe latter areconseqaently increased;
the snrface of Ihe bladder becomes comex, and if safGciently
thin will erentaalty burst. It has been demonstrated by Hr:
Graham, who has moat elaborately examined these phenomena,
that gaaea difler in tlieir tendency to difluse Ihemselves through
membrane* or poroua diaphragnu. This tenilencj diminishes with
*(IT. ofnlBeuid, and 7or8 (r. arprDtaaalphaleotima, each in 1) «.
le of density ot the gtn, boing inTeraely proportional lo
root of tbis deDBitj. It maj b« rBmarked. tlat the
ipplies lo tbe relative Telocity with which diSerent giweB
will be discharved from bd orlGce into a Tacnum : cLia fact seeme
to comiborale the brpotheaii of Dallon, that any one gu acta aa a
Tacnnm ia relation to another.
hydrogcD, oxygen, and nitrogen will b« aa fallowe : —
Oi;g«n . . Dene. 1105 . . Diff. 0'94S
Nitrogen . . „ (C972 . . „ 1014
Hydrogen. . „ 0-069 . , „ 3*807
If a long tahe be closed with a p)a^ of dry plaat«r of Paris,
inverted in a cup of <rater, and filled witb hydrogen |
will BO rapidly permeate the planter, ' ""
1 a plnp of dry plaat«r of Paris,
filled!^ witb h;rdra»n jrag, the gas
«r, to diffusa itMlfin toe air, as to
46. AnieS't -Fin Vamp Indicator. — The diffiuive power of
gasDB baa been ingenioaaly ntiliaed in the
Kg. to. conBtmction of this apparatiis. An inflated
film of caoutclioaB, a, snrrounded by a hand
of linen, to prevent ita expanding transvenely,
ia Qxed on a stand, r, no that the upper aurface
rests aeainBt a lever, b. Thtg lever bolds a
BCftpe-wiieBl, c, which is driven round by a
cord attached to a weight, d, in tbe hollow
stem. The two palleta of a cratch, to which
the clapper of the bell, e, is attached, work
agunst the wheel, t. If this appatatus
be placed in an atmosphere containing a
dangerous percentage of coal-gaa, the gas
will ra[udly permeate the film, and distend-
ing it longitadinally will press npon the
krer, and release the oacapement, and
thus set the bell ringing, to give tbe
reqaiaite alam). Tbe lever, b, may also be employed to complete
a voltaic circuit, and thus b; saitabla means a signal may be
conveyed from the bottom of a mbe to the mouth of the shan,
or elsewhere.
An analogous contrivance is tbat of makinfc the bottom of the
c»ae of an aneroid barometer to consist of a disk of parous eaiih-
•nwore, similar to that of which the porous cells of a voltaic
batteiy are made. With tbis the diffuaion is so energetic, that
the indication of the presence of gas is almoet instantaneonsL It
is to be hoped tbat these and similar coatrivancna may be inatm.
mental in migrating the fearful loss of human life tbat constantly
occurs amidst the perilous occupation of the miner.
D1FFT7810S OF LIQUIDS. 29
49. The following results also Bave been obtained* with regtird
to the transpiration of gases throagh capillaiy tubes: — 1. The
resistance of a capillary tube of uniform bore to the passage of any
gas is directlj proportional to the length of the tube. 2. The
Telocity of the passage of equal Tolumes of air of the same tern-
Serature but of different densities, is directly proportional to the
ensity. 3. Tllat rarefaction by heat has precisely the same effect
as loss of density bj dimimshed pressure^ in diminishing the
Telocity of the transpiration of equal Tolumes of air. And finally,
that transpiration is promoted by density, and equally whether the
increased density is due to compression, to cold, or to the addition
of an element in combination ; thus the velocity of oxygen is in-
creased b^ combining it Trith carbon Tnthout change of yolume, as
in carbonic acid gas.
50. The tendency of gases thus to diffuse themselves among
each other, is a property participated in by liquids. This is, how-
ever, not vrithout exception, as some, like oil and water, are not
miscible vrith each other; and others, as ether and water, are
miscible but in small proportions. In most cases of miscible
fluids, an actual penetration of the mass of one fluid by j^^, 31,
the atoms of the other seems to occur ; and the mixture
consequently occupies less bulk than the fluids did when
separate. Thus, if two glass bulbs. A, B, Fig. 31, filled,
one with water and the other with alcohol or sulphuric
acid, be connected by means of a tube, c, passing water-
tight from one to the other, the fluids will mix, and when
the mutual diffusion or mixture is complete. Trill no longer
fin both bulbs. On allowing the apparatus te rest for
a few minutes in a vertical position, a space unfilled by ^^
fluid Tsill be observed in the bulb, a, in consequence of /^v
the mixture having been accompanied by a diminution in ^^^
volome. If 100 parts of alcohol be added to 100 of ^^
water, the mixture will measure but 196 parts ; the same bulks of
sulphuric acid and water will, after mixture, measure only 135 parts.
51. It appears from the observations of Mr. Graham,f that
neutral salts and various other substances in solntion have a
diffiisive tendency, similar to that of gases. The results were
obtained by^ immersing wide-mouthed bottles, containiog any pro-
posed solutions, in glass vessels of distilled water; great care
being taken to avoid any mere mechanical mixture of the contents
of the bottle, and the surrounding fluid, by agitation. It was
found that, with most substances, when the quantity in solution
varied finom 1 to 5 per cent., the Quantities diffiised in the same
time ^usually a period of eight oays), were proportion^ to the
quantities in solution, the temperature remaining constant ; also.
Shot the diffusibility increased Trith increase of temperature. It
appeared also, that, of the whole quantity rather more than one-
• Graham on the Motion of Gmss; ** Phfl. Trans." 1849, Part n.
t On tlie I>iiRiai(m of Idqnids ; *• Phil. Trama." I860, Part I.
80
BXTEBKAL FOBCEB.
fourth was diffused dnrio^ the fint two days, the qnftiitities dif-
fused during each remaining period of two dajs being yery nearly-
equal. The following table exhibits the relative quantities of
rarioas substances diffused at a temperature of 60'5' F. daring
eight days, from solutions containing 20 parts to 100 of water: —
Sulphuric acid . . . 69'32
Chloride of sodium . . 68*68
Nitrate of soda . . . 51*56
Sulphate of magnesia . 27*42
Treacle 32*25
Glucose . . , . . 26*94
Cane sugar (crystals) . 26*74
Gum arabic . . ... 13*24
Albumen . . . .' . 3*08
Salts of different bases may be separated by difiusion, for the
quantities of the carbonates of soda and potash diffused in the
same time from a solution containing equal parts by weight, were
found to be as the numbers 35 and 65, very nearly ; and while the
quantity of magnesia in the salts obtained from sea-water was
6 per cent., the proportion in the salts diffuRed was only 4 per cent.
In some instances the difiusive power appears to be sufficiently
energetic to effect the decomposition of tnple salts, such as alum,
and the ammonio-sulphate of copper.
52. Friction is the resistance to motion which any portion of
matter offers to another portion in contact with it; and beinf?
analogous to adhesion, may be most appropriately considered in
this chapter. Friction is of two kinds, one of which opposes the
commencement of motion of one body in contact with another, but
ceases to act when the body is actually in motion, the other con-
tinually resists and retards the motion. To these forces the terms
»t(ttical and dynamiecd friction have been applied : but, to avoid
any error from the'confusion of terms. Dr. Whewell has proposed to
designate the former atiction^ a simple and intelligible term, retain-
ing firiction to express the latter force. We may therefore desi^ate
these retarding forces by their initial letters, J?* and /Sf respectively.
Between two plane surfieu^es, either of the same or of different
materials, F is proportional to
the pressure P by which the two
surfaces are held together, or,
in other words, p is a constant
quantity : but at the same time
.r'is independent of the extent
of the surfaces in contact. This
may be shown by the apparatus
represented by Fig. 32, m which
a weight, a, placed in a scale
attached to a strinz passing over
a pulley, b, is employed to drag
a mass, c, along the horizontu
surface of a table, de. The
mass c, whether of wood, iron, or other material, is in the shape
of a rectangular parallclopiped, .the breadth of which is B or 4
Fig. 32.
nuonoH or smrACES. 81
tiiDM tke height, and baTing one of the broad sides hollowed ont
BO as to leave only two narrow margins. K a is jtui iuffident
to keep c in motion, it will be fonnd to have the same effect,
whether c rests on the broad or narrow side, or on the two
maigins, here represented uppermost. ^ Also, if one or more weights,
each equal to c, be placed on it, in either jiosition, a proportionai
increase of the weignt, a, will be foand eqaivalent to tae increased
friction.
63. The retarding force represented by S increases from the
mstant when both sor&ces are qniescent, and attains its mazimnm
e&ct in the course of a few minutes. It may be yery readily
coanteracted by jarring the surface of the table slightly, but re-
peatedly, by the hand, during the time of the preceding experiment.
When the weight, a, has been determined ezperimentallv, and
the body, c, then allowed to remain qniescent for a short time, it
will be found to require a considerable increase of weight, in some
instances eyen greater in amount .than c itself, in oraer to start
the body, c, from its position of rest* The added weight will re-
present the resistance, 8.
Sh^B been found by experiment not to be independent of the
extent of sur&oes in contact, or proportional to the nressure, and
therefore not to follow the same laws as JFl No dennite law has
hitherto been assigned to this Quantity.
54. Between hard surfaces, P is found to be an uniform retard-
ing force ; but between soft surfaces, as those of felt or leather, it
increases with the yelocity of motion.
i^is fonnd to be diminished by coating the surfaces in contact
with any unctuous Rubstance. S may or ma^ not be diminished
by the same means. It appears from the experiments of M. Morin,
that if a complete stratum of the unguent be interposed, F is the
same for all substances. This result is manifest, as in this cafie'
the retarding force is the cohesion of tbe unguent., and not the
friction of tne opposed surfaces. Finely powdered plumbago,
either dr^, for sumces of wood, or inixed with pease for those
of metal, is found to have tbe greatest effect in diminishing friction.
In many instances ^is dimi- _.
Dished by polishing the surfaces *^'
in contact ; 8 is generally in-
creased by the same means.
56. The yalue of ^ may like-
wise be determined by the fol-
lowing method. One edge of the
tiulace, A B, under experiment,
is raised from the horizontal
plane until the body, c, placed
on it^ will just slide down, and
the angle of eleyation deter-
mined by a graduated arc, d e,
•i in the annexed diagram.
ir this angle be celled f, then ten ^ {the trigonometricel tan-
gent of the mgle) ia called the coefficient of friction, and the
angle itself bu been called the iliijin^ angie, or limiting angle
of railtanet, becaoite, If two jilane surfaces of any kind of
material rast againgt each otber, it is evident that no nmount of
preaauro will caute the surfacss to alids over each other, if they
•re inclined at an angle leaa than p to a plane perpendicular lo
the line of preaaure. The limiting angle of reaiatance ia important
ia the arts of conatruction, aa, lor instance, in delemiining th«
neceasar; direction of the joints of archea, and of the alopea of
The value of f having been thua determined, the value off +:S
ma; be aimilarly determiiied bj allowing the bodies to remain for
a abort time at rest, and then gradual!; raiaing the plane, until
Friction between cjlindrical .aarfacea is found to follow the aame
law, bnt S ia said not to eiist between cylindeia. This, however,
has been found not to be the caae, if a hollow and a solid cylinder
are very accuratelr fitted to each otber, as ia the caae in Whit-
worth's cyhndrical gaagea.
56. If a cord pasiing over the anrfac« of a wboci or cylinder
is emplojed lo soatuQ a weight, bb in the capstan, or to coroinnni-
cate motion, as in tbe driving parts of machiner;, the amoont of
friction dependa oa the angle of contact between the cord and
auTface on which it reals, and ia independent of the radiiia of tbe
surface ; it ia alao greater when the cord testa in an angolar
groove, than when resting on a curved auriocc, or on tbe aurface
of the cjlinder, aa may be thua abown : — l^ake three caat-iron
vlieels, one of two or three inchee, A, and another a foot or more
in diameter, b, with similar angular groovea, and a third, c, of any
oonveoient diameter, withsplunrim; take, alao, apiece of hempen
cord, vitb two equal weights,
Fif-U. II, K, attached lo each end of it.
"- Jjet the cord be placed in the
grooves *, b, aucceaeively, and
" It will be found that in both
cases the same weight, r, is
necBBsary, when attached to
one of the weights, n, to just
drag up the otber s ; but that
asmaller weight, f, will suffice,
' if the cord be laid on the plun
rim, c. When the weights are
^ freely suspeuded, it is mani-
fest that the oord will be in
contact with the groove through theeileut of a semicircle, or 180°.
If tbe cord be now carried from tbe wheel, a, horlion tally over a
pulley, a, it will bo in contact with the wheel through a quadrant.
a&lYlTATIOV. 88
or 90* ool J. It win now be foand that ■ wiU juH move with a
weight, F, Buch that, ifD+jraKs in the latter instanoe, then
i>+P=K*B would giTe the ralne of f in the former. Thoa, if in
the case of the strinf in contact with a quadrant of the circle, d
and B being each llb^ f (including the scale) were also lib., then
woold Ks2 ; and when the string is in contact with the whole
semicircle, d and b remaining the same, k' woald be 4, and the
weight, F (including the scale), would be Slbs.
Ctmaeqnentlj, in the capstan, the amount of friction depends on
the number of coils of the rone ; this amount for each sucoesrive
coil is, in fact, in geometrical progression, and between a wet
hempen rope, and a cylinder of oak, the common ratio is 8 Tery
neanj : thus, if the rope were held on with a force of 1 cwt. the
weights sustained by 1, 2, and 3 coils respectiyely would be about
8 cwt, Si tons, and 25i tons.*
57. In the working of machinery friction gives rise to a oon-
siderable expenditure of motive power, as well as wear and tear of
material : but as there is no friction between surfaces that roll on
each other without any sliding, this source of loss may be in a
great measure obviated by the introduction of what are termed
fnetionrToUerf gt frictionrwheeU. The axis of a fly-wheel, or other
heavy rotating piece of a machine, is sometimes made to rest on the
drcomference m a wheel at least ten or twelve times the diameter of
the axis, eiiher before or after passing through the bearing collar :
or if there be no considerable pressure on the circumference of the
wheel, the axis may rest in me obtuse angle formed by the cir-
cumferences of two wheels, placed near each other, and overlap-
ping each other about one-third of their diameter ; in which case
no bearing-collar or axle-box is required. The axes of the friction-
wheels must of course sustain a certain amount of friction, but it
will be small compared with the friction saved, on account of the
slowness of their motion.
▲TTBJLCnOirB AT 8BKS1BLB DIBTAHCBB.
Qraviiation,
68. When a heavy substance is permitted to fall from the hand,
every one knows that it rapidly reaches the floor ; and doee not
rise towards the oeiHng, nor move laterally towards the walls of
the room. A stone being mere inanimate matter, and consequently
absolutely inert, this phenomenon cannot depend upon anj iamaU
tendency to reach the lower part of the room, as one of the enmitial
* If Ian ^ be the ooeffldent of fHctioii between the rope and Bovlhea in
ooBtaot with it. mod 9 the angle of oontaot ; alio Px, the weight rastslnedby
Pi end the lirieiioa jointly, then
Pj=P,f*-*"* or logPi— k)gP,=«.ttti^.
8ee ICoeeley's " Meehanieal Prineiplei of Bngineering and Arobiteetve" for
farther infonoatJon on this sal^iect.
34
ETRBHAL VOBOB.
^.35.
properties of matter ia its utter incapaciW to change ita porition.
CoDseqQently, the rimple phenomenon of the falling of any hodj
towards the earth mnst anse from the exertion of an attractive in-
flnenoe or force emanating from the latter, and to this the name of
OravUatian is applied, m consequence of its caasing that efiect
which we recognise hy the term weiaht : the weight of any sub-
stance being merely a measure of toe attraction of the earth for
it. This form of attraction is exerted not only at comparatively
small, bnt at all distances, however vast : thus, this force acts as
effectually on the planet Herschel at the distance of 1,800,000,000
miles, as on the falling apple, in which Newton is said to have first
recognised its existence. If a mass of lead be suspended by a
string it will, when left iiee to move, point towards the centre
of the earth; now the same
thing occurs in India, in
America, and at our antipodes ;
a fact proving at once that the
lead does not obev a nahaxU
im%deney tofaU; tor the plum-
mets, A, B, point in opposite
directions, aa alao do c, d, ac-
cording as they are situated
at the opposite poles or at east
and weat ; all pointing towards
the centre, e, of the earth.
59. Gravitation, in common
with other attractive forces,
obeya most strictly the ffeneral
law already announced (31),
its intensity being inversely
as the square of the distance of the gravitating boay. Thus
our moon, which is placed at a distance of sixty of the earth's
semi-diameters from its centre, is attracted according to this
kw with a force of 60 x 60=3600 times less than bodies are on
the surface of our globe. The force of gravitation must always be
considered as acting from the centre of g^vity of any body from
which it emanates.
Fig. 96.
From this circumntance it is theoretically
impossible for two plumb-lines freely sus-
pended to hang perfectly parallel. Let
A and B be two lines, each having a
leaden ball suspended to it ; they will
point towards the centre, c, of the earth,
and of coarse, instead of being perfectly
parallel, will form an angle with each
other, which at small distances is so
slight that it may be almost neglected
_ in reality, althougn it can never entirely
vanish. In small distances, even to the
tK^^m^-:stmm^mmB^^Kmtrwr'^^mmr
ORAVrTATIOH. 35
extent of some hundreds of feet, the lines of gravity indicated
br two plnmb-Iines may, on account of the magnitude of the semi-
diameter of the earth, be regarded as parallel ; but when these lines
are some miles apart, their convergence must be calculated accord-
ing to the curvature of the earths surface;
this will amount to about one minute in a FSg, 87.
geographical mile, and consequently to one
degree m sixty miles.
LetABCDirbea section of the earth at
the meridian of Paris, and ▲ a; its axis of
rotation. Paris will be situated at c. and a
pltunb>line freely suspended there will point
in the direction of b o c. Dunkirk will be d
St an angular distance of 2*^ 11' 6" from
Paris, and its plumb-line will coincide with
P D e. Barcelona will be at b, at an angular
distance of 7" 28' 29' from Paris, and a
plamb-Hne there will coincide with the line
o B e, forming an angle of 9* 39' 35" with a
omilar plummet at Dunkirk.*
60. The intensity of the attraction of gravitation varies, not
onh- with the mutual distances of the attracting bodies, but also
vitn the quantity of matter contained in them. In this wav the
great centre of our universe, the sun, from its enormous bulk, its
mass being greater than that of all the planets taken together, \b
capable of attracting even the most remote, as Uranus and Nep-
t^me, although placed at the enormous distance of hundreds of
millions of miles. This force being mtUuaUu exerted between
bodies, they idways move to meet each other : hence when a book
or a stone falb towards the earth, the latter rises to meet it: this
motion is of course almost infinitely small, because the attraction
of these bodies for the earth being in the ratio of their masses, the
enormous preponderance in favour of the earth would prevent its
movinjT an appreciable distance to meet the stone, whilst it wonld
be sufacient to enable our globe to attract the latter at a distance
of several millions of miles. As a necessary consequence of this
mntoal attraction of masses, elevated builuinss and mountains
might be expected to gravitate towards each other, an effect pre-
vented by the superior attraction of the earth which tends to keep
them on their bases, and by the attractions at insensible distances
which firmly bind their integral portions together. For whenever
graritatton and cohesive or capillary attraction are opposed, the
tatter within the limits to which they are confined are most ener-
getic, instanced in the ascent of fluids in capillary tubes (37), above
their level, aud in opposition to the gravitative attraction of the
earth. Still, lateral attraction is exerted, for Dr. Maskelyne, in a
• PoaiUet, " moments de Physique."
d2
86 KXTSBITAL FOBCK.
set of ezperimentB peifonned in 1772 near the moantain Shehallian
in Scotland, found that a plummet was really drawn from the per-
pendicular by the attraction of the mountain to the extent of 54".
The same thing took place in the researches of the French astro-
nomers, whilst engaged in America in determining the measure of
the meridian ; nuinerous sources of fallacy arising from &e lateral
grayitation of their instruments towards the surrounding moun-
tains, opposing themseWes to the correctness of their results. The
lateral attraction of Chimbora9o, the loftiest of the Andes,
although much diminished by the existence of an enormous vol-
oanic cavity in its centre, was found by M. Bonguer to deflect a
plumb-line 7' or 8" from the perpendicular. The mutual attraction
of bodies free to moye is beautifully illustrated in the celebrated
Cavendish experiment,* which has lately been repeated by the
late Mr. Francis Bailv.t In this noble experiment the attrac-
tion of a large mass of lead for a given mass of light matter waa
rigidly determined, and thus by comparing the attraction of the
mass of lead for the light body with that of the earth, the meaa
density of the latter was determined to be 6*6747 times that of
water.
61. The ascent of vapours and balloons into the air, like that of
light bodies, as corks, m water, is produced by the attraction of
gravitation. For this attraction being greater in proportion to the
quantity of matter, the denser bodies, as the atmospheric air or
water, are drawn forcibly downwards ; and those containing a less
quantity of matter in a given bulk, as the balloon in the former
case, and cork or wood in the latter, are forced
•^'^^ to rise by the denser fluid bodies sinking
beneath them.
Let the vessel ▲ be filled with water, and a
solid body, as b, be placed in it ; both the fluid
and the bod^ b will be attracted by the earth.
If B be heavier than an equal bulk of water, it
will be more attracted by the earth than the
fluid it displaces, and will sink : but if it be
less heavy than an equal bulk of water, the
fluid will obey the preponderating fravitative
attraction of the earth, and b will oe forced to nse to the
surface. Thus the floating of light bodies in fluids of every de-
scription, is a direct and legitimate consequence of the law of
gravitation.
62. The spheroidal form of our earth, and of the planets of oar
system, appears also to result from this law. For as attraction is
equal at equal distances, and virtually emanates from the centres
of the masses, we may conclude, that the earth, when in a fluid or
semi-fluid state, must necessarily have assumed the spherical form ;
• ••rUL Trans." 17B8,p. 409. t ** Mem. Astrononioal Boo." vol xiv.
OBAYTTATIOV. 87
becaofle no^figore bas every part of the line bounding its penpbery
eqnidistuit* ttom the oenfane, except a circle : which would have
been the exact figure of a meridian section of the earth, if the cen-
trifi^^ force arising from its rapid rotation had not interfered, by
opposing gravitation in the equatorial regions.
63. As weight is an acquired property of matter, and prodaced
by an attractive force r58) emanating from the centre of our earth,
hot dinunishing as tne distance from that point increases; it
follows that a mass of matter would not appear so heavy on the
top of a lofty mountain as on the earth's surface, because it will
be there further removed from the centre of the earth. And ao
cordingly it is found that a mass of lead weighing 1000 pounds at
the level of the sea, loses two pounds of its weignt on being ele>
vated four miles above the surface : and if carried to the surtace of
the moon, and thus removed 240,000 miles from the earth, the
attraction of the latter for it would not exceed five ounces.
For this reason, bodies weigh heavier near the poles than at the
equator, on account of the former being nearer the centre of the
earth than the latter ; and if it were possible to place any body in
a cavity at the centre of the earth, it would be equally attracted on
aU sides, and consequently remain suspended w space, like the
fabled coffin of Mahomet.
64. It may here be mentioned, although the scope of this
treatise precludes a rigorous demonstration of the fact, that be-
neath the earth's surface the force of gravity varies, not inversely
as the square, but directly as the distance from the centre, and con-
B<H]aently ceases to exist at the centre. This is owing to the fact
that at any given point the superincambent spheroidal shell exerts
no attraction, in consequence of the attractions of all its compo-
nent particles being mutually balanced.
88
CHAPTER ni.
BTATIGBy OB THS HBCHAHIGAL BBLATIONB OF BODIES AT BB8T.
65. The science of mechanicB treats of the effects of physical
force on matter. If a force is counteracted or opposed in such a
manner that no motion ensues, the idea of , its existence is best
conveyed by the term pressure. Thus, if a weight be placed on
the extended hand, and sustained by it, we are conscious of the
existence of the force of gravity by the pressure on the hand, and
the muscular effort necessary to counteract that force : or if the
hand be placed between a heavy weight and a table, we then re-
cognise tne existence of gravitation oy the pressure alone : bnt,
when the hand is passive, the table appears to press against the
under surface of it, just as much as the weight does on the upper
surface ; and if the hand be removed, the doumward pressure of
the weight is sustained by the upward pressure of the table.
We may hence perceive the propriety of dividing mechanics into
two distinct hranches :
I. 8taiic8f which treata of the relations that must exist between
two or more pressures applied to a point or body, in order that no
motion may ensue ; and
IL Dynamics^ which treats of the relation between forces, which,
when acting on a point or mass, put it in motion ; and of the
nature and direction of the motion produced.
The former division, Statics, mil form the subject of the present
chapter.
66. When two equal pressures act in precisely opposite direc-
tions, as in the case oi the weight and table above mentioned,
thev are said to be in equilibrium : and the effect of the j)re8sure
is toe same at whatever point in the line of its direction it is ap-
nlied : thus the weight on the table might be supported by a string
from the ceiling, in which case the upward pressure of the table
would be transferred to the hook from which the weight is spa-
pended ; or it might be sustained by a vertical rigid rod, bv which
the pressure would bo transferred to the floor, or to the earth beneath
it, and the transferred pressure would be precisely the same in
amount, neglecting the weight of the rod and string respectively.
67. In order to enable us to estimate the amount of pressures, it
is necessary to employ some unit or standard of comparison ; we
cannot, for example, compare 1 hour with 12., or either with one
cubic foot. In this country, the unit of pressure is the weight of
SESULTijrr OP pbesbubbb apfusd to a poikt.
39
Fig. 39.
23*815 cdbio inchea of distilled water, weighed in air at the tem-
pentnre of 62** F^ the height of the harometer being 30 inches;
this weight is called 1 ponnd iroj, which is divided into 5760
grains, and 7000 grains make one pound (lb.) avoirdopoiB. ^
68. When two or more piessnreB are represented by lines or
munbers, it is meant that they bear the same proportion to each
other that the lines or nnmberB do ; and lines taken in the direc-
tion of any pressores, and proportional to them in lencth, are said
to represent them in magnitude and direction. If tne piessnres
cannot be represented by finite numbers, as» for example, the side
acd diagpnai of a square, tbey are said to be tnoommenmraibie,
69. When a system of pressures is in equilibriont, any nunber
of them maybe removed and replaced by a single pressure, called
the resultant of those it replaces ; of this the pressures replsced are
oaUed the eomponents, and the act of replacing them, eonqxmtion.
Similaiiy any single pressure may be removed and replaced by
sny nmnber of pressures, which would
jointly produce the same effect; the
pressore replaced is said to be reeohiedf
and the act of replacing it is called
nadhUum.
70. The resultant of two pressures ap-
plied to the same noint is represented
m magnitude and direction by the dia-
gonal of the parallelogram, of which the
s4i*oettt sides represent the pressures
in magnitude ana direction, llie truth
of Uiis proposition may be thus shown
by experiment. In the annexed figure,
A and B are two pulleys running on
pins in a vertical board ; o and i>, two
weights suspended by a string passing over them ; from any point,
I, of the strmg another weight, p, less than the suni of o and n, is
suspended ; b o, b h, s c, are three wooden rods jointed together at
I, and Ko and kh are two similar rods jointed to a clamp sliding
on BK, and connected by sliding clamps wiUi bg and Bfl. The
rods are all marked in inches measured fiom the paints of connec-
tion. Let us suppose the weights, a x>, p, to be 3, 4, 5 oa. reneo-
tively ; then take b o, b h, any lenjg^ths proportional to 8 and i, as
6 sad 8 inches, and xnake kh, k o equal to b o, b h, respeotiv^y ;
then BOKH is a parallelogram. Kow let the slidinj; piece, k, oe
moved im or down until the anfle o bh coincides with the angle
ABi^ which the string assumea when the weights were left free.
It wUl be found that the dia^nal of the parallelogram b k is in a
vertical position, and that its length is ten inches ; it therefore
besrs the same numerical ratio to the weight p, that the sides b o,
b B, do to the weights, c, n, respectively. Now the weight p would
evidently be supported by an equal pressure acting vertically up-
wards &t the point ■, which will be Tepraeetited hj s i, u 0 und n
r«preMiita in magnitude and dirsc-
tion the leenltant of two preHure&
which are themselTeB repreeeDtH
in magnitude and directian by the
±xat aidea, ta, an. If, then,
required to 6nd tbe reniltant
be obtained bj completing tha
parallelognun a b, of which the dia-
Ithe diagooal * n is the resnituit
of A B And 1 0 : then complsU the
panllslogram d k, and 4 F is the
resDltaBt of 1 D and A ■, that is,
of A B, A c, and A B, and so on.
Faan the comtniclioa it ii clearlf immaterial whether a b, a c^
A B, &c., ai« all in the same pUne or not.
72. It ibllowB from the preceding pnipoaition that a'point will ho
kept Bt rest, if acted on b/ three preuures, which are repi«Beiit«d
in magnitode aad direction b; the three aidea of a triande taken m
order, ij. is the aame direction, a> Aa ob, b a, in the anneied
diagiBm (ilg. 4S]. For if we complete the parallelogram ■ d, tbs
sideOD is equBlacd parallel to b a, and will therefore represent the
ot the joint eSect of o b
and 0 D, which is represented bjr
c A (70), it will therefore connter-
act the nniled eflecta of cb and
B A, and, if acting with them, will
keep the point oiaclion at rest If
the aides of a triangle are retpeo-
I tions of three preasnrea which keep
a point at i«tt, thej will repreaeat
the preasnrea in magnitnde, for if tbe three sides of one triaogla
are rsspectivelv peipendicnlBr to the thiM side* of another, the
triangles are amiitar to each other.
78. Bj an eiteasion of rimilu' reasoning, it may be ihown that
a pdnt will be kept at rest bj anj nninDer of prsssares rspre>
MDted in Du^itDd« uid du«ction b^ the aim of ft pnlj'cr
h U inunstenkl whether thi niiltw of the polygon Bra hi u
fbmt or not. Tha tmth of this
ni>7 be thus ihomi bj sxnri- ^V* ^■
nwnt. On * nrtickl bmrd draw
>nT polygpn, u i D, of vhich the
■idea are in any ■smmied nome-
rical proportion, aa 3, 6, 8, ID, and
7 inchea reepecdvelj; insert a
pin at linj point, o, and place a
tr it; attach polleyB
ilhatH&eapaadng
.. ._.» the point OEoaj
be pnaHel to the nqwctiv* ridee
of the paljgan, and in the aaoM
dinctiai ftom o, aa the lidee
taitm M order (bdicated b; the
AB,DitOBC,OCtOCD,OdtaDB,
ud 0 a to K A. Let aa manj atricgi be hooked on to the ring
at 0, ukI TeighCa attached to them, ptoportional to tha aidee to
the.T VB mpectirelf ouallel, aa S, 6, 6, 10, and 7 ooncea,
he ivnun^ from o, a
rlghla
weigh^ aa one ounce lor initance, tbe ring «
The mn maj dov he ivnun^ from o, atid the
"itifi'
But il either (rf the wei^hta be incraawd or diminiBhod by
•mall weight, ai
with nlScient
ima peaitioa, that ia, provided the polleje more
jitae. Let the angle a of the
polygvo be not in the aame plane
ae Ibee otbera, a, o, d, then draw
. ._p „_ JOD, i%. 44,
andjon d^bb; then the pres- J
me B A i« eqninlent to b ■, ■ a
pi); and ad is eqmTalmt to A ■, id; tharafoie ba, ad, which
are together eqniTalent to b K, B A, A ^ ■ D, are eqniTalaot to B ■,
■ D, bacaaae ABandiA neatriJize each other.
14 'Die method bj which coal ia raited Dot of the hold of the
cdlien ia a practical illaitmtion of the reenltant of «e*eral prea-
nrei in difiereat pUnea. Sereral email ropea are attached to the
end of a laiger one, which paaaea orer a pulley placed overhead,
ud ie then carried down to the coal-basket. Each email rope ii
beU b; one ohui, and all jumping down nmoltaneonaly from a
ttiKd step, the loaded baeket ie laiaed by a jerk ; hence the term
" ooal-whipper.''
75. Hie raenhant of two eqnal praeeorea acdng in parallel
a
tt
42 9TAT1GB.
directions, is a pressure equal to their snm, acting at the middle
point between them : thas, when the two sides m a balance are
equally loaded, the pressure of the beam on its support is its own
weight together with the sum of the weights in the scales. This
proposition, the truth of which is almost self-eTident, may be thus
illnstrated by experiment. Take a small rod, ▲ b, similarly shaped
J. ^ at both ends, and to its mid-
^' die point c, attach a string,
which, passing over a pulley,
D, supports a weight, k, which
=f====^="s«^^o just sustains the rod. If
1 D now two equal weights be
2' suspended from the eztremi-
Qf ties of the equal arms, and a
weight, F, equal to their sum,
be attached to s, the whole will remain in eouuibrium : for the
equal weights, ▲, b, being suspended from equal arms, cannot have
any tendency to preponderate on one side more than on the other ;
and the two weights, ▲, b, will have jnst the same e£R3ct in sup-
porting F, as they would have if suspended from c, the middle
_ point between them.
^' *•• 76. We may hence de-
duoe experimentaUy the
-resultant of two unequal
parallel pressures, and the
point of its application.
Let ▲ B be a balanoed bar
tumine on a pin passing
through the centre o, the
upper edge of which is
straight, and in a line with
the centre of motion ; and
let the edge be notched at
equal distances (of one inch,
for example) finom the cen-
tre, as indicated by the figures; also let egual weights (as of one
ounce each) be suspended Irom the odd diTisions i, •, &c. t, t, &c.
It is clear that each pair of weights, i, t: s, t, &c., being equal
and equidistant from the centre, will balance each other, and the
whole will remain in equilibrium. If now the weights suspended
from the points 6 and 7 be both suspended from 6, the middle
point between them, their weights will have the same effect on the
oar as before (75), and the equilibrium will not be disturbed.
Similarly the weights at t and t may be suspended from t with
tiie same recnilt, and the pairs of weiguts i and t, t and r, may be
successively collected at the same point, without disturbing the
equilibrium. We shall dow find that we have a ^ight, 6, acting
at a distance, 2, from the point of support balancing a weight, 2,
irmrn
Mctii^ at k dittaDce, 6; but
And benee ■we nuj inler gcnerkUj th»t >n j two panllel pre vam
vilJ balance each other, when tbej ve luTenelj prupoitioiul to
ibeir dulancea from the pmiit of giijipoit.
77. The nwmcnt of on; prewnre ■■ it* teadeDc; to more i bodj
to which it ii applied round any given ceotre, aai is meMur«d by
the prodnct of the prestuTe, ftnd x parpeadicattu' line dnwn from
the centre of motion to the line o{ directioii of the pTMsnie. It
■ppnmiB, fnm the preceding propoeitioa, that wbeo two premuM
an in eqmlibriiim, iheir momenta roond the cealre of motion, nr
wuDt of rapport, ve eqiul ; uid they «re alao in opponte direction*,
for each of the weights ainglj wonld turn tbo rod in adiraalioti
tmitztaj to the other.
78. And if any amnber of preaurea in th> eame plane, tending
to prodoce motion roond a
given point, are in e^oili-
brinm, then iho ram of the
monMnta which act in one
Erection ii eqoal to the ram
of tboae acting in Ibe con-
ttij directiou, •« ma; be
■hown by the following ei-
periment :— * a ii a circle
<rf wood tnming tm a jriii
in iti centre o; c„ c„ ftc..
an an; point* on it» inr-
&ee to which atringi are
attached ; tbeae pening over
the poUers F|, Ff, Ac., rai-
tain the weight* w,, w„
fto. If the board be allowed
te End ite own poeition of eqnilibrinm, and perpandicoJari o D ha
drawn from the centre o to the lines l-o, or po produced, aa the
caaa maj he, wb abaU find that the giun of the prodoola of the
weiriila and correapondiug porpendicnlara, o d, which tend to tnra
the board one waj, U equal to the aura of the limilHr product of
ttia TO^ita acting the oontrarj way. In ^e above fignre the
hiwaL^ ""™ '^^^ we^hla i« marked by arrowi, and wa
CUT to applj a ij^tem of parallfll preBsures to a rigid bodj, >nd to
find eipeniuenbUlj the position of the reBiillaiit. For this par-
Applj & wrieB of weights r,. P^ Ac, the itringi of thoM which «
intend to act upwards, passing Terticallj OTer pulleji. We ahau
find that there u some poiot at which a siDgte presBUTe majr bo
applied, which will maintain the rod in its honiontal poaitioD,
nnless the sum of the weights actins upwards sbonld equal the
■nm of those acting downwaida, in which case the lod will be ma-
tained without furUier rapport. But if them sunn of the weights
■IS not equal, the resnhant will act in the diivctiou of the greater
sum, and the single premire, to counteract it, must conwqnentlj
be in the oontraiy direction, and it will be eqoal in amoont to the
difference of the above sums of weights. In the example here
given, the weights p„ p„
^- *>■ Ty are 8, 3, and 6 oonoea
reapectiTelj, and are sus-
pended at diatanoei of 1,
I 12, and 17 inches from th«
end a; and the weights
F„ F„ are 2 and 5 onnces,
acting upwards, at dia-
tsnces of 3 and 14 inchea
from A. W« shall find that
the whole sjitem will be
Bnstained hj a support
placed under the diiision o, distant B inches from a, and the dis-
tances of op„ op- tc„ will be 7, 6, 4, fl, and 9 inches respectJTely:
and the sum of the poiilive momenls round the point □, or those
which tend to turn the tjstem tht sans looy a* the Aondi of a
dodc nuws, wiU be equal to the son of the negative moments, or
those which tend to turn the sTStam the eoMraty maj, for
(p^!x6 + W8x4 + (p^6>c9-(p,)8x7+(pJ6x6.
Btit the snm oT the weights acting downwards is
(p,)8 + (pJ8 + Cp.)B-17;
and the sum of thoM acting upwards is
(FjS + (p,>fi-7,
and conaeqnentlj the difierence of these >unu is
17—7 = 10;
that is, the resnltant oftbe sjrstem of pressures is a presinreof 10
acting downwaida at o; and if we attach a wei^tof lOoancM to
■qmLiBum o
I, Thsic is k puticalu cms of tbe i
eqtialit7 of oppoaite monieiits, tl»t
lequireB notice, on accooot of it* prac- ,
ticBl importiuice. If equal weighU, r,
be raapeiided from anj tbrse poinU, i
A, B, c, Bqniilistaiit Ktim each other ind
fiinn the oentre of tha moveable boaid,
thej ir!ll be in eqnilibriom in anj pod-
Let the diagram lepretent anj pom-
titm, joio B c, and bisect it in i> ; join
A D, and producs it to meet the Terbcal
Koe throngh b in ■ ; aim produce the
vertical throngh c to meet as in p;
then, because a b c ia an equilateral
trian^. A B is peipendicalai' to a o, and
fHiri Ihrnnrii n; also A o ia twice o d. Bnt bjniiaiUrtiiaDglaa,
V i>— D B, and therefore
and tlis momenta ronnd o in oppoeite directjooa are eqaaL
TUa eiplaina the advantage of emplojing three pmnpa, the
piltooi of which are attached to a three-throw crank, m all kioda
<if pnioMng machineiy, in which the crank-aiia ii driven round hj
•a anifonii foree, becaurn the Bom of the momeuta of ^ acting
nutancea ia the aama for all poeitiona of the crank.
■ If wfl eall tha npwird piuaum po^ttre, md ttia dcwnwmrd Dflgative^
" " ~B-hS-S+B-8— JO,
-B+I-HO-S+S-B^Ol
tl«tia,iatlieiHaor wndHbriBm, tha olgtirmlaal mm of th» pnmatf = 0,
fiat eonditiaii nwj be eipnwMl fama]^ by
-P,.O|ii-(-P»Op,-hP^Op,-Pt.0l.,+Pj.0l>,=0,
(fat ^ tb* o^itntaiJ HiHi cf tlie mmaiti roDBd O ^0, wUsli i> aiFrtaaaa br
HP.O,)-0.
niH ma Ibe nnoil «iiidltlinHofM|idUbriiiaiofa>iiv*r*t«ia of panUel
H—IIW ■iillin iiimiililliiiilj ir iinjlii iilmiiiiiil Mill lliii liiiiaiiiiialliiii
■■d aiiiiUwi, M eppliai] lo Uaa, mtnij busb npodte dliMiiiliHia i llaM
Moc eouldarcd niaiiliTe, irtuoli aie diavn is ■ duwtion oDntrar to tbu
pliiiTi ■iiMwiilie til ni nndtJTn Prom left to rifht, np«fd>» aad lowerda
a. m naaallj emiiiinl ■• th* poiltiv* XnaUoat g ud lh>m rifbt to lift,
Aoinaard^ tad UneUr baa lu, tht BafUJn.
81. Tlura ii a p*rticnbr cms of panllel prnmres in which no
reiultoDt can b« obtained, that it, when tb« ajatem of prenorea
maj be radncfd to two equal preuuraa acting ia opponte direo-
_. tiona, but not at tbe rame point, which
'•"^ majbelbmelnciJalad;— Lelp, p^ba
■ an; two preaiuren acting at the pomta
I Pi-Pf '" '''B same direction, and let
I p, xon, ^ rjXO^,; then o istbepoint
I to which the ntBulunt mult be applied,
I and a pressure p, -f r, applied at o in
I a direction conlmrjr to p„ and F,
■ will produce equilibrium (TG). Sun
I Fi " "Pi = ■•. " <??!
add to each of theie qoaotltieB f, x op,,
then i;xop, + e,xopi = r,xop, + r,Kop,,
or ^r,*f^'>Fl = f,''P>Pt■.
that ii, p, ii the pirint about wbicb the momenta of p, + p, and p,
are equal, and at which, conaeqaontl;, their reaultant mutt be ap-
plied : also, IB the 87Btem ia in equihbnuin, the remltant must be
equal in magnitude to p„ and mnat act in a contrary direetion to
p., that ia, in the direction of Pi + p^ We may, therefbre, remark,
that if two unequal parallel preamrea act at any two points in ap-
posite dlrectjona, ihotr resultant ia a preaanre e<|ual to their difier-
ence, Rcting net at an; point between the giren pointa, but at
wroe p int of the line joining them, produced in the direction of
tb« gireater preasure. But when p, ia diminiihed in vaiae, r, -l- p,
and p. become more nearly equal to each other, and at the aame
time iha point p, becomea more remote, becauae the product
P, X op, remaiua conatantly equal to (he aame quantilr P, x op^
_ .. and when f, becomea indeSnitely amall, and Op,
indefinitely large, the preasures at o and p, ap.
[ proach equalitv. We may henca conclude that
' - equal pi '" ' -• ■ -
I that is to say, there ia no point at which their
ol parallel prenures acting in oppoail
Itant,
I' ■■ .
B joint action can be Juatcounleracted by aaingle
I preaauic. Two auch preaeures as f, p, acting
■ Ht the points AB(Fig.51],arecalledaeoiit)^;
I A c, toe perpendicnjar diatance between their
I liaea of direction, the arm of tht onipb; and
™ p x * c, the moment of the eoupU.
82. As the IKO pressures cnnslituting a coaple are equal and in
-^ — - -_ - * familiarly illna-
ODlrifugol drill (Fig. 62), in which a rotatory impulse
ia oiimmDnicAted to the weight a, by forcibly unwinding the
strings &om the alem, a ; also spinning a lop, and trundling a mop,
Bat the b«(t practical flliiitra-
rf ooA or wcxJd, kept ir - "
. _ ._. ._, ■ loantuD jet ptajing against o
of it, bat witbont anj ebange of place, so loag aa the fore*
of lbs jat TeiDaiDB perfectly onifonn (Fig. 68). In this oaae,
the fwce of the jet acting opvaidi agwiut tlu loHhae of the
kail b exact]; eqnal to the fbrae of gntn^ ar'' — ■" ' — '
rft.a.
t
S3. It may be tlated geaerallj, that aaj namber of prenmreB
tctinf; in any direction, al the tame petal, maj atwaTS be reduced
to a nngle leanltant, which vil1= 0, in caae of eqmlibriimi ; but
if acting at diftrenl points, tbsj maj be reduced to a nngle
RfilUnt, and a rSBollant couple, either or both of which ina/ = U.
Ad eipenmebtal demonBtratioD of this wnnld, hovever, ho too
fomplieated to be raadily intelligible, and a mathematjoal proof ii
incompatible with the objects of this trsaliK.
84. Han; problenu relating to the coDdltiooa of eqailibrinm of
bodice, Tanouil; connected and supported, are of laat importance
in the arts of ooDBtmctioD ; and occaaiuDally verv curioui and unex-
pected reBaitBareobtained. Thus, for instance, U we insh to obtain
tlie but poaition in which any number of beams can be piactHi,
n s« to form a roof of a given epan, that it may be uniformly Btrong
in fitrj part, we have only to join loosely together an equal
iDmber of rods of unifurm . weight and proportional length, and
mapend their two free amis y^ ^^
fjoto two pointe at a pre-
pmioDal distance in the I
ume bonzonta! line, when I
it wit] be foand that the I
form which b;^ their own I
wei^t Ihey will spontane- I
ontly •nume, caTled thr I
finicolar polj^on, is, when
iavetted, lbt> strangeit fonn di which the beaioB can be placed
49 STATICB.
order to nipiMrt k weiglit nnifbmily digtributed orar tbem.
Agun, the rorm of the gracefiil calen&r; curve, that whicb, u its
^, ((, name impIieB, a chain umnies
when framj' auspcDded from two
points, is an impartatit problem,
as being the bans of the con-
atmction of sospensicD bridges ;
bnt for the foil inveetigation of
this Bobject OUT readers must be
refeired to the standard treatisea
86. Tbecentreof gravity of anybody, or mass of matter, is that
ptnnt about which the bodf will be balanced in all positioDB. This
point eridently coincidee with the centre of parallel preseares
(79), erery particle of the body being considered as a poiot sepa-
rately act«d on by grsvity. And as this is the point of apphca-
tioa of the resultant, that is, of a single pressnre Eaving the same
eSect 88 the individual pressures co^jomtly, it followe that the
veisht of the body would in ell cases have preciael; the same
Blalical effect, if it were all collected or concentrated at the centre
of gravity.
j^^ gg_ SS. To End the centre ef gra-
Ivitv of any body, let it be con-
sidered as a syetem or aaaemblage
of material points, and of theee taE«
any two, f,, p.; also lake any point
o, draw o p, horimntal, and there-
fore at right Angles to the verticals
throogh p, aod r,; let o, be the
centre of ^vitv of p, aod p, and
o, g, a vertical throagb o,,
then (76) 'i^Pi9i = ''t'^PiSi't
biitj',y, = oji, — oP|, andp,y, =op,— o j„ therefore
"■j xoy,-P,xoj), = p,xoj.,-p,xoff„
or ?,>toy,+p,Koj,-p,jcop,-hP,xoft,
«■ ('•. + '-,)'>ft = Pi'<op, + p,xop„ , (a).
by an cxtBEBion of precisely similar r« ^ ^
other point p^ and finding s, then «, and p, and k
find that
" p, + p, + p, +4c. '
liot OffU the diatance of a ftnia o^, a rerlical tbniugh o ; va3 tha
^■tatioe of a from op may he dslenaiaed in a limilar mKoner;
and kence the position of o, the centre of graritj of the ijitem,
tD*7 be foond. When the point; r,, p„ &c,, are not all in the eame
plane, we find the diatance of a from a horizontal plane, then finm
any vertical plane, and then from another Tertical plane, perpen-
dicular to the other two, h^ which means the poeitioa of the point
e in apace may be detennmed.
On refemne to the eqmition nurted (a) we may obserre that
one dde of the eqnatioa eipreaaea the niamenC round o of the
whole ayttem collected at a, and the other side the sum of the
momenta of all the aeparat« particles round the aame point ; hence
we lee tbat, in this inatonce, the statical effect of the whole sjstem,
ciJkcted at its centre of giavi^, ia equal to the aggregate eOecta
ol the Bcreral porta of the ayatem.
87. The centre of graritj of a material straight line aa, for
example, a atraight noiforrQ rod of any heavy matter, must evi-
dently be ita middle point; for, aa in Fig. 48, it may be conceired
to be dirided into any even nnmber of equal portiona, each pair of
wbicli, being equidistant from the centre, will balance each other.
88. The centre of gmrity of any material plane aurface, oa, for in-
(tanoe.slamiriaof metal, of nni- v,. ...
form denmty and inconaiderabie .
thickness, may be readSy deter- J
mined geometrically, if we can |
find two)inet,eachofwbii:ii will I
divide the fignre into two equal I
halrea, for the centre of gravity I
win bo the point of interaec" -
of theae two lines. Thns, if
take the parallelc^ism, a c (Fig. 57), and bisect ih« aides in the
points B, F, B, K, and Join a H, f k, tben □, tbe point of inl«r-
section of eh and Ft, will be the centre of gravity of (ha
parallologram. For the parallalogram may be divided into
narrow portioDn by lines, h e, b, e^, parallel to one of its sidea, n c ;
each of these narrow nortiona may be considered aa rods, of
each of which the middle point will be tbe centre of gravity.
Bat the middle points of all the roda are in the line fk, and
therefore the centre of gravity of the whole must he in tbat line;
and for n'milar reosona the centre of gravity must be in the liaa
K H, and therefore at the ptniit a.
m from which to tba langug* of Ui» diffemti
in the trianglB 1 B c, the Bides AC, CB,
te biMctetl in d, b, uid lines drawn
I right lines, the centre of gniTitj msf
be Tonnd hj dividing it into triutglea,
and finding the centre of gnvitj of
each. Tfana,UtABCDEbethefignra
in qnestion, divide it into the trianglea
A B c, A o D, and a D l, lind the centre of gravity of each in the
manner already described (88). Let a, S, e, be the ceatres. Then
join a b, and divide this line at d, in suuh a manner that the part
j_ p, (Jftwill bear the mme ratio ton 4 as the
* _ triangle a b c does lo the triangle a B c.
■ The paint d vill thus be the centre of
I gravitj of thi> figure A d c d. In like
I maonerpoin ft.with <; by the line c«<f
I and divide this at e, in such a manner
I that ce vill bear the eame propoTtiati to
I ed, that the quodrilatenJ figure a B o d,
I does to the triangle A l> ■ ; then e will
I be the centre of gravity of the whde
In a circle the centra of gravity is in
its geometric centre; and in an oval, at that point where its trana-
terae and longitudinal diametere intersect.
M. If a body be freely eaapended by any point, it will remun
at rent when a perpendicular line let fall
fnim that point paosea through its centre
of t^ravity. becaaae the upward preuure
which gupporte the body most be in the
same vertical line as the resultant of its
Ac a^regate grvvity. This law afiorda a
ready mode of determining the centre of
gravity of any bodyby experiment. For let
A C a D, Fig. 60, lie an irregnlarly.Bhaped
body, OS a board, &«ely aiuipenaed at a,
a plumb-line, abb, hanging on the same
support. The attraction or the earth will
cauHO the line a n to hang pcrpendicnlariy
downwards, and, acting ouacbd, will
"■ -TB of gravity to fall
BqtUUBUDH.
o of letl in another directioD, itill bsvins
the centre of ennt; ia Ihe conn« of the rgrticaJ line described
b; the plumb-line : let this line be a ■>, then the point o, where
A D anit ct D intermct each other, correepondi vitb the ceDli« of
graTJtj of the figure a b b d.
91. The centre of ^ritjiibj no menni Ko- «.
neceaurily placed within Ihe mosa of the ■
faodj itself; in a nag for example, aa i b, I
thia point will be the centre, c, and cod- I
■eqnentl; in the space midway fima eTei^ I
portioD of the solid. ■
92. If a body be not of nnifonn dendt;, I
the centre of gruTitj ia not sitoaCed in ihe I
places abore described. In a hom^^neona |
circnUi Spin it correaponda. as above
stated, with the Reometno cei
in difierent pula, it becomei eccen-
tric. Let 4 B c Kg. 6S, be^ an in-
clined plane and ■ bmij of cylindrical
bnt in one of unei|ual dunalty
figure i> s a, be placed upon ;
be composed of matter of eqnal den-
iitv, o will be the centre of grarity,
and being attracted hy the earth m
the direction na, falling below the
point aapported b j the plane, the bodj
will necesaarilj roll down.
Bat if the portion a of the figure
le composed of
i leuC the n
, __ .. C the remaining portion
bring [rf* tight wood, as alder, then a
bulk of the latter weighing 800 grains, will correspond in iiie to
a mass of the former weighing 11,360 graios: these nnmbcrs
being in the ratio of the reapectiTo specific gravities, or densitiea.
of the two bodiea. The attraction of the earth will now act Tecy
differantly on the cinmlar figure n, for the centre of gravity will do
longer be at the geometric centre, bnt at a point nearer o, as at a.
OraritatioD will act on t in the direction » t, cansing it to asanme
tike loweet point, Ihe noint s will obev Ihia attraction, and the
aicolar figure d will roll ap the inclined plane ^ remaining at rest
when a line let fall from the centre of grUTity B passes through
the point lopported by the ^lane.
93. A body reating on its boae cannot remain in a state of
permanent eqnilibriDm, nnless a perpendicular line, passing
through the centre of giaTitj, &11s within the base. Thus in the
figme a B, f^g. 63, s represents the centre of grsiity, and a line
falling ftom that point pM«™ through the b«o, wh^h » mpported
bTtlwt.ble: the figura tlierefore 8Und« wfelj. But pUce on it»
wmmit wither piec«, o n, tha oeutre of giavty T ' ^,»?f** *J
o ud u a perpeodiculir hne dr.wn from that point falli bejond
' tha supported base, the body necesisnly
Fit. in. falls. And the same effect w!ll_ be pro-
duced, if B portion of the material, as r,
ba removed near the base. Hence the
dangsr of loading waggons too high, and
of building walla, if necesaaril; inclined,
too loftv ; the leaiun^ tocera of Pi»»
and BoSogna, may, accident a^rt, stand
for ever, as long as perpendicular lines
drawn from their centrea of grarity Ml
vithin the bases of the buildioga. Thii
is indeed the case with both these re-
markable structures, for the tower of
Pisa ii S15 feet higb, and has an incli-
nation of ia-4 feet from Iha perpendicalar ; and that of Bologna,
with an elevation of 134 feet, baa an inclination of but 92 feet.
91 A body, not acted upon by any external fbrcea except
gravitation, will be in » state of tqiiiJibrium when ita centre of
iravity ia aupported. But the equilibnum may ba of cither of
three diffbrent kinds, vii., ttabU, umtabU, or lAdiffertnt. A body
ia «id lo he in a aUte of stable equilibrium, whenever its cenlra
of Rravity occupies a loiwr pooLion than it would do il Ihe Iwdy
were moved a little in either direoUon; for as the toUl weip;ht
would act in the same mwmer if collected at ita centre of gijvity,
that point would tend to descend if the body were moved, that is,
lo return to ita fonner position. And the amount of this tendency
to return, or the itabauy of the body, is measured by the amount
of a$ctnt of the centre of grtvity correapondipg to a given move-
ment of the body. . - L ,1 IS
Let J. B, Pig. G*, he the base on which the preceding fignre
rests, am) a its centre of gravity ; then if
Fig.t*. the body be tilled over towards A or B, the
I centre of gravity will describe the circular
ansae or UD, of which a is the loweat
point. Take the angle O i C ■— O B t>,
ihrongh o draw the vertical line o B, and
draw c E and D F horizontal, then o ■,
a r, will represent the relative elevationt
of the centre of gravity when the body
ia moved round the points ± b, respec-
tively : hence we see that the stalnlity
of tbe body will be much greater towanb
A than towards B, or, in other words, it will be more difBcult to
ovenum tha body in tbe fonner direction than in the Utter.
Hence tlie atilit; of eitendiiig the baie of a bnildiDg bj iiie4nB
of bottreneB, eapecisU; vhen the walla ore luliject lo the lateral
(hraat of a roof of wide ipan, aa in catbedraU ami otIieT loft;
edifieeB kavinff the iDterior apace entirely open.
9&. The Blabilitj of a ntapencled bod?, ami conaeijaently the re-
aiitanca it oppoaea to diatorbaace of itseqailibrium, increaaeB with
the distance of its cenire of gravity belov tht poiat of aopport.
Hence in the coaBtnictionof very delicate baiancea, it ia necesaary
for the centre of graTity to be but joat below the point of anpport,
otherwise ao great a force would be required lo disturb the equili-
bfinm of the beam aa to render its indicationa in the eatimation of
unall weighta nearly naeleaa.
96. The eqailibriuin of a body is unstable, when the centre of
graiity occupies a higher position than it would do if the body
were displaced in any direction. In thia ease, the body will, if
inoTed a little, recede still further from its poaition of reat, since
the centre of gravity will then, by the auppoailion, descend. Thia
if the condition of equilibriam of all bodies balanced, or supporled
on a lingle point ; and the art of balancing any heavy body con-
sists in repeatedly shining the point of support, ao as to keep it
CtnitiDoally under the centra of gravity.
97. If when a body is moved, the centre of gravity neither rises
Dor fklls, bat moiea in a horizontal line, that body is in the cod-
dhioti of indifferent equiiibriuni, and when disturbed from its
poaition, it has no tendency either to ad- Rn, m.
vaoce or recede. The equilibrium of a
sphere or cylinder, resting on a horizontal
phne, is of thia kind, for it ia manifest thai
tn theae bodies the centra of gravity will
atWBya move horizontally, since all radii of
s circle sie equal.
98. Whenever the path of the centre o<
eratity of a moving body is a curved line,
Uier« wilt be a poaition of stable squili-
brinm at the lowest point of the curve,
where the convexity in downwards, an4 one
<£ unstable eqnilibnum at the highest, where
the convexity is upwards. Tons, if any
body, AB, of which s is the centre of
gnnty, is raapended fram o, and capable of moving round that
pant, the path of o will be a circle of which* o is the centre;
there will be a position of stable equilibnum at a, the lowest point of
the circle, and one of onstable cqailibrinm at o', the highest poiut.
99. This point may thus be further illustrated : — Take an oval
board, thick enough to stand edgewise, with a hole in the centra
Inge enongb to admit a pencil, and let this be railed along a
■trkieht e<^, A b, ll£. 66, resting on a sheet of paper ; a pencil
pawiK throDgh the Eok will trace the corie, c d, the path of the
centre of gravity. When tha grsftter oxia ii honHmtal, u at b,
» .. Iho oentra of gTsntTvill oc-
. cupjr tbe lowert point of Uie
I curre, auiI tUe oquilibrium ii
■table ; when the aune axis
U vertical, aa at v, tbe centre
I of gravity it bigheat, and the
equilibriiim it unalable.
100. Stable eqnilibiiniii in
one direction tdkj, nnder
I certain conditions, coexist
I with nnstable eqnilibriam in
another. For example, in
the preceding figure, there are points of inflexion between the
J. „ bigheat and loweat ptunta of Aa
curve, OD, or pointi at which the
cnriB changpB Ironi conveiitj to ODD-
:a*i^, or vice vtrt&; lets be a point
It which the oval reita on a a, when
ts centre ia at a point of inflection,
and draw a line through the centra
and the point o. If now a be railed
above n tiDtil the line through o ia
vertical, the oval will then be aup-
ported in thia poaition.
In thia caie the path of the centre
of gnvitj IB horiaontal juat at the
point of equilibrium, a ; it uceoda
towards o, and the equilibrium ii therefore itable in that direc-
tion, hat \» nnatable towarda l>, in which direction the patb
deacenda.
Kg.w. Again, if we plaoe the oral
I board on a amall horiiotital
ojlinder, in. Fig. 68, with it«
greater biib parnllel to the axi*
of tbe cylinder, tbe equilibrinm
will be atabla towarda a or b, bat
tmstahln tranaverael}' beyond a
very small diilance from ita
vertical position, as indicated
by tbe dotted tinea. _ These,
however, which may be called cues of mixed equilibrium, are
. altogether exceptional. Qenerally tbe equilibrium of a body will
be (ound to be wholly atable, unstable, or iodiflerent.
tOl. If nn portion of the path of tbe centre of gravicv tM bori-
tontal, there will be no poailion of equilibrium, and tbe body will
tend to move in the direolion tovraida which the path deacendi.
Tbe motion of a doohie cone, or of a billiard-ball, ^/pareMly
■QDILIBklTBD 1
up mn incUned ^aue, coniiatiD^ of two rodi pUced at a imall
a^s, witli the open ends nuad, maj be eipUined on this
pnndple.
102. The cona premnta a good ezunple of a body capable of
beinE placed ander all the three oonditiona of eqnihbnum. When
lUooiDg on ita base, *, Fig. 69, its eqoilibrium ii ilAble ; when
praaed on ita apex, it is nniuble, b ; and when retting on iti
BlaDt lide, c, the eqailibrium ii indifierent.
103. One of Ibe moat important practical applicationa of the
comditionfl of equilibrium la in the conitraction of the arch ; a
atmeture compoaed of heai; masaea in contact, which eupport
each other b; the matoal preaanre arising from their weight.
The aepsnte maaaea of which an arch is competed are culled
eouaaotra, which collectivelj conrtJtnle tha archnng, and this
leata on two firm anpparta, the abvtmttiU. The external and in-
ternal outlines of the iirch-ring are called the extradoi and inf ratio*.
The lateral smfoce of the arch is called afaee, which ia supposed
to be a vertical plane, nnleaa otherwise eipreased.
IW. We will, in the first place, oonaiderwhat muit be therela-
tire weight of the vonatoin, and the direction of their surfaces of
The line of pressure at each joint of an equilibrated arch maat be
parpeiidicnlar to the Joint. Suppose Iti.jo.
the line of pressure a b, nc. TO, not •
to be perpendicular to the joint CD. I
Take aec.bb and di«w At, ea I
peipcmdicnlar to CD; then the I
pnMore A ■ is equiTalent to a r and I
pa, and bb to bo and bb, of I
which a r and b o will support each I
other, and tha Touaaoin v., t^ being I
OTged in opposite directiona bj the I
preasnrea P a, • E, will slide over
each other, which is contrai7 to the suppodtion, therefore a b
mnst be perpendicular to a d.
105. Let T„ y„&c, FI5. 71, be Ibe vouisoire of the aemi-arch
AiCD, and r, q„ &c. the loint*. Draw o x vertical, and ivhori-
■ratal, and o t„ &c. pantllal to r, q,, &a. Then for any Toussoii
M T, the comiponiling trinngle T, o T, has its rides Derp«DdicnIar
TeBpectlvelj to tlie directioni of ihe three preuureB that keep the
ToiuBoir at rest, for the preunrei sC the joint are perpeDdicnur to
OT,, OT,, oad the we[ght acts Tsrticalljr, that it, perpendicularly
to T, T„ therefore o i„ o t^ and r, t, represent raipectivelj the
17. ti tireMQreB on the joiota P,«,,
and the weight of T, (72).
id as precisely the same
reasoning will apply to the roc-
cessive triangles t, ot^ &c.,
the presaiirei at the joints will
be repreeented by the linesoT,,
&c., to which the joints are
reBpectiTcly parallel, and the
weights of the ToaBSoirs v„ v^
&e., by xt„ T, T„ &c. Bnt
the lines zt,. it„ Sk., are tho
tangents of the angles zot,,
XOT^ &c., ox being the radiun ; hence it,, t, t,, &c, are Ihe
diSarenteB of the tangents nf these aiigles, that is, the weights
of the Toussoin are as the diftBrenoes of the tangents of the angles
which their respectite joints make with the vertical.
106, If the depth of the TDuasoirs be small and onironn, and the
joints perpendicular to the curve, Ihe curve of the eqnilibntted arch
will be a catenary. Let Ad be the cDrre
^- ^*' A D, inverted with refer«DCB to a horiioDta]
"ine, and let PQ,, r'Q', be two of the joints;
raw the verticid lines rp, t'p. If a li b« (he
catenaiy curve (84), it is evident that anj' por-
tion, ^^, which may be supposed to havA be-
come rigid, after having assumed its posilioa of
equilibrium, is snppnited b; its Own weight and
'ly two tensions in the direction of the cnrro
It the points pij/; but the portion p r' of tho
arch Is kept in equilibrium by its weight and
' 0 pressures perpendicular to the joints, that
, in the direction of the cum; the form of
(be arch being datemuned by the same con-
diticna, must therefore agree with the cale-
The funicular polygon (84) is a case of equilibrium inverted,
precisely similar to the preceding.
107. £mitibriuni of Jre&ei in praefiM. — We have hitherto
conaidered the condition nf equiltbrium of an arch, supposing the
surfaces of the voussoirs to oBer no resistance to shi^ng. But
the surfaces of building materials offer a considerable resistance
to sliding, and it is therefore necessary to consider ihe practical
Gooditions of equilibrium. For this purpose it i« desirable to take
■ thut ckn exhibit tbe rarioiu pr»-
pertiea of an arch, vix., one connating of lonr TOUMoira, which, for
tbe convenieDce of tiperimeDt, maj bt purtioDi of m rectangiiUr
b«am of wood.
Let B D B*, Fig. 73, bo the web J^- »■
the TOUBoin of which mt on th«
kbntnieDtB at a b, a' b', and against
••ch other at the oUique pianei
p 4, r' q', and tbe Terlical plaDp
s t>. Let Di Snt BDnpaw the beami
to be withimt weight, «nd tbe joiniB
witboDt friction, m order to deter-
mine tlie cooditioni of eqniMbriam
when tlte areb ii acted od b;
vcighta at the erov* e, and tbe
lunmcJUf q, q'. Dnw o x t ai in
Iig. 72, tboD ihe weight* at ■ and
« tout be at z T to T T <106). Let
unawsappose tbe weight at the crown tobeiiicreMed,uid let the
we^u at X and qbe aixDto dt; join o d aod draw r i per-
pemticular to o o, then ai the pmaBnra of the weight at s was
And
D q[ will ilide inwards at ihe
with friction, if tbe angle d r a be lew than the Unudng Migle of
TCBiteuce (S5) for the giren materiaii.
It is *lao rtKiainte that the line ■ r ihonld not fall be;oild the
Irtoal enrtiico of either of the iobts,
Bnif ao. tbe Tonaaoir DDwill lum
inimd tbe outer angle at i or the
burt at p, ai in Fig- 75-
In like manneT, let the weight be
mereafed at the haonchei, and let
tbe weight at « be to that at q aa
T.. vTT inin oTand drawQD per-
or. We ihaU now
find thtt the haonchm will alide inwsrdi u in Fis. TS, nnteu tbe
angls DQE (Fig. 75) b« ]«■■ tbaathe limitinfr mngle of reiialADoe.
fff_ 77, It is aim nece«BBr; that ihe lioe
I QD should not fall beyond thcaurfaoe
ofeither of tbe joints; forifitdoso,
then the Toowoir will turn round tbe
point D or Q, or both, u in Fig. 77.
By an eitenaion of praciialjr
nmilar reiBoning, tbe aame piin-
dplcB may be applisd to tba lereral
parts of an u^^b, «t actually ooo-
Btrncted, If ire luppose the arch to
be in equilibrium, tbe line of prewur* will be a poljgoa pautng
through tba centrot of gravity of tbe »ou«aoir«, as repreaented in
Fis. 78 by a dotted line.
If a veigbt b« now added at tbe ctdwd ■, the line of pr«imiro
ascends towards a, and descends towaids r at tbe liaunches ; or
Hf,K. '^ *^ '"''^ weigbU at tbe
haunches, the Une of pres-
sure ascends towaids <],
and descends towards d, aa
showD in tbe figure; in
either of these catei the
arch breaks, us in the pre-
ceding Sgores, whenever
tbe hoe of pressure fall*
' beyond tbe snrface of tba
joints. If tbe line of pres-
sure does not pass beyoad
either the eitradoi or Ihfi intrados, it may pass very near either
of these boundaries of the areh, as in the ngure : dius by over-
loading tbe orown there is a tendency to crush Ibe material in
the inside under the banncbes, and by overloading tbe baunchea,
there isa teodenc; to croab the keystone in tbe inside at the crown.
iOS. From what has pracaded, we may gather that the two con-
ditions essential to the stability of an arch are: —
(1.) That tbe lines of pressure pass within the limits of the sur-
faces which are in contact.
{!.> That the lines of pressure meet the joints in such diracliani
that tbe angle contained between each and tbe normal to the jirint
may be less than Ibe limiting angle of rasistauoe.
We have here considered only the structure of tbe direct areh
in which tbe face is perpendicular to the iqffit* or inner carved
surface of the areh. For the construction of tbe oblique areh the
reader must b« referred la tbe standaid treacle* on eagmeetiug.f
59
. CHAPTER IV.
OF THE MSCHJLHICAL F0WBB8, OB SIMPLE MACHHOBB.
109. The mechanical powers comprise the most simple instni-
ments that can be employed for the purpose of raising or snp^rt-
Ibg weights, or communicating motion to bodies ; and all machines,
howerer complicatod, with which the ingenuity of man has
{ormshed as, are nothing more than combinations of these simple
machines. By means of these it must not be supposed that we
beget or increase force ; all that we do, is to apply force in a con-
venient and economic manner. Thus, if a man could raise to a
certain height 200 pounds in one minute, with the utmost exertion
of his stren^h, no mechanism could enable him to raise 2000 to
the same height in the same space of time. If left to elevate the
mass by his own unaided strength, he would be obliged to divide
it into ten different portions, and raise each separately ; whereas,
bv means of one of these simple machines, he will be enabled to
raise the entire mass at once, requiring, however, for the perform-
ance of the task, ten times as long as ne required to raise the 200
pounds.
Thus it is, in Umne, obvious that we exchange time for ])ower
in using simple machines ; and this is true with all the varieties
of impanatoB to which that term has been applied.
Toe simple machines may be divided into three classes : —
1. The lever ; 2. the pulley ; 8. the inclined plane ;
the theoretical properties and peculiarities of which, with their
chief modifications, we shall now briefly describe. The screw and
the wed^, commonly classed amonest the mechanical powers, may
be considered as modifications of the inclined plane.
1. THE LEVEE.
110. The lerer, theoretically considered, is a ^rfectly straight
inflexible rod, destitute of weight, and moving without friction on
a point of support called Afidarwnt.
Precisely tne same line of argument, and the same apparatus
that has been already employed to demonstrate the relations of
parallel pressures ^7^, may here be employed to determine the
conditions of eqnifiorium of a straight lever, acted on by two pres-
sures perpendicular to the arms. We may here remark that the
theoretical consideration of the lever being without weight cannot
be fulfilled in practice ; but we may arrive at the same results, if
60 iracnAncAL rowsM, ob siaru machihu.
the pin on which the levnr tarns be mide to cmncide with the
cantre of grarity of Iho leier ; for the Mmtn. of gr»»lty beinir nip.
ported iQ all pomtiona of the lover, its weight cannot, in any poM-
t(on tend to prodnce any motiou. We shall then find, Bret, that
equal preiBurea applied per-
peadicnlarly to the equal
I armsofastraight lever, will
I keep the lever at reet ; and
secondly, that any two prea-
I luree applied perpendica-
larlj to the armi of a lerer,
I will keep it at real, if the
presBurea are inversely u
the lenD;th of the arms. If p and w are the preaaarea Dsnally
wiled the power and weight, in relation to the lever, and p, w,
their pointa of application, alao f the fulcrum, we shall have
nple given, the diitanoes o(p and ui from f are 3 and 7
i the weight! of r and w are 6 and 14 ooncei ; and
3yl4 = 43 = 6x7.
111. The pointa p and to
are not neceaaarily on oppo-
[ Bite Bidet of f : the only ii»-
ceaaary condition is that f
and w mnit tend to tnni the
' oiind T in contrary di-
IS, coDsequeatly if ther
I act both on the same side of
IP, mn»t»ct
„. 80.
<ly the caB«« in which the
power and weight act peTpandicQ.
larly on the anne of a atraight leror;
1 may now nroceed to the general
K, ia which tbey act in any di-
I rection on the arms of any lever.
I The only aeceasaiy condition is, that
I ■•"■if momenta about F must be
«1 (77), and act in oppoaite direc-
s. That is, it p and w act in
' any direction ji r, id w, on the arma
P p, P u, of any lever, and Tj/,rtif, be drawn perpendicular to
p p, w IB (either beiug produced if neceaaary), the Isver will be
kept at r«Bt when
THE BALAXCB. 61
113. An experimental proof of this may be readily obtained as
follows : — X F B is a bent leYer, moring on a pin at f ; it consists
of two straiglit arms, inclined to each ^^ _
otber at any angle, and perforated with '^* '
hdes at eqnal distances of one inch
from the centre and iinom each other,
for the attachment of strings. A
connterpcnse o is added, in or£r that
the centre of gravity of the lever may
be made to coincide with f, in which
esse it will remain at rest in any posi-
tion. With the centre f, and with any
ooDvenient radii, as 3 and 5 inches, de-
scribe two cirdes on the board, in
which the pin at f is inserted.
Attach two weights f, w, by strings to any points of the arms,
asp, v; and attach to the board two pulleys d, e, in such positions
that the lines J>p, s w (produced if necessary), may tonch the
eiTcles in the points j/, u/, respectively ; then if the weights be in
the ratio of 5 : 3, ks 10 and 6 ounces, and the lai^r be attached
to the string touching the smaller circle, and vice versdj the lever
will remain at rest when left to the action of the weights, or, if
diitnrbed from that position, will return to it more or less exactly,
acondiDg to the amount of friction at the points d, e, f. Ip this case
we have p (10)x Fjp'(3) = 30 =* w (6) x f «/ (6).
In the experimental determination of the equilibrium of moments
(78) a carefol measurement of the radial distances is necessary.
If, however, it be desired to obtain integer results, this may be
effected as above, by describing concentric circles on the board, the
rsdii of which are multiples of one inch, and attaching the several
strings, so as to touch these circles respectively, and selecting
suitable weights, as in Fig. 82.
114. One of the most familiar practical applications of the lever
is the common balance, or beam and scales. In its commonest
finrm this instrmnent consists of a flattened bar of iron or other
hard metal, tapering a little toward both ends, resting on its sup*
port by an obtase knife-edge in the centre, and having an eye at
each end to which the scale is attached by means of a hook. In
sll beams of bettor construction, each scale is suspended from a
knife-edge that passes through a box at each end of the beam. The
centre of gravity of the beam is usually placed considerably below
the point d suspension, in order to give due stability (92) to the
balance; bnt the points of support of the scales when in the same
horizoDtal plane, are a little above the point of support of the
centre of the beam, that it may acquire sufficient sensibility, by
the diminntion of its stability, when loaded.
115. The most perfectly constructod balances are those which
ea timCBAXKAL POWKU, OK atKPt.B MAOBim.
an iwed in chemical inTesligationB, for detarnuning atomic ««ij^ta
IT eaDiWenU, and ipeciEc graviliea; or in aiBa;iiig, tLatiB.de-
^rmiDJnc the amount of imparity in the precious meUllj theae
■ ... - tjn^ anneied diagram
(Fie- 83)nipreBeutsone of the beat deicription of these, coostructod
bj Mr. Oeitling. The beam, l, is ao acute pieresd rbomh, com-
bining liehtness with sdSiwsi. This ii fhmiihed with a fine knire-
edge at Uie centra, which reit* on an agate plane cemented to the
top of the pillar, b, and two kiii(»«dg«« attached to its extremities,
~~ which the scats planes, oemented to the snspeiinoli of the
. „.P . .. '-iMoftheicr'
■cales, rest. When not in nse, the agate plant
linit lifted oCT their knife-edges br a rising sufmo . __
m of which, the beam itKlf U raised off its bearing. This
anpport if raised bj au ecoentrio at b, rooted bj the batton, d.
The scal^-pana, when out of gear, are kept stoadv bv a tDOrahle
support benealh them. B; means of a sliding liAjng-piece, r,
which passes out through the side of the glazed case, o h, a noaU
weight ti of wire maj be deposited on the upper edge of the beam,
gr. placed in the scale-
le whole rests on three
—J „— - , , ate seen in the drawing.
The inmoation of horiiontal position of the beam ie given b; a
kmg slender indsi, l, attached to the beam, beneath the pomt of
wbuh a scale of decrees i> ^aced. In the adjastmeat of a
baUnca of this deicnption, it is necessary that the knife-edges
pan ; and smaller parts in proportion. Tl
adjusting screws; two only of which, k, i,
The inmoation of horiiontal position of
63
attached to the heam ahould he parallel, and in the same plane,
and that the tenmnal knife-edsea should he equidistant from that
at the centre of the heam. T^e centre of ^yity of die heam is
sdjnsted to ahont 0*01 inch helow the knife-edspB, which causes
the heam to oscillate in ahont 40*. The drawing represents a
12-inch beam, designed to bear 1000 grains in each scale, and to
indicate 0*001 grain.
116w Aa a smaller weight may be made to counterbalance a
greater, by lengthening one of the arms of the lever when arranged
as a balance, the dishonest vendor is thus frequently tempted to
cheat the unsuspicious buyer. This is readily detected, by weigh-
ing the substance to be purchased first in one scale pan and then
in the other; if the balance be correct, it will weigh the same in
both, but if inoorrect, its apparent weight will be different in each
Kale-pan. To determine the true weight of a substance with such
a bdance, weigh it first in one scale-pan, then in the other ; multiply
thew two wei^ts together, and take the square root of the product.
Tkm if a sub^ance weighed 253 pounds in one scale and 251 in the
other, 1^251 x 253 = nearly 252 pounds, the true weight.
This may be readily proved algebraicjdlv. Let a, b, be the un-
equal arms of the balance, A, b, the weights which, appended to
these anns respectively, balance x, the true weight ; then (110)
a : h :: X : Af
also a : 6 :: B : a;,
therefore b : x :: x : a:
hence o^ = a x b, and x = ^axu.
117. Another process for weighing accurately with a false balance
haa been devised by Borda, wnich indeed furnishes us with the
most accurate mode of ascertaining the exact weight of any sub-
stance, even with a good balance.^ For this purpose accurately
coonterbalimce the rodv to be weighed by means of any heavy
matter, as fine leaden shot, or sand. Then remove the body and
leplaoe it by weights carefully introduced into the scale-pan,
until the shot or sand be counterbalanced, and equilibrium re-
stored. These weights wiQ give the true weight of the body firee
from any error arising from imperfections in the balance. Ijiis is
called the method of double weighing.
118. In the lewr with un- j^, 94,
equal arms, it is obvious that
the spacea through which its
extremities move are very
diilerent. Let the line a f 0
represent a lever, turning on
the folcram at f as on a
centre, and suppose a weight
to be attachea to the end, c,
64 ¥BCH^KICAL POWBRS, OB SIMPLE MACHIXES.
and a power applied to a, sufficient to move it Then, while the
end, C| describeB the arc, d c b, the end, a, will jpass through the
arc, E A o, the length of each arc heing in the direct ratio of the
arms of the lever. The power applied at a, and the length of the
arm, a f, remaining the same, as the weight applied at o is in-
creased, the arm f c must be proportionablj diminished, and the
length of the arc, b d, or the space described by the weight, will
be aiminished, as the weight increases ; consequently, the time
occupied in causing the weight to describe the same space will be
proportionablj increased. From this reasoning we become convinced
of tne truth of the statement we set out with, that the application
of the mechanical powers is an exchange of time for power (109).
119. The difference in the spaces described by the unequal arms
of a lever, and consequently of the time required by the ends of
each to traverse a given space, is well illustrated by the solution
of the celebrated case assumed by Archimedes of Syracuse. Iliie
philosopher, seeing the immense power capable of being exerted
oy a lever, declared, that if he had a place to stand on, and were
provided with a sufficiently l(»ng lever, he would move the world.
If it be granted that he could exert a force of 30 pounds in pulling
an arm of a lever through 10,000 feet per hour, he would, to raise
the earth a single inch, nave to cause the end of the long arm of
a lever to pass through an arc which would require the continued
labour of 8,774,994,580,737 centuries to accomplish, supposing
Archimedes worked 10 hours per day.
120. The common steelyard is an example of a straight lever
with unequal arms. In this instrument the weighing is effected
by attaching the substance to be weighed to the short arm, and then
shifting a constant weight along the longer arm, until equilibrium
is obtained. In the steelyard. Fig. 85, p is the movable weight, Pi^p^t
&c., the divisions, each representing one pound : f, the fulcrum, and
w, the weight, suspended from
^' ^' the point to. As the short arm
is not made sufficiently heavy
to balance the Ions arm, let o be
^ ^ , the point at which the weight,
p, must be applied in order that
the steelyard may balance itself,
then the graiduation must com*
mence from that point, and not
from F. Let us suppose w to
be 1 pound, and to be balanced
by P at^*, then w is sustained partly by f and partlv by the weight
of the long arm, which by the supposition is equivalent to p x o f,
therefore wxwf=pxof + fxfp,=«pxo/},;
and iipiPtj P%P» ^i ^ taken, each=io f, since to fsoj),,
we shall have 2wxwF=p>«2o|)i=Pxop„
8w X w F=p X 3 0^1 ■ p X o|)g,
S
DIFFEREHT KIKDB OF LBVXBS. 65
and 80 on; that is, p, raspended homp^jp^ &c., will respectlTely
Bopport 2 lbs., 3 lbs., &c., at tr.
JA tlie steeljards in common nse there are two hooks for suspend-
ing anj substance to be weighed, at difierent distances from f, and
two corresponding gradnations on the opposite edges of the bar,
which may then bs emplojed either way upwards. When f tr is the
ihorter, a mater weight maj be ascertained, bnt with less accnracy .
121. The levers jnst described have been termed levers of the
fiist class, and are characterisEed by having the Ailcrom at some
poinl between the power applied and the resistance to be over-
come. Those leveiiB in which the fulcrum is at one end, and the
resistance at an intermediate point, liave been termed levers of
the second class ; whilst ihoae m which the power is applied be-
tween ihe folcram and resistance are placed m a third class. But
it appears unnecessary to make more than two classes : the only
leai CDstinction that it is necessary to make, is between levers in
which the fulcrum is between the power and the resistance, and
those in which the fulcrum is at one end. The proportion between
the pressures to produce equilibrium is expressed in the same
tenns in each case (110, 111), the chief difference between them
being, that when uie fulcrum is intermediate, as in the lever
already adverted to, the pressure upon it is equal to the sum of the
pressures api^ed, and to their difference, when the fulcrum is
lenninal.
122. The power, weight, and resistance of the fulcrum being
oooflidered as three pressures by which the lever is kept at rest, it
may be remarked generally, that the two terminal pressures must
always act in the same direction, and the intermediate pressure in
the opposite direction ; and further, that the intermediate pressure
must always equal the sum of the other two. This consideration
irill frequently be found useful in determining the direction in
which s^ gi^en pressure must necessarily act.
123. That modification of the lever, in which the power is applied
between the fulcrum and resistance, is not very frequently met
with; indeed, on account of the mechanical disadvantage at which
the force is necessarily exerted, it is never used except to gain
considerable velocity, or to overcome _
smaU resistances. Kkpf, Fig. 86, '^•^•
represent such a lever moving on a
hmge as a fulcrum at f, and the
power be applied at f, it is obvious
that whilst p moves through a small
■pace, K will describe a larpe one ]
and as both are performed in equal
times, the velocity of the end, r, is
considerably greater than that of
the point, p. The common tongs,
used to supply the fire with fuel,
66
mBOSAHICAL POWERS, OB SDtFXM ICAGHnTES.
afford an example of this kind of lever; the sheep-slieara and
sogar-tongB are similar examples.
124. Of the first described leyer, in which the fblcmm is inter-
mediate, examples are met with in the crowbar, scissors, pincers ;
and in the ordinary poker, when it rests on the bar, in the act of
stirring the fire. Of the seoond kind of lever, in which the fulcrum
and power applied are both terminal, an oar will aflbrd an ex-
ample, the water being the fulcrum, the boat is the resistance, and
the hand of the rower is the power. The chipping-knife used hy
druggists, in which the end is fixed to a board, the common nut-
crackers, the chaff-cutter, and the treddle oi a lathe, are also
instances of this kind of lever.
The following figures of the crowbar. Fig. 87, chipping-knife,
Fig. 88| and forceps, Fig. 89, afford examples of the tnree forms
Pig.SV,
Fig,B9.
i^.88. ^
of lever ; the letters p, f, b, respectively point out the position of
the power, fulcrum, and resistance.
«.. ^ A good practical illustration of the bent
*^' lever is met with in the truck. Fig. 90, by
which sacks of com. or coal, or other heavy
goods, are removed irom one part of a whan
or warehouse to another. In this machine
the axis of the wheels, f, is the fulcrum,
against which the foot is placed while the
weight at n is raised off the ground by the
hand applied at p.
125. The compound lever is a system in
which two or more levers are made to act
on each other; by which means a vast in-
crease of power is gained. The system re-
presented in Fig. 91 consists of three levers, a b, b c, o d, of which
the arms are respectively 6 and 1, 4 and 1, 5 and 1 ; and they are
so placed that the short arm of a. b acts on the long arm of b c,
and the short arm of the latter on the long arm of c d. Thus a
^w
pBnrciFLE OP TnnuAL telocities. 67
yreastae 1 at a will siisfain 6 at b ; and as 1 at b will iMtlaDce
4 at c, 6 at B will balance 6 x 4 or 24 at c ; consei^aently, 1 at a
will balance 24 at c. Similarly, 1 at a will sustain 5 x 24 or 120
at D ; thus, for instance, 1 ounce troy suspended at a will sustain
10 pounds suspended at d. The short arms of the levers here
LW W W W ***
represented are made heavy, that each may separately balance
itself on the pin which forms the fulcrum. If o^, i^; a^b^; &c.,
be the long and short arms of any system similar to tne above,
in which b^ acts on a,, &, on o^ and so on, we shall have equi-
librium when
p : w : : &, X (, X &c. : a^xa^x &c. ;
that is, in any system of combined levers, the power is to the weight
as the product of the short arms is to the prcduct of the long ones.
126. Prineiple of Virttud VdocUies. — ^There are two other modi-
fications of the lever which it is desirable to notice on account of
their important practical applications ; but the action of these cannot
be readily rendered intell^ble without the application of a very
important mechanical principle, that of virtual velodtieSf which
may be here explained, although a rigid general demonstration is
inadmissible. The velocity of a moving body (Ch. YI.) is measured
or represented by the space described in a given time, but in re-
ference to statics means only the space that would be described, if
ihe body were put in motion ; and tne virtual velocity of any point,
p, acted on by a pressure, p, is the space which that point will
pass over, measured in the direction of f'b action. The general
oondition of virtual velocities is, that if p^, p,, &c., are any number
of pressures acting on any system or macnine at the points ^i,j},,
&a, and Vj, v, &c., the virtual velocities of these points respec-
tively, then in the case of equilibrium,
p, xr,+p,xt;,-f- &cao;
which may be expressed by Z(p x v) a o.
The aiffebraical sum of the quantities (p x v) must be here un-
derstood, for it is evident that in the case of equilibrium, some of
the virtual velocities must be in a direction contrary to that of the
others, and consequently that some of the quantities, v, must be
negitive (vide 79, note).
nut we may here confine our ideas to two pressures acting at
F 2
> machiDe, mi tendiDg to prodoM tDOtion in oppoule
There >re many cues in practice in vrbicfa it unmld be tediooa
or difGcult la follow out tbe relations of preHares from their poiota
of appll cation, to other potnta at which tbeir efiecta must be eiti-
nial«d ; but in tlieae cajea, on the contriin, the lirlDsl velocities
may without difficulty be determined. Thii will be readilj nikder-
rtoodfrom the foUowmg '—
137. The principle of virtual Telocities is manifestlv tme in the
case of the Btmi^ht lever; for if
'y**- thelaver, prK,Fig.92,kept«t
it bj p, ij acting verticBll;,
e moved into (be pontion,
'ra*, and vertical linea be
I drawn through r' and k',
! meeting r s i" P, f, than
t'p, t!T will be the ipacee
traversed br r, s, respecavetv
io the direction of tlieir
actions, and will therefore represent their virtual tolocitiet. But
b; similar triangles,
and b; the topposition pf : pk :: a: p,
therefore f*; ; s'r ; : » : p.
In this case the ratio of the virtus) relociHes is constant,
wbstber tho space described bv the lever be great or small ; but
in manj initaaces, the ratio of the ipacei auccesaivel; deBciibed
bv p uid K is not constant. The ratio of the virtual velocities is
then repreaenlfld bj the ratio of very small cormBponding spaces,
or Diore Btrictlj speaking, of indefinitely amall B[iacei; in otber
words, the ratio oT the virtual velocities is the limiting ratio of ths
corresponding spaces described.
We will now proceed to the explanation of the two examples
proposed.
'*'■ W- 128. SobetvoTi Balance.— Two
ir .^; levers *b, od, Fig. 93, are attached
at their middle poiols, E, p, b; pins
on which Ihej turn, to a verti-
cal support, I r. The arms, a e,
BB, cr, FD, are all et^ual, and
their extremities are jomled to
two vertical supports at the points
«, c, B, n, so that the diatancea,
A C, B r, B D, may be all equal. The
sapport* AC, BD, ~ ' *
bf Iioriiontsl tables, o, b,oq vbich the veighti are placed. Joid
■ r, r D, D B, B E, then i r D ■ it a parallelogram, and wiU coDtiDoe
to oe UiB same, in whateTer pooition the leTen, jk B, c D, maj be
pbceJ ; coneeqaentlj A c, B D, will in atl pouCiona remain parallel
to ■ r, that n, will be alwajs Terlical ; and therefore the tabtei a, B,
will moTe parallel to tbemwlTei, and all points nf their surfnces
will moTe thnragb eqaal spaces, which wiU likewise be eqnal to
tiKMe passed thraogb 07 a and b. If, therefore, weights are placed
■njwbeie on ihe tables oh, their virtaal Telocitiea will be equal
to those of the points a b, that ia, thej' will be nibject lo the con-
ditions of equilibrinin oF the ordinarj balance ; but with this ex-
ception, that, nnlike the arms of a connnon bulance, it is imnw-
terial on what parts of the table the weights are placed.
129. The weighing machine is comtmcted on this principle
-rbea heavj goods are
reqoiitid to be weighed, '*•*■
as io the weighing no- m
chines at toll-gates and I
nilwaj stalioDS, the I
arms, a K, b b, are Tar; I
■maqpal, attd Ihe effect I
of the connlerpoise is I
rorther incresied b^ its I
being attached to the |
koser arm of a lever, as
E. m Fig- W.
130, The Anou.— This term has been applied to a combination
of lerara b; wbich a preasnre is effected un precisely the same
principle as that on which the weight of the body ii ■ • ■ -
the erect position by straightening
the knees- Machines are thus
constructed in which a iarge
amonnt of Dresmre is required lo
be exercised through a nttali space,
•Dch as ooptiuR and printing
preaMB, and the tike. Let a b c d
be the ftameofapress, Bthe centre
of the riidng piece, snd rou a
bent lerer tuning on o. the middle
[Kant of A D, and jointed at n to a
fink, which is also jointed lo the
IV- M-
rf
ijet F, s H, I, tig. 96, F,QH,B,. be fonr positions of the
knee and hwidle, snch that the angle r, e r,— r, a r„ and conse-
E, E„ and I, Ell then
!,ap and p,f„^i,
qoentlj H, OH. = H,aB,. Join r, r-r,r,
when the angle Fj a f, is very small, r, »■« =1 "p ,
will represent the cMresponding virtnal velocities of tl
70
r and e. Bat while p, f, - p, f, , b, e, , acid s, i. are manireatlj
Tei7 tmeqiuJ ; let p be the oousiHiit preuure on the handle tt r,
J.. ^ Aod R, B*, the comaponding
^' preasum eiertedbeCireanK,^
■od B, Bj respectiTelj, ttieQ
(137)»x»,B,=pxr,F„
= b'x^b'';
orii:B'::B,E.;E,B,;
coDBeqaiinlJjr, ai e,b, becomea
eicaeoingly atatll, compared
with B, B, when the knee, h,
approaches very netr]y to the
stmit^ht poaitjon, the preaamv
k' becomea exceedingly great
relatively to k; but it mnat
be romanibered, that thia great
preesnrs is exerted through
oalj a ver; small apace, and
thereFore in practice, the presa
requires carafal a^jaatment, in
order to develop it» fall power.
131. The wheel and axle is a modificatiou of the lever, in which
conaiderable mechaoical adiantage is gained. The machine consists
of a cjUnder, A, Fig. 97, termed the axle, tnnuDK on a centre, sod
connected with a larger circle of wood or other snbataDce, a, called
the wheel. Sometimea this ia replaced hj a ^^e, as a, fixed into
4, to the eod of which the power ia applied. The resistajice to be
_ „ OTercome is Gied to one end of
^' a rope wound round the small
cylinder a, whiEat the power is
applied to the circi^mfeience
of B, generallj by meana of a
rope r, acting in the direction
* of a tangent to b. Here the
radiua M the smaller circle, er
axle, maybe conaidered ascor-
reeponding to the abort arm,
and the radius of the larger (or
wheel), or the length of the
apoke fixed into *, to the
leneer arm of a straisht lever.
And accordingly wa find that
equilibrium ia obtained when
the power applied is to the
WsisUnc ■ '
the a«n:e ratio aa the radiu irf the axle ia
0 that of the wheel.
THB PULLET.
71
OtDmg tlie ndhiB of the wheel w, and tliat of the axle w, we have
PXWaBXtO.
The winch, wmdlefl% capfftan, and crane, afford examples of the
practical application of this naeful modification of lever. In the
nDowing ngana, representing several varieties of the wheel and
axle, the same letters of reference are nsed as in the diagram last
described.
JV-9S.
Jl^« 9Va
^.100.
JV* 101-
2. TBB POLLBT.
132. The sinaplest form of pulley is naed onljr to change the
dirsction of motion. As osaally constracted, it is a small wheel
moveable aboot in its centre, in Ihe cir-
cuniiBrenoe of which a groove is formed,
to admit a rope or flexiUa chain.
^ In the single fixed pdle^ moveable round
itscflotre, G, Fig. 101, no mcrease of power
is gained ; it is merely a convenient in-
stnment for changing the direction in
which a given pressure acts. If a rope
AB be pasKd over the pnDev, eqnui-
briun wiU oocnr when the weights n and
p ne equal ; the tensions of the string
win be equal on both sides of the pulley,
uid as these tend equaUy to tuip the
poOeyin opposite directions, it will ne-
oasianly remain at rest.
133. Bv a single moveable pulley we
sxe enabled to sustain a resistance of 2
^ a power of 1. Let the rope a o, Fig. 102, be fastened to
afixed pomt at ▲, and passing under the groove of a moveable
poUey D, be brought over a fixed pulley b, so placed that the
Kveral portions m the string may be parallel to each other :
tbe <mly use of b is to render the application of a weight more
convenient (111). Let a weight b be suspended from the axis
of the moveable pulley, d, and a weight or power, p, applied
at the «nd of c; nader tlieM circumstancea b will obrioiuly ba
supported equally by the power r tnd the beam a, which aidi
in auataining the weight by the tennon of the atring in the
aame manner aa r doea, and accordingly ■ will be supported
by a pressure p, equal to one half
tif.lot. itj own weight. Hence iu the nogle
□oveahls pulley, eqailibrium ia obtained
chen the power ia to the reaiitance ai
1 loa.
In the puJUy, aa in the lere
lostai „ .
tion will abow, that for a to be railed
line inch, p must fall through two
incbee, aa the end at A ia immovable,
and each of the atringa between A, D,
and D, E, will be shortened by one
inch. It is to be remarked that in this,
as in other modificatdona of the pulley,
the weight of moveable parts, as well
as of the string, ia not here taken into
the account. ,
HDgIa pulleys. In soch an ansngetaent each fold of string
■ustains an equal abare of the weight, or resistance ; and the
portiona of the string being all panillel, equilibrium will result
n being the number of atringa at the lower bloclc,
_ ,„ Kg. lOSTfte folda of atring in
'*- "*■ being 4, a power of I will anatain - .=-= ,.
If the diameters of the pulleys 1, 2, 3, &c., OTer
which Che string passes in aucceuioD, be as the
numben 1, 2, 3, ftc, as in Fig. 103, the polleya
will revolve in the same time, and may therefora
be united ti^elher in each block, that ia, may ba
merely grooves in one solid piece. For when ft
length of string equal to one circumference of
the pulley 1, has passed over it, that pulley has
made one revolutiou, and as the string 3 is
shortened aa much as 1, double the quantity will
have passed over the pulley !, that is, the length
of one circumference (the circumferencoa of
circles being as their diameters), and conse-
quently the pulley 2 will also have made one
revolution in the aame time, and bo on for the
Some practical advantage is gained by this
anaogement in the aaving of power spent in overcoming the frio-
tion of separate pulleys, but there is a greater disadvantage in the
amna of KiLum, 7S
luliQi'lj of the itrin^ to diaplacement. Id practice thii mftcbine
unuUj conwata of aeieml separate pu]l«;B of eqnal aize, called
liema, working io sep&rale giooTeH. or mortieei ia a, Moet, on a
)HD or &iie poanog tbrougb theia aod the block. A peir of blocks
:• need for niiaine oeaTj weights, bdiI tbe extent of their action is
Emited only hj the length of the rope employed.
In this iiutBDce again we recognise tbe truth of the principle of
nrtaal retocities, for the striag to vhich e is attached ia lengthened
M modi as all the strings snp^rting B are sbortened, and coDse-
qoently (p) r, the TiitnaTTBiocitj of p, = n (ti) K ; but ■ = n p, and
race (126) .xW» = Px(o)p,
weebtan [«=] npx («) e= ry [«!■ = ]«(«) r.
135. Instead of tbe string folded on the pulleys being entire, it
i< ranetimes divided iato several portions, each pnlle; hanring by
1 separate string, one end of which is attached to a fixed point, and
tbe other to the adjacent pulley. In this system, the tension of
ihertringiBjFig. 104, = theanmtJthe tensiona of the two atringa
■bich support A, each of which = p ; therefore
tension of a b = 2 P, similarl;
lensionof BC = 2xteDBioDof A6 = 2x2 p = 3*p,
weight of ■ = 2xtenBionofBC' = 2x2'p = S*Fi
and liniilari/, if there were n moveable pulleys, vte should find that
B=2" p.
In inch asjstem, thersfbrKj the gain of power may 2^. loi.
le determined by calcubtmg that power of 2, of
ubich the index la the number of moveablepuUeys.
In the lyatem of pulleys represented in Fig. IO4,
there are three moveable pulleys ; now tbe third
power of 2 is 8, and accordingly, with sucb an
srrangenienl, we can, with a power of 1, counter-
balance a resistance of 8. The Gxedpulleyin this
■yilem does not increase power, bnt merely afibrds
aoMte convenient mode of applying a weight.
136. When tbe pnllejs are traversed each by
a sepoiate string, the ends of the atringa being at- j^_ jfi^^
lached, not to ^ed pcouta, as in tbe last case, but
to tbs resistance to be overcome, as in JTig. 105,
eqiulibrinm ia obl^ned when
,:.::.:{!■"-■),
+ ■).
-■)■
74
KBCHAaiCAL POWBBS, OR 8I1CPLB MAOHINBS.
Fig. 106.
Wben the first moveable pulley is drawn up to the fixed puney in
the former of these two systems, or down to resistance in the Utter,
no'fnrther action can take pbice ; coosequently these systems aro
applicable only when a large amomit of power is applied to be
exerted through a small space. Their most important practicd
application is m tightening the rigging and sails of ships.
137. In the preceding cases, the several ^rtions of the stringy
have been supposed to be parallel ; when this is not the case, some
alteration takes place in the condition of eqnilibrium. Taking the
case of a single moveable polley, eqni-
libriom occnrs when the power is to
the resistance, as radius to twice the
cosine of half the angle contained by
the directions of the string. Let o n,
Fi^. 106, be the direction m which the
^^ weight or resistance b acts ; prodace b i>
Cj^J untu it meets o b at i. Then, if d e be
^-^ taken to represent the amount of power
at p, it may, by the^ resolution of pres-
sures (69\ be considered to be the re-
sultant or two pressures, one acting in
the direction e c, and efiective in raising
the weight b ; the other, c d, being coun-
teracted bj an equal and opi>08ite pres-
sure arising from the tension or the
string E o ; and as the two folds of the
string, OB,' be, are equally active in
■K ) suBtamingB,2cE will represent the whole
weight sustained by the power f, and
p : b:: de: 2 CE ::rad : 2 cosdbo.
When the strings become parallel, the angle dec vanishes, and ita
cosine becomes radius, then p : b : : 1 : 2 as already explained (1S3).
The pulley has been referred with great justice to the lever, of
which indeed it may be considered as a modification ; the diameter
and radius of the single moveable pulley, representing the arms of
a lever, on which the power and resistance respectively act^ by
means of the string.
138. The DifferenUal PuUey. — ^A combination of the wheel axle
with the pulley, to which this term has been applied, affords an
advantageous means of employing considerable power. One end
of the string passes over the wheel a d, and the other is wound
round the aue, b. The tensions of the strings b b, d f being each
equal to ^b, a D may be considered a lever kept at rest by three
pressures, p at a, and 4b at b and d, consequently
iBx[DC=] CA = iBXCB + PXCA;
theiefore iBx(oA~CB)=pxcA,
4bxab=pxc a;
or
whence
p : B :: ab : [2ac =]ad.
I
\
k difleTcntuJ pnlleT worked fay an endless chaia. Wie. 108, hw
htflj come connderablj into use in buiJilina; ; in this tiie gnxirM
of tin doabls piJley are hollowed oat to fit the Hueceiaii'e Unks of
the chain, ami the awenJinj; chain is prevented bwa ilipping 01
theinil^y bj passing under a — "—
Ine Cbinese capstan, of
»liich Dr. O. Gregory niet
vilh anne drswiiKS mora
than a centniT old, acts
npoi thii prisaplB: the csp-
•Ud coDsuto oftwo parli of
diSerenl sine, and the rope
is Boqiid on to the latKer,
while it is nnwoiuid rrom
the nuaDer, as in Fig. 109.
The power will iocreaM as the difierence between the two portions
of the capstan is diminished.
139, aigiditji of Cordagt. — The conditions of ttatieal eqmli>-
brimn of the tarioiui sjatems of polleje have been correclly deter-
nuted ; bat when an eiceas of power, beyond what is required to
^nce eqnihlnium, is applied to raise the weight, or to OTeroome
the Kiiitaiice, it must be borne in mind that a certain portion 1^
this win be oonsmned in orercoming the resistance of the machine
iUel^ ofwhic^ the rigidity of the cord f<Hina the principal psLrt.
76 HICRunCAL FOWBBI, OS BDIPLB HAOWm.
Tha mode b vbich thji rigidity acta prejndidall}' will b« beat
tuidentood bj a reference to Fjg. 110. Let A be employed to
niu B bj meuiB of a cord A o D b, pssaiDg over ■ puller, of vbirh
B it the centre. Tb^n, ai the cord is leaTing the pnUef *t c, it
fla. 110. *'''^ i'' consequence of its rigidit;, usame, on
nnwinding, a aligbt convexity toward) ibe
EnileT, iLnd will Babseqaently become vertical.
imilu'ly u tbe cord is beins wound on to
the polley at d, it will deviate nom the vertical
direction by a aUght concavity lowarda the
pulley. Draw r e a, a horimntal line, throogb
B, meeting the directions of the cords at a and
B in the points r, a ; then it is evident that the
power and weight may be supposed to act on the
arms of the lever, F B o, and the requisite ratio
of A : B will be that of G o : B p. It has been
roughly estimated hj Taschel that if a coid O'l inch in diameter,
passing over ■ pulley one inch in diameter, be employed to raise
ope pound, then b&lf an oance will be required to oTereome tbe
rigidity of the cord. This amonnt will be diminiBhed bj increas-
ing the liie of the pulley, but increases with the diimeter, and
•Im with the tension of the cord, or if the cord be wetted.
3. THE mounxs flame.
140. The action of this mechanical power depends upon the
simple principle, that a body free to move can be supported only
by a force equal to its own weight, unless a portion of this weight
is sustained by a Gied obstacle, in which case it can be suppoited
by a smaller force.
An inclined plane consists of any plane snrface, a d (Fig. Ill),
J. ,,, sufficiently hard, inclined at
'"'" a given angle to a boriaontal
plane, in which three elements
an distinguished ; its height
A B, its length a c, and base
B c. In our theoretical cod-
siderations of its action, it«
EUT&ce must be conddeml
as perfectly hard and amoalb,
comUtions to which the best corutrucled instruments afford, of
OOUTve, but a distant approximation.
141. Let ABC, Fig. 112, be an elevation of the inclined plane,
and let tbe point d be kept at rest on the inclined plane achy th«
pressure p in the direction o r, and a pressare B acting vertically,
and let a be the angle adf, and 3 the aaf^v acb: draw the
horisontal base ■ c, and the vertical height a >, also b e and a p
pnpendicalar to a a Then the pres-
•ure D p maj be resolved into Iwo
(72], D A in the direction of the plane,
and Ap perpendicnUr to it, of which
1 r CAD have no oBect in moring tha
point D along the plane, and i f d p
npnsentB the whole presnire p, a s
will repRMDt the eSectife part of it,
■hich therefore : p :;AD:DP,aiidcon-
>e<]aenU7
Similarij the vertical fKtean d a may ho reeolTed into d b and
B X, oT vbicb B B acting perpendicalarlj to tlie pluie bu no
tendency to move d along its sorfaca. Therefore the eflectivs
part of B : B : : D E ; D a, and canseqaenU}
= RX— = KX^(hy aim. triangleB}=«, lin fi ;
itat D being at reat, the preisorea JD oppoaite directions mn^t be
«|Da!, thcr^ore p. cos a = B. ua 0,
If r acta parallel to the plane, we have aimplj p — s. nn J3;
T p : K ; : height of plane : its leogth.
If p acta horizontallj, a^fi, and then
p.coa^ = R.BinAorp = B.Un(3;
Of p: b; : height of plane: length of base.
Hi. The conditioDB of equilihrinm on the inclined plane maj be
eanlj ihown experimentallj. Two boards a b, b o, are hinired to-
Ecilier at B,_Rg. 113; one of "
tKem, B c, is placed borixon-
tilly, and has a gradoated
arm, de, allached to it, to
vhich A B inar be clamped bj
a KTCw. The edge ab is L
dirided into inches, aa well as '
a narrow slip of wood or metal,
A T, attscbed at A Ini a pin on
wbich it can moTe &eelj ; thii
slip hazing Tertically by its om gravity, shows the height of Iha
plaw when Aia rawed. A weight k is placed on ayerj light carriage,
coiaiiling of a thm plate of wood with fnnr small light wheels, sjid
IS BBatainnl hy another weight r attached to a siring, which passes
owr a pulley at A, and is fastened to the carriagB. When the
wiglita are as the numbers of inches in A B and * p, they will be
m esmlibnnm but if the plsne is raised or lowered a little from
thu pomtion, the equJilnum wiU be destroyed, and « wiU ascend
78
MBCHARTCAL FOWXB8, OB BnOlM MACHnrn.
or deecend accordingly on the inclined plane ; especiallj if aided
in starting, by a jar on the table on which the pUuie rests.
Here, as in the other mechanical powers, time is lost as power
is gained, for the Tertical height to which the body is raised by
means of the inclined plane, u e^nal only to the height of the
plane, while the space through which the power descends is equal
to the length of the plane ; and the less the height of the plane
the greater the weight that can be raised on it by a gi?en
power.
143. The Screw. — If an inclined plane be supposed to be wound
spirally around a cylinder, in a manner similar to thst in which
IW. 114. spiral paths are carried round mountains to
^ lessen the steepness of ascent, we have a
9creWt one of the most useful of simple
machines. The edge of a flexible inclined
plane a, Fig. 114^ an angular piece of naper,
for example, wound round a cylinder b,
represents the thread of the screw, which
projects to a certain distance beyond the
cylinder on which it is supposed to be wound.
In order to appl^ the screw, a hollow spiral is
carved in the mside of a block of wood or
metal, termed the female screw ; this hollow
spiral must be of such a size as to admit the projecting thread of
tne first, or maJe screw. Thus constructed, the male screw is
generally turned by means of a lever,
fixed into its head ; thus, indeed, form-
ing a compound machine, the power of
the lever being added to that of the
simple screw. The power of the screw
increases with the circumference of the
circle described by the lever, l, Fig. 115,
to which the power is applied, and with
the diminution of the distance between
two conti^ous threads of the screw,
measured m a direction parallel to the
axis. Calling this distance d, and the
circumference of the circle described bv
the lever, l, equilibrium will be obtained,
when
p : B : : D : L.
This is true by the principle of virtual velocities, for as the
screw is raised or lowered tnrough the space of one thread, by
one complete revolution, d : l : : (v) b : (o) p,
and hence p : b : : (v) b : (v) p.
144. JBunter's, or the Differential Screw, — If a very large amount
Fig. 116.
79
Fifi.iie.
of pnmue h required to be exerdaed, the threads of the screw
QULit be Tery ^aie, and are therefore more likely to be torn off the
crlinder on which they are cut. To obviate tnis inconTenience,
the laige screw that works throueh the head
of the presi^ as in Fig. 116, is nollow, and
bas a thread inside, less coarse than that on
the oatside of it. A male screw fitting the
preceding hollow screw is firmly fixed in the
rising piece. In this machine it is evident
that while the larger screw descends throoeh
the space of one thread, daring one revom-
tiooofthe handle, the smaller will ascend
vithin it through the similar space ; conse-
rtljT the descent of the rising piece, or
▼irtnal velocity of b, will depend on
the ^fiBsrence of the spaces between the two
thretds, wluch may be made as small as
we please : and consequently the resistance, b, will be limited only
bj the strength of the materials of which the press is composed.
145. The Wed^. — ^When two inclined planes are placed with
their bases approximated, as a b, Fig. 117, we have a wedge ; which
is a triaogolar prism, bounded by plane sides, of which two that are
Fig. 117.
opposite are equal and Pj^ndlol trian^es, the others
wing parallelograms. This is occasionally used as
t mechanical power to lift heavy weights to small
elevations, but is more generally used for the puipose
of splitting timber; the edge being introduced into
a deft maoeto receive it, and the wedge forced in by
repeated blows of a haojmer upon its biusk. The great
saTaatage of tiie wedge appears really to depend
upoo the percussion used to urge it into the mass of
timber, &c., exciting vibration between the particles
of the solid, and thus permitting the edge to insinuate
itself between them. Certainly the diiect action of a weight
preoing upon the back of the wedge, can bear no comparison
with the immensely greater effect gained by
percussion. The amount of weight necessary
io^reu a common nail into a board, compared
mth the weight of a hammer that will readily
drive it, is slmost incredible.
146. Theoretically speaking, it has been
Boppoeed that the power gained by the wedge
bean the same proportion to the resistance
to be overcome as naif its back does to its
height; but this applies only to the equilibrium of the wedge when
at rest : for friction enters so largely mto the consideration of any
motion that might result from pressure on the back of the wedge,
that the only part of this theory which is really supported by
80
MECHANICAL POWEBfl, OB BIlCPLB MACHIKBS.
practical observation is the fact, that the power of the wedge in-
creases as its width or back diminishes. Many of our domestic
instmments are modifications of wedges ; a saw is composed of a
series of them, and knives, scissors, razors are nothing more than
fine saws. Needles, pins, &o., may be considered as acate circular
wedges.
147. Another important modification of the inclined plane, of
lig, 110.
constant application in machinery,
is the cam; this is a projecting
piece attached to a revolving axis,
on which an arm, or lever rests,
which is periodicaUy raised as the
cam passes between it and the
axis in the course of its revolution.
The eccentrics of a steam-engine,
and the stampers of mills for crush-
ing ore are examples of the ac-
tions of cams ; large shears are moved by the same means at the
iron works, by which iron boiler plates half an inch or more in
thickness, are trimmed into shape with no more personal labour
than that of holding them.
148. In that elab<)rate and wonderful part of the animal economy
the muscular system, we have much to admire in the adaptation
of power to the movement of the bony levers constituting the
skeleton. Here, where great strength, rapidity of movement, and
elegance of figure, are equally attended to, we find evidence of
infinite wisdom in the adaptation of mechanical power, apparently
the least advantageous, to the most important motor functions of
the body. In considering the mode in which extension of the limbs,
especially of the upper extremities, is performed, we see a set of
levers of the first kind (121) called into action; or those in which
the power and resistance are at opposite ends, and the fulcrum
intermediate. In the flexion of the limbs,
we have a set of beautiful examples of
levers of the third, or that kind in which
the resistance and fulcnim are terminal,
and the power intermediate. And in
some other muscular efibrts, as depressing
the lower jaw, we have examples of levers
of the second denomination, m which the
resistance is intermediate between the
fulcrum and power. The action of raising
the body on tiptoe has been commonly,
but erroneously, considered as an illustra-
tion of the second class of levers, which
ic could not be unless the upper end
of the muscle, instead of reacting on the
parts from which it arises, were attached
lig.UO,
LEYEBS ni THB ANIMAL ECOHOKT.
81
to a post or some otber fixed poiut external to the irame. For
let A F be the bones of the leg, f b the foot, b p the muscles which
raise the heel, and for oonTenience, let b p, the line of action of
the moscles, vgpwards at p and downwcnrds at b, be parallel to
A F, in which the weight of the body acts ; then p actiog
apwards at jp sustains the parallel pressares p and w acting
downwards at f; consequently
or
PX pBsPXFB + WXFB,
P X [p B — F Bs3 ] p F=B W X F B ;
that is, the conditions of equilibrium are precisely those of a lever
ofthe first kind (110).
149. By the insertion of a muscle near the fulcrum, we gain a
mat increase of velocity at the further extremity of the lever
(123), a rapid motion best fitted for many of the purposes of
animal life is thus obtained. In the act of flexing the arm
for example, the fulcrum f is formed by the condyles of the
humerus at the elbow j«int, the resistance is the weight, b, in the
hand, and the power is applied at p, by the contraction of the
muscle attachea to the radius. When this muscle (biceps flexor
cabiti} contracts, the hand b describes a much longer curve in a
Ftg. 121.
given time than p, therefore, although the power of the hand is
much less than the contractile force of the muscle, still that power
is capable of being exerted through a much larger space.
150. The following* are some among many examples of levers in
the human body.
>
A. Fvlcrwn behoeen the Fewer and Bmstance.
POWBB.
Muscles arising from
tuberositiesofischia,
and inserted into the
lower extremities.
Muscles connectinff
the occiput and
spine.
FULCBUM.
Heads of femora.
The Atlas.
BESISTANCE.
Weight of the trunk,
- when flexed upon
the thighs.
Weight of the head,
acting at its centre
of gravity, in front
of the fulcrum.
82
IdSCHANICJLL FOWBBS, OR BDfPLV MJLCHINEB.
B. Ffdcrum terminalf Beaietance intermediate.
POWER.
Digastricus, and other
aepressors of the
lower jaw.
FULCRUM.
Articulation of the
lower jaw.
KE6I8TA1ICE.
Action of the tem-
poral and masse ter
muscles.
C. Fvlcrum terminalj Power intermediate.
Biceps flexor cuhiti
and hrachialis.
Deltoid.
Condjles of hume-
rus.
Glenoid cavitj of
scapula.
Weight of arm and
hand.
Weight of the arm.
151. Of compound pulleys we should scarcely expect, where all
is characterized hy beautiful simplicity, to find any examples ; of
simple pulleys, merely to alter tlie direction of motion (132), we
have a few instances. The structure of the pulley-like or^n is
always extremely simple, usually being merely a groove m the
bone covered with cartilage, sometimes a bony hook, and in
another case a tendinoas ring. The tendon of the obturator in-
ternus, which, in passing out of the pelvis, glides in a groove in
the ischium, so as to alter its direction ; and the hook-like process
through which the tendon of the circumflexus palati glides, so as
to alter its direction to a right angle, and the tendinous ring in the
depression of the frontal bone, through which the tendon of the
obliquus superior muscle of the eye glides, being thereby bent
to an acute angle, are examples of the simple pulley in the human
body.
152. We have no illustration of the inclined plane, or its
modifications, in the human skeleton. The sacrum is certainly
not an example of the wedge, notwithstanding its figure. The
only approach to a wedge in animal structure which the authors
aro acquainted with, is the bony apparatus discovered by Sir
Philip Egerton, in the neck of the ichthyosaurus, an extinct ante-
diluvian reptile. Three wedge-like bones have been described by
him as connected with the cervical vertebrae, and fitting into
spaces between them ; these wed^s are supposed to have been
withdrawn when the animal flexed the head upon the trunk, and
to be introduced between bodies of the vertebrss when the head
was raised: so as to diminish that vast muscular effort which
would otherwise be required, to keep the enormous and dispro-
portionate heads of these animals extended.
83
CHAPTER V.
PRI2ICIPLE8 OF MECHANISM.
153. In the preceding chapter the conditions of equilibrium of
the gimple macnines have heen ascertained, some of their leading
mudiHcations explained, and their various applications in the
ftniual economy illustrated. The object of the present chapter is
to point out the various relations of tne parts of machines to each
otkBr, the objects desired to be attained by machinery, and the
mean s adapted for attaining them. A cursory notice of the leading
features of so wide a subject is obviously all that the limits of an
elementary treatise will admit ; and it will be found convenient,
in the following treatment of this subject, to confine our ideas as
much as possible to the mere relation of motion as existing
between two parts of a machine, without any reference either as
to tbeir actual velocities, or to the force employed in moving, or
required to be exerted by the machine, or the mode of connexion
of the parts in question with the framework of the machine.
These points belong more properly to a practical treatise on the
c<»i8truction of machines.*
15}. Any combination of mechanism is called a £ratn, consist-
ing of several parts, OT pieces, variously connected with each other
in succession. Of two successive pieces, that which communicates
niotion is called the driver^ and that which receives it, the
follower.
155. The line in the direction of which the action of the driver
on the follower takes nlace, is called the line of action; and when-
ever the driver and follower are moveable in tne same plane, about
fixed points, the line joining those two points is called the line of
centres.
156. Motion may be communicated from the driver to the fol-
lower either by direct contact of their surfaces, or by some inter-
mediate communication. If the surfaces roll on each other without
rubbing, as the circumference of one cylindrical surface on another,
the action is called rolling contact ; but if the surfaces do not roll
on each other, as when a projecting pin rests on a cam or an
eccentric (147), in the Btamping machine, for example, the action
is that of sliding contact, in many cases the contact partakes of
* The Authors are indebted for the subttance of this chapter to the
** Pdadplefl of Kechaaiim/' bj Prof. WiUit. Cambridge, 1841 .
Q 2
84
PRIirClPLES OF IfECHAKIBM.
Vig. in. both characters : thua, let A c, Fig. 122,
be the driver, and b d the follower, their
surfaces being in contact at fn, and let
A e, B <i^ be their new positions when the
points n, p come in contact at r; then if
the lengths of the sorfaces mn^mp are
not equal, as their various points must
have successively come in contact, slid-
ing as well as rolling must have taken
place.
^ 157. If, in the communication of mo-
tion from the driver to the follower, the
action is oue of pulling only, as in driving
the mandril of a lathe, a wrapping eon-
nector of some flexible material is used,
as a rope, chain, or band ; but whenever
a pushmg action is necessary, either
constantlj, or alternately with pulling,
as in a common pump, tne connexion is
bj Unk-work^ the link consisting of some ngid materiid.
158. The ratio of the velocity of the driver to that of the fol-
lower, or, as it is called, the vdocity-ratio^ may be either eonstarU,
as is the case with ordinary wheel-work, or variabUf as in the
action of cams, eccentric wheels, or links.
159. The relation between the direction in which the driver and
that in which the follower is moving, or the directional relation,
may be either constant, as in the wheel and mandril of a lathe, or
changing periodically, as in a bottle-jack, or the piston-rod and
cnmk-axis of a steam-engine. The directional relation must be
either constant or changing periodically.
160. By the various combinations of these three elements,
namely, connexion, velocitpr-ratio, and directional relation, all the
requisite varieties of mobon may be expressed. The following
Synoptical Table of the Ebmentary Combinations of Pure Mechan-
ism affords some general illustration of their application.
rSee Table, p. 85.)
Before proceeding to oescribe some of the more important com-
binations of mechanism, it is necessary that the following propo-
sitions of very general application should be established.
I. To determine the Ratio of the Spaces described hy two corre-
sponding Points of a Driver and Folhwert when the Velocity-
ratio is constant,
161. Let y and v be the velocities of the points, if constant, and
8 and s the spaces described in any time, t, then
B:s::y :v.
If the velocities are not anifonn, let b, 8„ &c., #, f^ &c., be the
'■f- ■-^»-.-,-
^-•"
.. ■ ->-. __
TABLB OP BLEKBlfTABT OOMBniATIOXS.
85
spices deseriW in the timeB t|, <,, &c., then since the yelodtj-
ratio is constant,
s, : #1 : : V : V.
nmilarlj
therefore,
8, : «, : : Y : V.
&c. &c.
8,+aj + &c. :»,+«,+&c. ::y:v.
And this is equally tme when the qnantities, s, «, and t, hecome so
small that the changes in y and v hecome continuons ; and hence,
The vdodty^ratiot when conttant, is obtained hy comparing the
entire spaeee deecribed in the eame time^ whatever ekanges the
aetwU vdodtiee mt^ have undergone during that time.
SIXBOnOVAl MMhAXlOS—
(2) CHAirewo
Mova ov
(1) COaiXAVT.
mxonxcALLT.
coavaziov.
dans A.
Class B.
Class C.
Vtloeiif^ratio
rOoei^'ratio
varffing.
eoiutant or varp'
ing.
Pxnsioir a.
BoDing cjlinden.
Boiling carves.
Mangle wheels.
BjBottiog
Cooteet.
oones, aod hyper-
and rolling ourre
Mangle racks.
bokridft.
wheels.
Kscaping gearings.
Geaeral ftmiige-
Eccentric wheels.
nentandfocmof
Wheels with inter-
toothed wheels.
mitted teeth.
Pitch of wheels.
Boiling corre
levers.
Dimiox' b.
Forms of the indi-
Fin and slit lever.
Pfa» and slit lever.
Bt Sliding
OoDtoci.
▼fdoal teeth of
Cams.
Cams in general.
wheels.
Unequal worms.
Bwash-plate.
Cmdb.
Geneva stop, and
Double screw.
other intermit-
Bscapements.
Bndlees screws
tent motions.
Propehnenu.
and their wheels.
Dinnov e.
Bends in genersl.
Conical poUeys.
Curvilinear pulley
Bt Wnppiog
OoDiMotorfl.
Forms of tbeir pol-
lers.
Curvilinear pul-
leys.
and lever.
Guide pulleys.
Fusees.
Oesring chains.
Bxpanding pul^
Modes of oommn-
leys.
nioating limited
^
Dcnsiov d.
Cranks and link-
Link-work.
Cranks, eooentrics.
work for equal
Hooke's joints.
and other link-
rotations.
work.
Cranks for limited
Batchet wheels
motions.
and clicks.
BeD-crank work.
Intermittent link-
work.
86
PRIKCIPLBS OF MBCHAHjSlf.
Hg. 1S3
II. To determine the Vdodty-raiio in Link-tcork.
162. Let A p, B Q, Fig. 123, be two anns moving on fixed centres,
A and B ; and let them
be connected by a link,
PQ, jointed to their
extremities, p, Q. L«t
AK, BB, be perpen-
diculars from A and
B on PQ (produced
if necessary), and
let AP, PQ, QB be
moved into the new
positions, a p, pq,
a B, very near to the
former. Draw pm
and -Qn porpendica-
lar to PQ and join
^ A B, cutting p Q in T ;
then in the ripht-
angled triangles,
p J) m, A p R, P|7 is perpendicular to a p, and p m to a r ; therefore
the angle jpp ma the angle par, and the triangles are similar.
In like manner the small triangle, Qng, is similar to bqb;
whence,
p^ : pm :: AP : A R,
and qn: qq : ms :Bq\
also
at:bt::ar:b8.
(1)
by similar triangles art, btb; and vm^^n ultimately, since
p Q and p a are tlie same link. By compounding these proportions,
omitting iaentical terms, and inverting a t, b t, a p, b q, we obtain
AP * Bq'
BT : AT
. (2);
but — ^ and ^ are the angles simultaneously described by p
AP Bg
and Q,^ and may therefore be taken to represent their angular
velocities; also t is the point at which the line of centres (155)
cuts the link p q, therefore in any g^ven position of the link.
The angular vdodties of the arms a p, b q, are to each other in-
versely €u the segments into which the link divides the line of centres.
Cor. 1. By compounding the ratios (1) and (2) we obtain
—2 : 3Ji : : BB : ar:
AP B^
that 18, The angular velocities of the arms a p, b q, are inversely
YKLOCnr-RATIO. 87
Of the jperpendiculari firom their eeatrei of motion on the line of
aetkm.
Cob. 2. Produce ap, qb to meet in x, and draw kl perpendi-
cular to F Q, then
pm : pm : : pl : kl,
and qn :Qn ::kl:ql;
whence bj componnding, |>m:Qn::PL:QL,
and conseqnently l is uLtimately the point of interseetion of two
conneeutive positions of the link.
Cos 3. If the paths of the points of action p, q, ha^e no fixed
centre, the results obtained above are inapplicable, but pj7, q^,
being gmall portions of the paths described simultaneously, repre-
sent the linear velocities of p and q, and
vpxcoapvm=Fm = qn-qqxcos<iqn,
therefore rp : qq : : cos Q g n : cos ppm;
that is, The linear velocities of the points p, q, are to each other
inversely as the cosines of the angles which the link makes with
their respective paths.
m. To determine the Velocity-ratio in Contact Motions.
163. Let A,B, Fig. 125, be the centres of motion of two pieces
connected by the contact of curved ^, 124.
edges, and m the point of contact in
a given position; and let p, q, be the
centres of curvature at the point m,
common to the two curves, that is,
the centres of those circles which co-
incide most nearly with the given
curves at that point ; and join p q,
which must evidently pass through
the point of contact, m. The motion
of the pieces through a very small
angle may hence be considered to take place round the points p, %
as centres, and therefore the line of action, pq, will be equivalent
to a link, pq, connecting the arm<(, ap, bq. Join ab, meeting
PQin T, then by the preceding (162) the angular motions of the
anns, a p, b q are to each other as the segments, b t, a t, and
p Q is the common normal to the two curves, that is —
In the communication of motion hy contact^ the angular motions
of the nieces are inversely as the segments into whicn the common
normal divides the line ^centres.
lY. To find the Amount of Sliding m Contact Motions.
164. Let A, b, Hg. 125, be the two centres, m the point of con-
88
PRIKCIPLE8 OF MBCHAHI8M.
Tig, 126.
tact of any two pieces a m, b v, and m d
the common normal; then suppose the
curves to move into new positions shown
bj the dotted lines, ana let m be the
new point of contact, p and n the new
positions of the points that were in con-
tact at M.
Since every point oi mn must hare
been in contact with some point of mp,
during the movement from the first
position to the second, a sliding of the
surfaces on each other must have taken '
place eanal to the difference of mp and
mn. Join pn, which will ultimately
represent this difference, and become a
right line perpendicular to the nonnal
is. D ; also with the centres a and b de-
scribe the circular arcs m|>, m n, which
are ultimately perpendicular to am, bm.
Hence in the small triangle upn the
sides Mj7, M n, j9 n, are respectirely per-
pendicular'to am, bm, md, and conseouently the latter lines make
with each other angles equal to those oi the small triangle, therefore
pn sin jp M n sin a m b ^
p M sin ^ n M sin b M D '
in which expression -^ is the ratio of the sliding to the entire
pu
motion of the point of contact in one of the pieces, b m d is the
angle between the nonnal and the radius of contact of the other
piece, and
sin AMB=iBin (bam+abm),
as the sine of the sum of the angular distances of the radii of con-
tact from the line of centres.
a> n 1 \.L • P^ sin AM B
Similarly we obtain • ■^— ^-r- .
' flM SmAMD
165. l^Vom these expressions it appears that in the small triangle
pnUjpn can become indefinitelv small, compared with n m or jp m,
only when sin a m b vanishes, that is, when the radii of contact
coincide with the line of centres ; but when p n vanishes, there is no
ilidingj and the contact becomes rolling contact. Hence it appears
that
In rolling contact^ the curves must he 8o formed that the point
of eoTitact shall always lie on the Une of cevires,
166. Also, as the line of centres and common normal both pass
through the point of rolling contact, it follows from (163) that
In rolUng contaetj the angtdar velocities are inversiy as the
segments into which thepoiiU of contact divides the Une of centres.
BOLUKQ COHTAGT.
89
167. In wrapping connexions, the yelocity-ratio is the same
a£ has been already foaod in link- work (162);. for in any given
pod^n, the action of two pieces connected by a band is the same
as that of two rods drawn from the centres to the points of contact,
connected by a link ; hence in this case also,
7%e angular velociHes of the nieees are to each other inversely
as the segments into wMch the oonneetor divides the Une of
centres.
168. If the Une of direction of the link in link-work, of the
common normal to the curres in ooutact motion, and of tne con-
nector in wrapping motion, be severally termed the Une cfactionf
the preceding propositionB may be considered as particnfar cases
of tne more general condition, that
I%e anguuar velocities of any two consecutive pieces are to each
other inversely cu the segments into which the line of action di-
vides the Une of centres; or inversely as the perpendiculars from
the centres of motion upon the Une of action.
169. In the preceding propositions, each pair of connected pieces
has been supposed to have circular motion in the same plane
round fixed ^mts ; many of the conditions existing with regard to
circular motion may be extended to rectilinear motion, by consi-
dering the rectilinear motion to take place in a circle with an
indefinite radius. Thus the conditions of motion of a rack and
pinion are the same as those of a wheel and pinion. These general
conditions having been established, some of the more important
elementary combinations may now be separately considereo.
CL. A : Drv. a. — Communication of Motion by BoQing Contact.
170. As in this case the point of contact must always lie in the
line of centres (165), and ifivide that hne
into two segments haying a constant ratio,
the yelocity-ratio being constant (167), it
is clear that none but surfaces of reyolu-
tion can fulfil the required condition. If
the axes of motion be parallel to each
other, portions of two cylinders, the axes
of which are parallel, and the radii of
which are in the required yelocity-ratio,
iviU attun the proposed object.
If the axes be not parallel, but in one
plane, two cones, or any portions of them,
the vertices of which coinoide with c,
Fig. 126, the intersection of the axes of
motion ag, bo, and of which the corre-
sponding radii, a d, b d, are in the given
velocity-ratio, will fulfil the required conditions ; for it is evident
that the drcumferonces of all corresponding sections of the two
JV.126.
{
90 PRniCIPLES OF ME0HANI8M.
cones, made parallel to their bases, that meet each other at anj
point in the hne cd, will have a constant ratio, and will therefore
roll together.
171. If the axes be neither parallel nor meeting each other, the
required surfaces of contact will be traced out bj the revolution
of a line, intermediate in position between the axes, round each of
them successively. These surfaces are called hyperboloids.*
172. In the practical application of rolling surfaces it is found
necessary to cover them with leather or some other yielding ma-
terial, and also to allow a sufficient mobility to one of the axes to
ensure their contact by means of pressure : but the application of
such means of communicating motion is very limited
Fig. 127. A new mode of fnctional gearing has been devised by
Mr. Robertson of Glasgow, which is represented in
Fig. 127. In this a series of equal acute angular grooves
are turned in the circumference of the wheels, which fit
^ into each other, and possess a considerably increased
amount of holding surface. The amount of hold, or
resistance to slipping, will depend partlv on the acute-
ness of the wedge-shaped edges, and Partly oc the
pressure on one axis towards the other. This will pro.
oably work more satisfactorily than any kind of rolling
contact hitherto proposed; but the necessary mobility
of the bearings of one of each pair of contiguous axes is
always objectionable.
If great accuracy in the relative motion of the driver
and mllower be required, as in clock-work, or if any con-
siderable resistance must be overcome, as in mill-work,
the contact motion is communicated by toothed wheels ;
the employment of which is as extensive, as that of simple rolling
contact is limited.
173. The total action of toothed wheels upon each other is
analogous to rolling contact, because as equal lengths of the cir-
cumferences contain an equal number of teeth, they must evidently
pass the line of centres in the same time ; but the action of the
individual teeth upon each other is by sliding contact, and will be
subsequently considered. In reference to large wheels, the por-
tion of the circumference occupied by one tooth and one space
between two consecutive teeth, is caUed the pitch of the wheel,
and two circles which would roll on each other m the same manner
as two given wheels actually roll, are called the pitch circles^ or
geometrical circles; the latter term is used by manufacturers of
clock and watch-work. Oearing is a term applied to trains of
toothed wheels : they are said to be tti gear when their teeth are
engaged together, and out ofgeoTj when they are disengaged from
each other.
* See Hjmen' " Analytical Geometry," p. 142, or any other tareatiie on
the same labjeot.
TOCrmED WUEIU. 91
IT4. Toothed vheeli with fevteetb its termed ^'nioiu, tnd the
tcFtb of these, Iranei, because thev are iiiu»U; made much longer
in Ihd direction uf the aiia ihaa tie teeth of larger wheels, for the
uke of rtrength, 'JTie teeth of
vheeli may be made either in ooe ■W*- "*■
piece with the rim, as in vatch-
vlieela, or(;onnatoriepsrat« piece) —
(nmed into the rim ol' the wheel,
an in large mill work ; the teeth are
then cslli^ aw». The former me-
thod is miiaUj adopted in metal
nhee] work, and the lalter in
w^ioden wheela. The small prwbeela,
or pinions, in wooden wheel-work,
fr«|uentl7 cnniiat of two parallel
disrs, separated b; an inlerval a
little wilier than the thickness of
the wheel : this space is traversed
hj ■ series ofeqoi Ji slant ejlindrical
pins, called ttava, between which
the o^ saccessiveij pass; awheel
thus cnnstmcted is called a trutulie,
nr lamtm. The con are made of some well-seasoned hard wood,
SI mouD tain-beech, tiombeam, or hickory, tha grain of the wood
being in the radial dirccti^in ; these are driven into grooTes or
tnortioei in the rim of the wheel, and secured by pins passed
throneh ihem inside the rim. Fig, 128 represents a large wooden
o^-wueel. A, aad trundle, a, as usuallj constructed in mill-work.
The pinions of Dolch clocks are umilarl/ constructed ; in then
the pins are iron wire.
175. In modem mill-work, the wheels are nniall; of caat-iron ;
bnt when the wheels are very large, and the power transmitted
considerable, they are found to wear better and to work much
more smoothly, if one of them, uaoally the larger, is supplied with
wooden digs, instead of iron teeth;
a wheel thus constructed is called ^- '••■
a mortUe-vh^.
1T6. In the wheels hitherto con-
sidered, ihe leeth have been sup-
posed to stand out radiallj from the
rim ; these are called tpurvhetU.
If the teeth project sideways from
tbo face of tha wheel, '
pinion ofao ordinary vertical walcli :
m which the axis a, is at right
angles to the szi* b. In crown-wheels tha rei^aiaite s
92
PBIKCIPX.E8 OF MECHANISM.
obtained by giving snfficient depth to the rim, but its thickness
mast be inconsiderable, otherwise the amount of wearing would be
sensibly increased by the oblique action of portions cf the surfaces
in contact. For the same reason, the diameter of the pinion must
be small compared with that of the wheel.
In some cases the place of teeth is supplied by equidistant pins,
standing out perpendicularly to the face of the wheel, as in Fig.
130 ; wheels tnus constructed are ctMed pin-wheeU ; Uiey are now
rarely used, except in clock escapements.
Fig. 190. Fig. 131.
If a wheel be required to drive a pinion in the same direction
in which it is movinfi", the teeth are cut in the intide of the rim, as
in Fig. 131. A wheel thus constructed is called an annular
wheel; the action is very smooth, but it is seldom employed,
owing to the difficulty of construction.
177. If the motion required be that of two conical frusta (170),
the teeth are cut on their surfaces, as in Fig. 132 : these are called
bevil-toheele. In these, the teeth and
•^* 1^ spaces are all directed towards the
apices of the cones, a, and thus con-
tact takes place along the whole sur-
face of the tooth, and not in points
only, as in the crown-wheel and pinion
(176).
^. U3.
..A^
178. If the path of one of the pieces be rectilinear, the teeth
are then cut on the edge of a straight bar, and the piece is called
a rackf the extent of motion of which is practically limited by its
length ; Fig. 133 represents an ordinary rack and pinion.
179. A much greater smoothness of action may be obtained by
cutting the teeth obliquely on the rim of a wheel and pinion, as in
Fig. 134, but the construction is more difficult than that of ordi-
naiy teeth, and the obliquity of the teeth produces an endwise
rOBMB or THE TUTH OP WHBBLB.
93
preasare on the axiB, which would in man? cases be ^g- 134.
objectionable ; especialJy where considerable power
is required to be transmitted.
Having now sufficiently considered the general
formfl of wheels, the forms of the individaal teeth
belong to the second division of the subject, as
the action of one tooth on another, is that of sliding
contact.
Cli. A : DiT. h. — Chmmumeatian of Motion
hy Sliding Contact,
180. The axes being supposed parallel, it has
been shown (163^ that the angular velocities are
in the inTerae ratio of the segments, into which the
normal to the curves at the point of contact divides
the line of centres. Hence when one toothed wheel is driven by
another, it is necessary that the normal to the point of contact of
any two correniionding teeth should always pass through the point
of contact of the pitch circles, that being tne point at which the
line of centres is. divided in the re-
auiied ratio. Let a, b, Fig. 135, be ^V' 135.
ie centres of the pilch circles h l, k m
in contact at t, and let the tooth h d L
be generated bv the revolution of the
carve t s d on the outside of the pitch
circle h l ; and the corresponding space
KDM, by the revolution of the same
curve, T X D, on the inside of the pitch
circle k m ; then if the tooth and space
be in contact at o. the normal to the
point D will pass through t : for if the
generating curve be brought into the
position THD, so as to touch the circle
H L in T, T D will be a normal to h d at
d: and that the curves hl, km, may
be in contact, the generating curve must
toQch K M in T, that is, it must be in
the same position for both the curves,
H L, K M, and consequently t d must be a normal to both, that is,
they wiU touch in d, and the line of action will pass through t.
181. In order, therefore, to find the form of the tooth of a wheel,
that will work correctiy with a given tooth of another wheel, it is
ceoesaary to find the curve which by rolling on the convex surfaoe
of the pitch circle of the latter, will generate the curve of the given
tooth, which may always be done ;* then the same curve, by rolling
on the eoneatfe surface of the pitch circle of the former, will eenerato
a space in which the given tooth will work correctly. In order
• See Airy on the Teeth of Wheels ; Camb. FhiLTr. vol ii. p. 279.
94
PRIMCIPLBS OF UECHAMISir.
Fig. 136.
that the solution may be practicable, it is necessary either that the
convexity of the tooth shoald be greater than the concavity of the
space, or that the two curves should be convex towards each other.
182. This problem also admits of a simple mechanical solution.
Let the curved edges of two boards, a b. Fig. 136, be portions of
the given pitch circles. Attach to one
of them. A, a piece of card-board, or
other convenient material, c, having
the shape of the given tooth, and to
the other, b, a piece of drawing-paper,
D, the piece, c, being raised a little, so
as to allow o to pass under it. 1 hen
keeping the circular edges of the boards
in contact, and making them roll to-
gether, the outline of c may be traced
on D in several successive positions. A
curve, e/, which touches all these suc-
cessive outlines, will give the required
form of the tooth of b ; for, from the
mode by which it has been obtained, it will, if. cut out, touch c in
every position, and therefore the contact of the two curves will be
equivalent to the rolling action of the pitch' circles. A solution
thus obtained is evidently impracticable, if the convexity of any
part of either curve be not greater than the concavity of the
opposed portion of the other curve. Having shown how the general
solution of the problem of the requisite forms of corresponding
teeth may be obtained, we may now ascertain the forms which
may be most conveniently applied in practice.
183. Two particular solutions of the preceding general problem
have been employed in practice. In one of these, the generating
curve is a circle, and the curves described
by a tracing point in its circumference,
while rolling on the convex side of one
pitch circle, and the concave side of the
other, are called respectively Bpiey-
doida, and ffupocycUnds. Let a, b, be
the centres of the pitch circles, otto,
6 T n, in contact at t, and Thkthe gene-
rating circle ; and let t c be the epicycloid
described by the point t, when the circle
Tbk rolls on T m, and t f the hypocy-
cloid similarly described, by rolling on
Tn. Now suppose the three circles, ar m,
erfijTbkio roll together until the points
which were coincident at t, assume the
positions a, e, 6, then it is evident that
the point 6 must be common to both
curves, also they are in contact at 6, and
rOB10 OF THB TEETH OP WHEELS.
95
(t IB a normal to both, for an indefinitely Braall arc of each curve,
of which h is the middle point, may be considered as a circular aro
described by the radius t 6, round the centre, t : since, therefore,
the common normal to the point of contact, 6, always pisses
through t, the point of contact of the pitch circles, the Teiocity-
ratio will be constant when motion is produced by the pressure of
one of the curves, ae^ «/, upon the other. It is also evident that
6, the locus ofeotUaet of the curves a 6 c, e &/*, must alwavs be in the
circumference of the generating circle, rok. The following two
individual cases of this solution have been frequently employed.
184. When the diameter of the generating circle is equal to the
radius of one of the pitch circles, b t, the hypocycloid, e & b,
Fig. 138, becomes a radius of the pitch circle : consequently,
when epic^cloidal teeth are described on one pitch circle, by a
circle of which the radius is half that of the o4her pitch circle,
these teeth wiil work correctly with radial teeth, placed within the
ciicnmference of the latter pitch circle.
Jf^. 138.
185. When the generating circle is one of the pitch circles, the
hypocycloid is reduced to a point in its circumference : consequently,
when an epicycloidal tooth is described by one pitch circle on the
oUieT, as a 6 c, Fig. 139, it will work correctly with a small pin, 5,
in the circumference of the describing pitch circle.
186. The second^ particular solution is that in which the radius
of the generating circle becomes infinite, or, in other words, when
the curve is generated by a straight line rolling on a circle. This
is most readily effected by unwinding from the circumference of a
circle, a string with a tracing point at its extremity, and the curve
thus generated iB called the InvoitUe of the circle.
96
PBIKCIPLES OF MECHAKIBM.
f ^
Let A, B, Fiff. 140, be the centres bf.the pitch circles in contact at
T ; through t araw d t e at any angle, and ▲ d, b e perpendicuUr to
it, and with radii a d, b e draw the circles, d h, e f ; then by similar
triangles, ad : be : : at : bt.
Through the point t draw htk,
the involnte of the circle d r, and
F T G, the involute of the circle, b f ;
then by the revolution of the circles
DH, BF, let the involutes assume
the new positions, /f^, htkf which
last, in fact, are involutes described
by the point t, for from the mode of
describmg the involute, the line
D E must be a common normal to
all involutes of the circles, e f, d h,
that intersect it; hence, when the
action takes place between the in-
volutes, the^point of action is always
in the line' d e, and consequently
the line of centres, ab, being always
cut at the same point, t, by the line
of action (168), the velocity-ratio is
constant
187. If the circles, ef, dh, l<ig. 140, were made to approach
to or recede from each other, the velocity-ratio would continue the
same ; for the common tangent, e d, would always intersect a b in
the same point, t. This is a property of some importance, as it
follows that a pair of wheels witn involute teeth, described as above,
will work.correctly, whatever the distance between their centres may
be, a property not possessed by wheels with epicycloidal teeth.
As the position of the line de, was arbitrary in the first instance,
an indefinite number of diflerent pairs of involutes may be de>
Bcribed, corresponding to the same given velocity-ratio.
188. The theoretical principles on which the forms of the teeth
of wheels depend having been given, the methods b^ which the
teeth are practically constructed may now be explained. The
simplest case is that in which teeth in one wheel act against
pins in the other, as in the lantern or trundle, already mentioned
(174). In combinations of this kind, it may be observed, that the
toothed wheel is o/tpays the driver, and the pin wheel the follower.
It has been shown (185) that an epicycloid will work correctly
against a pin, which was not, in Fig. 139, supposed to have any
sensible magnitude. In order to apply this principle to practice,
with a radius at=b, Fig. 141, describe the pitch circle of the
drivinfl^ toothed wheel, and with the radius btt, that of the
following pin wheel ; and let the pitch be the arcs of equal length,
T a, T c : round the centres t and e describe two equal circles, to
represent the staves, the diameters of which are osaally somewhat
leu thin half the pitch ; and {ram the points t and a dMcribe in
opponM directians the epicy- ~ ,,,
dmd.Ti,ai. meetin^int; **'"■
joia Aiaod bc, cutting a tin
f, »nd thfl circumference of the
BtAic in b. Take any namber
of points, fun, &c^ in the
tpicjcloids, T fc, Qit, anil roncd
tun points dcacribv circular
am with radii equal la be;
and drav the curv», pb, ba,
loncbing all the circlea in the
pdnCa, q.r, &c^ then vill the
uliun of this curve npoa the
ilBie be the same as that of
llie epicjcloid upon the pin ;
ibr if any point, n, of the
ep'cvcloid were brought to co-
indde with e, then the curve p 6 and the Emrlace of the itaTewonld
tridentlf touch each other at r.
189. Id order tbat there may be no intermpUon to the action of
tlie teeth on the staves, it is neceesary that when the point ( of the
tooth p 6b it on the point of qnittine contact, and therefore on the
lineftf action re, the centre, t, of the next »tave muat be on the
line of centres a b, and that stave commencing contact with the
Mit tooth at (.
LetTBe,tbe pilch-angle of the pin-wheel=^, at — e, b T=r,
theiiBA&.half the pitch angle of the toothed wheel will be )-^,
Wanse the pitch-arcs of the two circles are equal; and let k
be the ratio of the diameter of the pin to the pitch, then c b, the
ndina of the pin,'^ i krf. In the triangle b as
?giii=(^.)-_i^^ 0)
im7«6" siB(ABe-hflAe)
br aDcbe = tm{bee+bee),
-ain^I-lABO + ABS+BAsV
= CCs(iAB« + ,A«);
tlienbjgnbstitiitiDg the valoe of CB in (1), and redDciDg,nc obtain
98
FRIKCIFLES OF XBCHAJnSM.
B7 Babstitating in this equation any particular yalue of ^ expressed
in parts of radius, and also of—, the necessarj value of k may be
T
obtained, wbich will cause one tootb to commence action, at the
moment that the next is ceasing to act.
Should the value of X; come out negative, the case is thus shown to
be impossible; and if X;=o, the pin cannot have any sensible thick-
ness. In practice it would not answer to construct wheels so that
one pair of teeth should quit contact at the instant of commencing
contact of the next pair; for in that case any error in the form of
the teeth would be very injurious; if the error were in excess, the
teeth would lock into each other and break ; if in defect, either
originally or from wearing, an interrupted jarring motion would
result. The constant difference between the width of a tooth or pin,
and that of the space in which it is to work, is termed h<ich-Uuh.
The following table shows that diameter of the stave in parts of
the pitch, which will just allow one tooth and stave to quit contact.
Value of?.
r
Number of Teeth (T) or Staves (S) in
the Pinion.
8
4
5 6
7
8
if nick
0-34
0-73
0-61
+
+
+
+
+
+
+
1
i
1
—
0-68
+
+
+
+
_
0-61
0-46
+
+
+
+
+
+
+
+
.T
—
0-37
+
+
+
+
4
«
018
0-59
0-37
+
0-63
+
0-75
+
+
1
—
—
—
0
0-38
0-67
2
—
—
0-20
051
0-66
+
8
—
—
0-39
+
+
+
4
—
0-01
0-46
+
+
+
5
—
0-10
0-60
+
+
+
/<%
6
—
016
+
+
+
+
1
8
—
0-22
+
+
+
+
10
—
0-26
+
+
+
+
m f r*©^
^ (drives.
—
0 38
+
1
+
+
+
/
when the adjacent ones commence. The impossible cases are
marked — , but when the si^ + is inserted, the least necessaiy
diameter of the stave is considerably greater than half the pitch ;
and consequently all such cases may be employed in practice. But,
for titt reMUta abore giTen, it ia oecenar? in pnctioe to lUov
mm leatli to the wheel, or to gire ths ttan lera diameter, tbkn
ihs table ilioin to be •diniNible.
ExampU. A wheeliiieaoiredtodriTe apimonof one-foiuthof
iti diuneter ; reqaired, tlie leut number of tseth and ttams that
MS be employed.
Od lefening to the Hne in the table in wbicb — = 4, it aj^iean
tint if (our itaTM aie given to the pinion, and conxequentlflilleen
teclb to the vheel, the diameter of the itaTS cannot exceed the
hvidredth part of the pitch ; and if five staves ere given, their
diameter mmt be coaiiilerably less than half tlie pitciL. In pnc-
liM, therefom, it would not be lafe to etuploj ka namben thu 6
ud t4, or 7 and 2r
190. Teeth and etavee of the form ieet described wov ta mush
ire eenerai nae fnmerly , wben irood vat more niiplo;ad than it
it the preaent time, in the oonatmctioD of nuQ-wiuk. Li vbeel
weA la now conotmcted, both the driver and Eollowef an oaaaUy
ni]>pti«d with teeth ; and the form commonlj adopted i* a ewn.
bioatian of the radial line and epicycloid (183, 184). A paur of
■heela the teeth of which are Ihna
eeortracted U repreKQted iii Kg. **■ ^*
Hi, where A and B are the centre!
gf the pitch circles in contact at T ;
tbe complete side of each tooth, as
CTO, dtAto, coneista oftwo parts,
one of which, ct, ot&t, liesuttjUii
Ibe ratch circle, and ia called the .
fan; and the other, i a, or tg, |
Bee ioitlioat, and is called the /oee )
Df the tooth. The Saake of the
teeth ia both wheels are radial
lines, and the faces, epicjcioide ;
of which T a is geoeraied bj the
dnzltTJbB, andT^ b; t/a. The
tonn of the curve ede, which con-
oectt two coneecntiTe teeth, is
immaterial; it is onlv necesearf
that in the action of the teeth it should keep clear of the pointy;
it is iberebre called the cUaring. As the teeth of both wheels
an similarly conBtmcted, and are ejmmetrical with regard to a
radial line produced to their points, either maj be emplojed to
drive the other, and in either direction.
191. To examine the action of two coiresponding teeth, let the
lower wheel drive the DpperinUiedirectionofthearcow; thenthe
senicirda a/t will be the locni of contact in approaching the lino '
of centres, and TjfcB,iD receding. The contact, tiierefare,Degins at
tbe root irf the driTer'a tooth, and ends at the point, and ^ reverie
100
Uksa place vitb the follower ; ud tbe length of f>ce is inneb
greater thin the correapondiDg teagth of flank, forifvith ■radius
B^BoiroleiB described cnltiiigT/A in e, and rith a rsdins as, a
drale 8 (, cutting a t in (, then t and t will be the extreme points
of contact of the flsnlc tc, and t( is manifeatly mach ihorter
192. It bos been ibown (IM^ that BpicTcloidul _t««lh de-
»crib«don one pilch circle, by s circle of which tbe diameter is
half that of the other pitch circle, will work correctly against
_ ,„ ndial lines on the latter. In Fie. 143 let
**■'"■ A be tbe centre of the driyer, and a that
of the follower, and t the point of contact
of tbsir pitch Hrclea ; t d a a tooth of the
former, and ndm a radial line or tooth of
e latter, wilh which tbe face ad baa
ten in contact during its motion trnoi T to
The semicircle rda is the locus of
contact ; let the apex d of tbe tooth rad
be quitting contact at tbe moment that the
leit tooth ia commencing, (hen d will be
n tbe aemicircle rdB, and the base of the
leit tooth will coincide with t. Join bd.
hen if ij were a pin in a wheal rds, Tbd
woold be the pitch angle ; but in the pre-
t inatance Tnif is the pitch angle, which- ^Tiii; it follows,
tberafbre. that
The taut numher of radii that mill vmrk icith a given mmiher
ef epieydoidal teeth it tqaal to twice the leatt RmnMr of puw.
193. The line radim of a wheel exceeds the geometrical or
pitch radios br the distance of the point of the tooth from the
pilch ciirle ; this quantitj is called the addendum. If it ia the
true radius, and athe nnmber of teeth of tbe driver, andu, w, those
of tbe follower, the Tshie of- in mill-work is usnallj — -„ nearly,
in which case the addendtim is abont y'j of tbe pilch for both
In dock-work, however, a different valoe U aaaignod bj a recent
antbor on this snbjecl,* namely,
which gives the addenda lo the driver and follower | and J of tbe
pitch ieiipectiTalj,+
194. The following practical roles are Iboie commonly adopted
in the tonitmction of mill-wheels, a portion of a pair of which in
gsaris represented in Fig. 144, man, e as, being the pitch lilies i
• B«ld-| Bomlogj, p. 114. t Bea WiUli' PrlMiplM of Uinhudsm, p. ST.
^^ •'
LBSeTH OF TBITB.
101
Fig.l4L
n
>f
tt
tt
Addendnm, or depth
to pitch line, Je=^ pitch.
Woilang depth, df=^
Whole depth, <^ ^ = iV
Thickness of tooth, 00=^
Breadth of space, he=^
It here appears that a back-lash of
^th pitcQ is allowed to preTent the
teeth from locking, and ^^th pitch
is alloired in depth to prevent the
teeth from httUuig, or striking the
bottoms of the spaces: these pro-
portions, however, differ slightly in different localities.
195. The necessary length of the teeth of wheels will depend on
the conditions of the contact that takes
place between them. Let a, b, be the
centres of the driver and follower, t the
point of contact of their pitch circles,
and d the point at which the tooth of
the driver is just quitting contact. Join
xdf cutting the pitch circle iny, and
join Bd^ err, then, as contact is ceas-
ing at d^Td must be perpendicolar to
dm. The arc t/ is called the arc of
receding actum. If the diagram were
reversed, and the wheel b were sup-
posed to be the driver, then t/ would
be die arc of approackkig action.
Fig. 145.
LetAT=B, BT=r,/rf=:B, andTBD=tf. We have
Ad"=AT"+Td"— 2 ATXTcicoe atJ;
snbetitadng the valaes of these quantities, and reducing, we obtain
by expanding this in series, and neglecting the higher powers of 9,
we obtain »^2Br+r'
B 2 B« •
B and n being the nmnber of teeth in a and b respectively, and c,
the pitch, we have 0=?^?!?=?^** .
n
103
PRI1IC1PLB8 OF MBOHAmSM.
and if the arc of action t m, or r 0=f c, by sabBtitoting and re-
dncing we obtain - == "* I ~+ - ) •
0 \ll 11/
Similarly, b^ reverBing the diaffram, and making the arc t m that
of approaching action, e the addendum of the follower, /the ratio
of the arc T m to pitch, and interchanging n and n we shonld obtain
whence
from which formula the relative values b and e may be obtained
by Bubstituting appropriate Talues for f and /, n and n, it being
remembered that at least f + /must= 1.
The friction of two corresponding teeth that takes place before
reaching the line of centres is accompanied by greater pressure,
and ifl of a more injurious character than that which takes place
during the receding action. It might hence be supposed desirable
to render the are m approaching action as small as possible ; but,
on the other hand, the amount of sliding, and consequently of
friction, increases rapidly with the distance of the point of contact
from the line of centres, and hence (since the sum of the arcs of
approaching and receding action must at least be equal to the
pitch) as much, or perhaps more would be lost than gained, by
g'ving a very small value to the are of approaching action : — ^the
>st method, then, is so to adjust the adaenda, that there may be
less action before the line of centres than ailer it.
196. The following table of the values of -is calculated for
c
Value of 11
n
Correspondingvalues i
of- when
e
• V
F = 2/.
F=V2/.
1 F=/.
Back follows . . .
0
2
1
0-5
rxV
2-3
11
0-6
2*4
1-2
0-6
Pinion drives . . h
^
2-6
1-3
—
•^
2-8
1-4
0-7
^
3-2
1-6
0-8
f 1
4
2
1
2
5
2-6
1-2
Wheel drives . . .
4
6
8
1-6
•
6
6-5
3-2
1-6
iio
7
3-3
_-
Back drives . .
QD
8
4
2
rOBMS OP TBB TBBTH.
103
thiee different ratios of the two arcs of action, namely, when the
arc of approachine action is equal to, or two-thirds nearly, or one-
half that of Tece£ng action.
Example, — In clock-work tiie wheels always dri^e the pinions,
and the ratio of their nomhers Taries from 8 to 10. From the last
colomn of this tahle it appears that 1*5, the ratio of the addenda
in Mr. Reid's nile, is scarcely enongh to gire an equal action he-
£)re and afYer the line of centres, and that it wonld he hotter to
take a ratio of 3, which would give the simple rule
D w + 3
tt^n+l'
This role gives an addendum of ahoat k pitch to the driver, and
^ to the follower, and may safely he adopted when the wheels
drive ; but when the pSnion drives, it appears that it will be safe
to employ
197. The forms of teeth determined by the preceding articles
are those most commonly employed in practice ; Imt they are sub-
ject to this inconvenience, namelv, that any given wheel will work
correctly only with that for which it was desiffued, since the form
of the teeth of each depends on the radius of the pitch circle of the
other ; and in the manufacture of cast-iron wheels, now mostiy
employed in heavy machinery, this would be a serious inconve-
nience, as it renders a multiplicity of patterns necessary for wheels
even of the same size and pitch. In order to obviate this, when-
ever a series of wheels are required to work with each other indis-
criminately, it is^ found desirable to employ the radius of the least
wheel of the series, as the diameter of uie common generating
circle for all the curves of the teeth, consequently (180) the faces
of the teeth will be epicycloids,
and the flanks hypocycloids, and ^' ^^■
any one wheel will work co^
recUy with any other of this series.
198. Involute teeth (186) di£for
from the epicydoidal teeth al-
ready described in having . the
entiro working surfSsu^ of the
tooth, both &ce and flank, formed
of a continuous curve ; the side
of an epii^cloidal tooth being
made np of two difierent curves
jcnned at the pitch circle, f^g. 146
rroresents a ||ortion of a pair
of^ wheels with involute teeth, in
which A, B, are the centres of the
pitch circles in contact at t ; a d,
B 1^ the radii of the bases of the
involnte^ and ]> e their conmion
104 PBIHCIPLE8 OF MBCHAHIBM.
tangent, which is the locus of contact of the teeth (186). As in the
preceding forms of teeth, the point of approaching contact lies
within the pitch circle of the dnver, and of receding contact within
that of the follower. Let the point b, of the tooth e m of the driver,
be jast quitting contact at e, then r is the extreme point of action
of the follower. With a centre a, and radius ▲ e, describe an arc
K kj cutting A B in ib ; then the point b will coincide with k on the
Une A B, and a clearing hollow of at least the depth of k must be
formed within the base circle, as in the figure. Let h be the point of
the tooth H F of the follower ; with centre b, and radius b h, aescribe
an arc H A, cutting d t in A, then h will be the first point of contact,
and the approachii^ and receding actions will be as ^t : te.
The chief objection to involute teeth rests on the obliquity of
action, which is always in the line d e, and by which a consider-
able amount of pressure is thrown upon the axes of the wheels ;
this is not the case with epicycloidal teeth, in which the action is
perpendicular to the line of centres, at the moment of crossing
that line. Their peculiar advantage consists in the property of
working correctly at different distances of these centres firom each
other. The distance of the centres of a pair of involute wheels
may be so a4justed by trial, that they will just pass each other,
by which means the back-lash is reduced to the least possible
quantity : this is an evident advantage in the cgnstruction of
those machines, such as dial-work, in which the object is the
transmission of correct motion, not of working power.
199. In the practical construction of the teeth of wheels, a cir-
cular arc may be found which will differ from any proposed curve
by a quantity quite within the limits of error of workmanship ;
but the methods commonly adopted for finding the required centre
are merely tentative. The author already mentioned has fully
investigated this subject ;* and has devised a very ingenious in-
strument, the OdcnUogrqphj by means of which the forms of the
teeth of large wheels may be traced with as much accuracy as is
practicable.
200. In concluding this subject, it may be remarked that the
chief points of excellence in the construction of toothed wheels,
are iodformUy of action^ durdbilUy^ and strength. The first is
attained by the epicycloidal form of the teeth ; tne second, by so
arranging the form, that the curves may roll on each other, with
the least attainable amount of sliding, and also that the approach-
ing may be smaU, compared with the receding contact ; and the
third, oy enabling as many teeth as possible to engage with each
other at the same time.
201. In the preceding investigations respecting toothed wheels,
the wheels have been assumed to be in the same plane ; when this
is not the case, bevUled wheels must be employed, of which the
pitch surfaces will be two conical frusta (170), which will roll on
• WflliB, Prinoiplet of MeohaniRD, p. 123 tt ««29*
THB EKDLESS BCREW.
105
Vig. 147.
each other willi the required velocity-ratio. The forms of the teeth
of these may be determined by deacribiag the requisite curves for
the neater and lesser bases of the conical frusta, considered as
pitcS^rdes. By tracing these curves in corresponding positions
oo Uie bases, and cutting away the substance of the cones until a
strsight line, passing through the common apex of the cones, will
touch both curves, and the intermediate surface, a series of teeth
will be formed that will work correctly with each other.
- 202. When the axes of the required motion are perpendicular to
each other, bat do not meet, a constant
velocity-ratio may be obtained by a screw
and nut (143), in which the nut will
advance throngh the space of one thread
during each complete revolution of the
screw. A screw is called right- or left-
kaoded, accordingly as a line touching the
|hread rises to tne right or to the ien, as
in Fig. 147 : the direction of the motion
of the nat, in relation to that of the screw,
win be determined accordingly.
203. The threads of the screw may be
in contact with a rack instead of a nut, as
in Fig. 148, in which case the linear movement of the rack will be
tbe same as that of the nut. When the teeth, instead of being on
a rack, are cat on the circumference of a wheel, as in Fig. 149, the
I^,\4»,
ng,\4B.
combination is called the endUia icrewj which is frequently em-
plojed in machinery. In the rack, the motion is limited by the
tsngth of the rack, or of the groove in which it works, but in the
enUesB screw the motion is unlimited. The form of the teeth is
di&rent from that of ordinary teeth, in consequence of the oblique
action of the screw : the best mode of obtaining the requisite form
ii, after roughly catting the requisite number of teeth in the wheel,
to convert the screw itself into a cutter, by making one of steel, and
cnttine notches in the threads of the screw in the direction of the
axis. By rotating this with pressure against the teeth of the wheel,
the oeoessaiy ibm of the teeth will be obtained.
204. If lbs incliDAtion of the tliread of
Ihe screw to the aiia be conBiderable,
one or tnore intemediatetbreadB ma; be
added, aa in Fig. 160 : in which caae the
screw is said to be double or triple, ac-
cording to the number of separate spiral
threads. As each otie of these will pass
its own wheel-tonth across the line of
centres in each revolutioa of the screw, it
follows that as many leeth will pass the
line of centres during each revolution of the
screw, as there are threads in the screw.
20D. If we suppose the number of threads to be ver; great, for
Fta- W. example, equal to that of the wheel-teeth, then
the screw and wheel may be made exactly
alike, as in Fig. 151 : this is an example of
le of the disguised forms, which some com-
gement maj be made '
S06. Where great smoothness, and the
' ' ' Dt back-lash are required in a
„ ati»e motion, they may be at-
1 by means of the ingenious device of
Messrs. Callen and Riplejr, the principle of which may be thus ex-
plained : — Describe two circles touching each other at c (Fig- 152),
,, and let 1 0 be the radius of the larger and the
"'' diameter of the smaller- Draw any line A B
I cutting ths two circles in d and E, and having
bisected a c in a, join i> a. Then, because the
angle d a c is double of k i c, but the whole
circumference DC equal to half the circum-
ference EC, it follows that the arcs CD, CB,
are of equal length ; and cousequently, if the
inner circle rolTin the outer, the pomt a in
the circamfcreDoe of the former will always lie
in >. a, the radius of the latter- And conae-
qnently if d b were an arm revolving round a, with a prqjeding
Tig. Ita. P"* •' "• "'^ ■*" "S" * pi*** t*-
volviw roond a, and having a dia-
metnu groove bf, tbe pm will
aiwan bo in the groove, if they
revolve with unifbira velocities in
9 the ratio of two to one, and if
■ there be two or mo)^ correspondinB^
[Hns and grooves, eiUier of the axes
carrjing these may be employed ta
drive tie other.
207. Fig. 1S3 represenU an ar-
tkngemant of thia kind, in which tb*
mupnvo oonrecTOBB. 107
mM A curies three eqoidiBtaiit arms, fnrniBhed with friction loilera,
and the pUte on the axis b, three equidistant grooves : this is
probahlj the most convenient number ; and either ▲ or b may
be advanta^naly employed to drive the other. If the leqnind
veioci'tv-ntio be anv other than 2 : 1, the grooves will be nypo-
cydoids, but the velodty-ratio will always hd that df the number
of pins and grooves respectively.
CL. A : Dmsiom e. — (hmmunieation of Motion by Wrapping
Connectors.
206. Any two curves revolving in the same plane, whose wrapping
eonnector cats the line of centres in a constant point, will main-
tain a constaot angular velocitv-ratio (167). In practice, sarfiioes
of revolution are employed, which revolve rouna their axes, and
niaiiifettly possees the required property. In
order that the communication of motion may ^' ^^
be cantinuoufl, the two ends of the wrapping
ooonectoc are joined, so as to form an endu
leas band, which embraces a portion of the
paQey, and is stretched sufficiently tifht,
that the friction of the band on the pimey
(56) mayexoeed the resistance to be over-
come. The band may be direct as at a, or
crossed as at b, in Fig. 154 : in the former
case, the axes will revolve in the same, in
the latter, in the opposite direction, as indi-
cated bj the arrows. Motion communi-
cated in this manner is remarkably smooth
and free from noise, and any sudden slight checks or inequalities
of motion are relieved by a yielding of tiie band. Endless bands
aie commonly employed in all kinds of machineiy, in which a
very eocact velocity-ratio is not necessary ; and they are capable of
tnnsmitting large amounts of force.
309. Bands may be either round or flatf and are formed of
various materials. The round bands most commonly employed
in machinery, are either cat^t or gutta percha ; the ends of a
catgat band are sometimes umted by splicing, but more frequently
hy a hook and eye, both of which have a screwed socket, into
which the ends of the got are forced by twisting, having been
pcevionaly dipped into a little rosin, and the hook or eye warmed
to keep mb rosin fluid while the^gut is twisted in. The dn^gging
out of the socket when in use may be further prevented, by searing
the end of the gut protruding through the socket with a hot wire.
Crutta percha Mmds have this advantage, that their ends may be
completely united, without any bulging or increased diameter, by
meltmg the extremities with a hot iron. The use of hempen
ropes in ooane machinaiy is now frequently superseded by the
108
PSIM0IPLB8 OF KBGHAVXSM.
I%ff. 165.
f?Ofl
rMi
m
M
C
O
e
emplojinent of iron-wire ropes, which poflseas &r greater dura-
bihty, if exposed to much attrition.
AVhen considerable power is required to be transmitted, flat
bands are commonly employed. Leather belts were formerly
almost universally employed in large machinery, but are now
frequently superseded by those of gutta percha. ^ In mining work
flat bands are employed to prevent twisting durine the ascent or
descent of the backet : the hempen buids formerly employed for
this purpose, are now generally superseded by woven wire bands.
210. The form of the groove in the pulley is important, as it
affects the adhesion of the band : the
principal forms are represented in
Fig. 156, in which the groove of a is
angular, of b, circular, and of c, flat.
' The groove may be supplied with a
series of teeth, as d, or a series of
pins, as e ; both these forms give
more hold to the band, but at the
same time they greatly increase the
wearing, unless the connector be a
chain, the links of which are adapted
to the successive pins or notcnes.
The pullepr p has no groove, but, on the contrary, a slight con-
vexity at its middle point.
211. If a tight flat band run on a revolving oone, the direction
of the approaching band being perpendicular
to the axis, as in Fig. 166, it will generally
advance towards the hase of the oone, instead
of receding from it, as might have been ex-
ricted at first sight ; for the edge of the band
towards the base is more stretched by the
increasing diameter of the cone than the
other edge, and consequently has a constant
tendency to become convex, and to assume
the position b h ; the commencing convexity
at b thus continuallv advancing towards the
base of the cone. Advantage is taken of thia
curious property in foimine pulleys for flat
iMinds, which are made a little convex in the middie, as f, in the
nreceding figure, and the band, haviuff equal tendencies to recede
from either edge, remains in the mid<ue of the pulley. This form,
besides its greater simplicity, enables the band to be more easily
shifted from an attached to an unaltachtd pulley, and thus thrown
out of gear. This convex form is much more effectual in retaining
the band, than the edges or flanges of o, Fig. 156; in &ct, if the
form of c be employed, the band will ^^enerally make its way to the
top of one of the flanges, and remain there, or be jammed into
one oomeri and will rarely be induced to maintain its intended
position.
MODE OF BBIFTXHa BABD8. 109
212. In order to render the angle of contact between the band
lod pplfej aa large as poedble, for the pnrpoee of increasing
the friction necesaarj for transmitting a given force (56), it is
deniable to cross the band, whenever the nature of the machinery
▼in permit it; it is also desirable, when possible, to incline
the axes of motion slightly to each other, in order to aUow the
two mrtions of the band at the point of crossing to pass withoot
touching, and oonseqtiently nibbing against each other. When a
flat band is crossed, it is desirable to twist it half a tnm before
placing it on the second pnUey : for in conseqnence of the twist,
the flat snrfaces of the band will be opposed to each other at the
point of crossing, and will be less likely to come in contact than
the edges ; also, if a leather belt be employed, it will enable both
poIleyB to be in contact with the rough side of the belt.
213. An endless band of any kind is easily shifted, during its
action, into a new position on a cylindrical drum, or from one
cyiindrical pulley to another of the same size
(210), if the advancing ^portion of the band
be drawn aside in the required direction ;
bat the same lateral pressure on the re-
tiring side will have no such effect ; if the i
beh^ A B, Fig. 157, which has been in contact
at B, is drawn aside at a, the point a will
come in contact with the dram at a point
to the left of its original position in the
figure, and during a semi-revolution the se-
veral points of the belt will be successively
laid on in the new position, a h ; but if the
Erection of motion were from b to a, the displacement of the
band at a could have no effect in altering its position. ^
214. In order that a band may maintain its position on any
surface during revolution, it is necessary that it should approach
the Borfacein the direction of a tangent to the circle of contact, at
the point of contact, for it is manifest that then only will there
be no unequal stretehing of either side of the band : this applies
both to the case of a conical pulley, and to that in which it passes
&om one cylindrical drum or pulley to another, the axes not being
parallel. In the latter case the receding band is forced to make
io large an angle with the plane of the pulley, that it looks as if
it would slip off every moment, and it would do so immediately, if
the motion were reversed.
If the machinery be at rest, it is very difficult to shift a band,
owing to its tension, but, on the principle just explained, it is per-
fectly easy to alter its position when in motion. Ihe same remark
applies to round bands running in grooved nulleys, such as lathe
bands, which may be readily laid over the eoge of the pulley on the
advancing side, and thus put out of gear.
215. When only a limited number of revolutions of the drum is
required, and the power is employed in rotating the drum, the slip-
ping of the bandmay baeflectn>lljiiTe*eDt«dby<i(uBngit>iinsny
timei u maj ba Deoeuar; OD the dram, and fiiung tbc end* to the
drum: thiu tb« oarriue, b, Fig. 168, nmi backwudi and forwarda
on the rollen, «/ and deiirea iti Bkotioii fiom the dnim, a, moanted
on an aiii abota it, by neani of two bands fixed to, and wonnd
round th« dnun and attached to the carriage at eand d: thia is
tbo conitructioD of Ute common manglo.
Fig. IK. IV- !■>.
216. In order to maintain a conrtant velocitj-ratio, it ii neces-
aary that the band ihonld not be permitted lo become heaped op
on the dmm. This maj be readilj pretented by cutting a spiral
groove on the drmn, in which a cord or chain will wry readily
arrange ileelf, aa in Fig. 159. Thie plan is unuaUj adopted in the
bwrels of weigbt-clooka.
CL. A : DiTmum d. Chmnuutieatifm of Motion &y Link4eort.
317, It haa been shown that when two anni, BtoreaUe round
liied centres io the same plane, are connected
'V- "«>■ by a link, their angular vclooiliei aro inveiMly
as tho segments into which the link dindcs tbo
lineorc«ntiosn62). This relation is cooataotly
changing, as the onns reTolve, [uless the point
of inlenection, T (Fig. 123) be removed tou
indehnile distance by making rq parallel to a >
in all podtiana, which can happen only when Ibe
arms are equal, and the link eqnal in length to
the distance between the centres, in which caae
the angular Telocilies of the arms will always
be eqnaL Let a, b, be the centres of motion,
A 1, c D, the equal arms, and c d ( = a b) the
link ; with tho centres A and a, and radina Ac
B D, describe circles cutting the Une of cen-
:s in o and h, and the same line prodnced in
>nd 6. If a D be carried round the drcle,BC
will always be a parallelogram ; but in any
giTOn position of one of the arms, h d, not coinciding with the line
of centres, there are two poasible corresponding position* of the
JV- in.
■nn AC, tor i. cirele with oeatn Duid radini dc will eni the circle
a c somawherc else, u &t ■, ftnd D s will bo tha ucond possibla
poaitioD of the link ; bnt in thii poaitioa the velodtj-retio is not
tODitiuit, becum the pniat r the intereectioa af a b and o ■ ia not
■ fixed poial. When the liak c d coincidss with thelineofoeotnis
in either of the poBlioni ao or ft H, the an^u- moremaDt of oDe
ana hu no power o( commanicating motion to the other arm.
'I'heiie two poiitioni of the link are called the dead pmntt of ihe
218. When the preceding arTaQftement ia emplojed in craniau-
nicjting m cunBtant Telocitv-tatio, lome rapplemeDtaiy coatHnnce
if requisite U> preTont the link from ■hifhug frota the pAnllel to
the cnne pomtion at either of the dead poiuta ; thii may be eSected
hj three different methoda.
I. Bt introdQciiig a third arm equal to the other two, the centre
of motion of which maj be od the line of
centres, aa e. I^^. 161 , and the moreable end
connected with the link c D at r ao that % r
maj be parallel to ic or HP: Drtbac«n(re
of molion E maj be in an; other place, aa
t, and the end. /, cunnectM with the points
c, D, br links reapectiTel; equal to a A, e B.
In ihe' former case B F, E c mnit alwavi
cimtiaue rectangles, and in the tatter, tbe
triangle, en/, formed b; the links is con-
strained to move patalkl to ihe equal tri-
angle, ABC. It maj be remaned, that the
latter arrangement obTiatea the difficulty
of the Ices m power at the dead points, tor it ia ol
link onlj can coincide with a Hue of centrea at an; one time: the
former does not poseeaa thia adTantage.
II. A a, B fc an the two norallet sies, a f, b q the parallel anus,
and r q the link ; let equal parallel arms, ap, ~^ .„
69. be attached to the other eod of the axes *' _
and connected bj a link, pq, equal to pq, in
auch mannerthataplane pasaingtbroagb A, a,
and p ma/ be perpendiciilar to aoolher plane
Hssing through n, i, and r. In this caae both
links will cflospire in communicating nnifuna
motion from one axia to Ihe other, aod when
either link ia at one of the dead points, the
other ia perpendicnUr to the arms with which
it ia connected, anil is iherefbre in the moat
faTOurable positian for action.*
It is immaterial whether the link pq ia car-
ried by two anna, or by discB, aa in Kg. 1S2,
the equality of the isdial diitaooes of tbe
• Thii priiviFle ku beaa silaptWI bj Uu
112
PROICIPLES OF VECHAMIBM.
Fig. 163.
points of connexion from the centres of motion being the only-
essential condition.
If either axis be carried across the plane of motion of the link.
Fig, IM. ^^®7 ^^^ come in contact,
and continuous motion would
be impossible; this is ob-
viated by bendine the axis
into a loop, called a cranky
Fi^. 163, beyond which the
axis may oe indefinitely
prolonged. This is the prin^
ciple on which power is con-
tinuously employed in rota-
ting the cranlc axis of a lo-
comotive or marine engine.
In these cases, however, the
links are not driven by
another crank, as in the figure, but by the piston rods.
III. In the third method, two or more links are attached to two
discs at the extremities of the axes, at points equidistant from the
centre, and from each other, as in Fig. 164. The planes of rotation
of the discs must be removed to a sufficient distance from each
other to allow the links by their obliaue position to pass clear of
each other. Tnis method is rarely, if ever,
employed in practice.
219. It has been shown that a continu-
ous constant velocity-ratio can be com-
municated between two axes by link-work
only when they are parallel, and revolve
in the same direction in equal times;
if, however, motion be required through
a small angle only, it may be communi-
cated with an approximately constant
velocity-ratio, whatever may be the mag-
nitude of the ratio, or the relative position
of the axes.
It has been shown (162), that if two
arms, at and bq, Fig. 165, or b^,
moving in the same plane, be connected
by a link, p q or p ^, and placed in such a
position that the intersection, t or f, of
the link and line of centres, shall coincide
with the perpendicular, kt or Kt, upon
moreable ourved needle for pMsing a ligatare oonseeatiTelj thrrmgh both
sides of a deft pelate, in the operation of atapfajloraphj : » mo^eabJe pieoe
at the end of a stem eanyinK a curved needle ia connected by two linki with
another nOTeable piece at the junction of the atemand handle, firom which
motion prodooed lij the linger ia oonunonioated to the needle.
1^.186.
X. H3
tha intenection of the
) will be monientarilj
coastant, and will continoe laSciently so Cot practiciil pniposes,
if the moliaD of the arms be conGnM to a imall angle on each
nde of Ibe mBui notitiini : and the angolar vekicitieH of the
■mu will be iaTereel; m tT.BT, or it ■ Bt.
Wben pnctioble, the ampleet mode of arnaging t^M poeitioae,
IB to make the lick perpeodicnlar to both anne, aa l P and on or
r D ; in thia caie, the angnlar Telocitiea are inTenely ai the .aima
theiaaelTea.
210. If the aiea be neither parallel nor in one plane, as a e, >/
Fig. 166, let ef be their comntoo peipendicalar, draw eg panllel
to B/ arrf in tSe pUdo ieg ^ ,„
draw BO, oc wrpendiGnlar ,
to 1 e, e jr, and let the ratio
of B A : c 0 be inTerulj aa
the angular Telocitiea of the
axes a. e, b/ reepeclivelj.
Draw bK, eu puvtlel to ^
xe, ce respectiTelj, and
cntdng each other in ■;
join e ■ and prodnce it in-
definitel/ to A. From anj
point, A, draw Aa, A^, per-
pendicnlan to A e, e ^, male
Bf=«g, draw bI — and pa-
nllel to; A, and join AI, which is parallel to ef, since aghl, and
B«e/ aie parallelcgrama ; and ■■ the planes e i A, /b I are there-
iif^ , 1I~I LI i- ^-^A^^A. __._J.'.-I i-.i- .. . I > - >
theai
that is, invenel; u ob: ce, or in the reqim.,
221. Hie mecbaniim of organs, pedal haipii, bell-hanging, and
Tarions other portions of machineiy, cominonly called i«ii-eroni
vonbi&lUaDdertbisclsBS of sensibly eqoal imall angular motions.
Thia dMB of work frequently re- j(^_ Igj
qniiea a cfaan^ in (he direction and
lelatire velocity of small molions,
which may generally be efleoted by
a single axis with two arms. If the
motion be in one plane, let da, ab.
Fig. 167, be the lines of direction ^
meeting in a. Draw a b, d f, pei^
pendicnlar loab, ad, and take b x :
Dr in the ratio of the required ts-
locitiea in the directions ah, da,
nmectiTely, and draw Bi, fa i*-
nllel to ba, da, and intaraecUng in
114
PRIH0IPLE8 OP MXCHAHISlf.
A *, join a A and produce it to o. In a c take any point, c, and from
c draw cb^cd perpendicular to abf ad^ then a crank consistingof
the two arms, ch, cd^ will communicate the required motion, dj
remoying the point c to a greater distance, the angular motion of
the arms will he diminished, and consequently the inequali^ of
the velocity-ratio, through the required extent of motion. This
oontrivance is called a heU-arankf as the term crank is commonly
restricted to a hent axis (Fig. 163).
222. If the given directions of motion intersect, as in the pre*
ceding figure, we obtain four angles round the point of intersection,
in one of which the directions of motion are hoth towards the point
of intersection ; in another, hoth diverge from the point ; hut in the
two remaining angles, one motion approaches, and the other
recedes, in either of which the axis c may he placed. If tho
directions of motion are parallel and opposite, tne axis will lie
hetween them ; but if paitdlel and similar, beyond them. In tho
latter case, if the required motions are likewise equal, the axis is
removed to an indefinite distance (refer to couples, 81), and the
crank becomes practically impossible ; but the reouired change of
motion may then be efiected by the following metnod.
223. Let the two directions of motion be a (2, c&, not in the
ifmie plane ; find their common perpendicular, cd^ draw ee parallel
to a df and in the plane bee construct the re-
quired crank, 6 b 6 (221) ; draw bx perpendionlar
to the plane bc, and take BA=ae; draw a a
parallel, and ^ necessarily equal to b «, then will
A B be the axis and a a, b o the arms necessary
to convert the small motion in ad, into the re-
quired small motion in c b.
If the arms a a, b 5 are parallel and equal, the
construction will meet the case which was shown
to be impossible by a crank in the same plane
with the two motions.
The preceding arrangements are of constant
occurrence in the construction of organs; the
crank is termed a back-faU when its arms are in
the same horizontal straight line, and a iquare
when they are at right angles. An axis with arms as in the pre-
ceding figure is a roller^ and the links are stiken when they act
by a thrust, and trackers when they act by tension.
JV^. 168.
GLASS A.'^Elementary Combinatwtu m Trauu.
224. Whenever the required velocity-ratio of the driver and fol-
lower is very large, or veiy small, especially in the employment of
toothed wheels, to which these remarks principally apply, it is
found practically more convenient to employ two or more elemen-
COXBISATIOHS IH TRAIirS. 115
tai7 oombiiiations, the product of whose velocity-ratios will be the
required ratio. If any number of axes bave each a wheel and
pinioD moonted on them, and be so placed that the pinion of the
nnt axis shall be in gear with the wheel of the second, the pinion
of the second with the wheel of the third, and so on ; and m?i, ^,
w radii of the pitch circle of the first wheel and pinion, to., j?-
those of the second, and so on ; then the velocity-ratio transmitted
hj tlie train will be
fg, X tPg X &C. X Wn .
p^ X^, X &C. Xpn*
or if the wheels be supposed to be all of the same pitch, then the
qoantitieS) «7,|), may M the numbers of teeth and leaves in the
wheels and pinions ; for it has been shown (125) in a system of
«*«« acting successively on each other, that
w : p : : product of long arms : product of short arms ;
^^ w : F : : virtual velocity of p : virtual velocity of w,
>pd as these levers may be supposed to represent the radii of the
pitch circles of the wheels and pinions, the truth of the above pro-
position 18 obvious.
. ^5. In a train of wheels consisting only of ^r wheels and
I»^ons with parallel azep, the direction of rotation of the first,
^'i^ &c, axes will be the reverse of that of the second, fourth,
•c. ; consiequently, if the train consist of an even number of axes,
«« extreme axes will revolve in opposite directions ; but if of an
^ number, the revolution of the extreme axes will be in the same
"'Bctioii. If a wheel, c, be placed in gear, with two other wheels , a
JJ« >, the velocity-ratio of a and b will be
we ttme as if they were in contact, for it ^' ^^•
"erident that for every tooth of a that
PjJ'es the line of centres of a and c, a
woth of B will pass the line of centres of
* and c Bat the direction of rotation of
«e axes of a and b, which would be re-
▼ereed if they were in contact, will be
rendered the same by the action of the
jntennediate wheel, c: such a wheel is
wnned an uUe wheeL When the shafts
•^'wo wheels, a and b, lie so close together
that they cannot be placed in the same plane without making
^m inconveniently small they may be fixed as in Fig. 169, so
J*i ^°^^^ o°® another, and may be connected by an idle
'■wel, c, of the thickness of a and b, t<iether with the space be-
tween them. Such an idle wheel is called a Marlborough wheel :
? ®°^P}oye<i in the roller irames of spinning machinery.
226. By intermediate bevil wheels parallel axes may be made to
fe^olve either in the same or in opposite directions, according to
I 2
116
FBIHGIFLE8 OF IfECHAinBH.
Fig, 170. the relative poBition of the
wheelfl. In Fig. 170, a drives
B ; and b, on the same shaft as
B, will drive either c, or d. The
wheel c, on the same side of
the intermediate axis, will
revolve in a direction contrary
to A ; but D, on the opposite
side of the intermediate axis,
will revolve in the same di-
rection as A. In this combi-
nation the axis b e may be nrolonged to any required extent ; it
is an arrangement frequently adopted in English mill-work for
transmitting motion to several macoines on the same floor.
227. In mill-work it is considered desirable that any given tooth
of one wheel should oome into contact with a given tooth of
another wheel, in ^ar with the former, as seldom as possible,
since the irregularities of their figure are more likely to be ground
down and removed by continnallv bringing different teeth into
mutual action; and there can oe little doubt that the same
principle is applicable in clock-work, as irregularity of wear most
evidently affect the nniformity of the velocity-ratio ; some clock-
makers, however, entertain a contrary opinion.
Let a wheel of h teeth drive another of h teeth, and let — = —
m and n being the least nnmbers in that ratio ; then m ns m h, and
n is the least whole number of circumferences of the wheel m that
is equal to a whole nmnber of circumferences of the wheel ir. If
then we begin to reckon the circumferences of each wheel that pass
the line of centres after a given pair of teeth are in contact, it is
clear that after n revolutions otir and m of n, the same pair of
teeth will again be in contact ; neither can they have met oefora,
for since m and n are, by the supposition, the least multiples of
the respective circumferences that are equal, it follows that the
two given points of the circumferences cannot again meet, until
these numbers of circumferences have rolled on each other.
In order to make the interval of recurring contact as large aa
possible, m and n must have their largest value, that is, equal to
M and N, or, in other words, m and v must be prime to each other,
that is, they must have no common divisor. If, for example, a
following shaft were required to move three times as fast as tho
driving shaft, a pair of wheels with 72 and 24 teeth would trans-
mit the required motion ; but the mill-wright would add one tooth
to the wheel, and thus obtain 73 and 24 : two numbers which are
prime to each other; and any given tooth would not again meet
its fellow until after 73 x 24 teeth had met successively, that is,
until after 24 revolutions of the driving shaft ; this extra tooth is
called the hunting-cog.
1IT
8SS. We cumot oKr a dinpler illiutntioii of the >ction i^a
tawn of iriieel-work, than bj eipUiniiig the conitrncticai of & clock
of Ihe iinipleit kind, which i« repruwated in Fig. 171. The
weif^t w is attached to a cord or chain that ia wound Toead a
groond bairel, ± ; npon tha aama adf^ ot arbor* ig fixed a
toothed wheel b, which drirea a
piDiDD ( on the aecood arbor b c, ^' ^^•
which also carriea a wheel o; ''
thia wheel driTca the piaion e on 1
the tfaird arbor, opon which ii
ahio fixed a toothed wheel, D,
with teeth of a peculiar form,
termed a iwiag-whtel, or letm-
Khted. AboTe the aca,pe-wheel ia j
an arbor d a, taimed tbs verge,
which ia oonuected with the pen-
dnlniD, i> r, bj means of a forked
piece, K. The verge also canim
a pair of knni d, with a toolh at
the end of each, called jiaUeit,
which are altematelf engaged
with th« scape-wheel, d, in such
a maimer that at each oscillation
of the pendulum one tooth of the
wheel escapee, and aiiother fsjla
oo the opposite " - -' ' ■
karing passed t
osile pallet, the wheel
^ J Med through a apace
equal to half the pitch, i t, m n,
are the pUlea, which are msin-
taXned in a parallel position b;
pOlan, which are here omitted, to aroid cenfimon : the small endg,
at mmlt, of the arbors run in holea in the platea.
If we hare a aecoods' pendulum and 30 teeth in the swing-
wheel, it will reTolTs once in a minnie ; and if b have 45, and c
48 tee th^ and the piniong, b, e, each 61eatea,then r,,T, being the
times of iTochronal rotatioQ of the arbor e d, and the barrei.arbor.
bnt the clock is also required to indicate the hours and minutes bj
hands on a dial, g k ; this is effected bj wheels placed outside the
frame, klvtn, called molion ahedt, Tha barrel-arbor panea
• ^rkr ii (bs watoluuksn' term f« in Hi*.
118
BRINOIPLES OP MBGHAHlSlf.
through the plate, m n, and two wheels, b, f, are fixed on it. Be-
low these a stud, «, is fixed in the plate, and a tuhe is placed on
this stud, to one end of which the minute-hand, m, is attached,
and to the other, the wheel e in gear with e ; a second shorter
tuhe is fitted on that of the minute-hand, which carries at one
end the hour-hand, h, and at the other, the wheel f in gear
with F. As the barrel-arhor revolves once in an hour, tne wheels
E and 6 must be equal ; and as the hour-hand usually revolves
once in twelve hours, the number of teeth in / must be twelve
times that in the pinion f. The dial, g A, is placed in front of the
motion wheels, and if a hand, «, were attached to the arbor, c d,
prolonged to pass throngh the dial, it would indicate seconds, but
m the train represented in the diagram would move backwards ;
for as there are two intermediate axes between those of i>, and of
e and/, h and m will move in an opposite direction to s (225) ; to
make them move in the same direction, a second axis must be
introduced between A and d, or the wheels e and / must be on
the barrel-arbor, and e, f, on the stud s.
229. In order to exhibit the relative motions of the different
parts of a machine, without regard to their relative positions,
which can only be represented oy a drawing, some kind of nota-
tion may be employea. Various modes of notation to represent
clock-trains have been proposed by different authors; the following,
which differs somewhat from precediug methods, appears to be
well adapted for the purpose :
I
40-
I
-72 — hour-hand
-40 — ^minute-hand
I
48-
Barrel
6 30— swing-wheel.
45-
This diagram represents intelligibly the preceding clock-train ;
the long horizontal lines are the frame, the short ones between
figures represent gear, and the vertical lines, axes. The figures
represent not only the numbers, but the juxtapositions of the
wheels and pinions.
With the train just described, a clock would require to be
wound up every day, on account of the number of revolutions of
the barrel : it is not found convenient in practice to allow more
than 15 or 16 turns to the barrel, consequentlv, if the barrel-
arbor make one revolution in about 14 hours, the train will go
for a week ; for this a different train from the preceding wiU be
required.
230. In clocks of the best kind it is usual to employ high nnm-
OOMBIHATIOSS Df TRAIHB.
119
beicd pinions, sncb as those with 10 or 12 leaves, on account of
tlie smoothness of their action: the following will be a good train
lor an 8-daj dock : —
10
I
54-
•80 — ^hour-hand
36 — ^minnte-hand
Bar
I
i*el
108-
•12
108-
I
-12
I
30— swing-wheel
100
I
10
13*»30". I'^SO".
10'
1™. — ^periods
in which the barrel will revolve once in 134 hours, and will go for
rather more than eight days with 144 turns.
In smaller clocks, the pendulum will make 100, 120, or 150
oscillations in a minute, the numbers will then require to be pro-
portionabljr altered.*
231. Much ingenuity has been in former times bestowed on
finding the requisite numbers for trains which will approximately
represent the motions of some of the heavenly bodies, such as the
period of one year, which is 365* 6** 48°* 50* very nearly ; as, how-
ever, such machines appertain to the extensive category of inge-
nions, bat useless mechanism, any details may be considered un-
necessary ; the more curious reader is referred to an able digest
of this subject in the work previously quoted.f
232. Calctdating Machines. — ^The present subject would be in-
* As a farther illustration of the above-mentioDed notation, the foUowinff
traia it given, wh^ch has been employed by the Author in the oonBtroction ot
tine^eoeSj in which the boor-luuid is employed in carrying round the cylin-
ders oekmging to his aelf-regictering raagnetie apparatus : the hour-hand
one rerolntion in 2i hours; and in order to avoid the nnsteadinesB of
the hoor-faand, which in ordina* y movements results from the necessary play
of the teeth of the motion-wheels under the dial, the central axis, whicn
carries the hour-hand, carries also an idle wheel in the train, and the axis
which carries the miottte.hand is placed out of the centre : also the numbers
of the teeth of the wheels and pinions in gear are as far as possible prime to
other.
hour-hand minutes seoonds
r
r
I
I
•IS
I
72-
I
-10
60-
eel 62 11 76 9 14~icape- wheel
L_ I I I I
J
13i>48">20*. 24>>. lb. !«. —periods,
f See Willis, Prindplei of Mechanism, p. 223.
120 nmroiFLKS or kbohahism.
complete without some aoooant of these eminently ingeiiiouB and
practicallj uaefbl contriyanceB. All these xnachines act on the
principle of Boccessive addition; one wheel carrying forward
another wheel in gear with it or connected with it hy some in-
termediate mechanism, so many teeth or divisions as there
are units in the digit or figure it is set to represent. But they
ai'e of two essentially different kinds; one bemg designed to ac-
complish merely the multiplication or division of numbers, the
other, to which the name of difference engines has been applied, is
designed for the calculation of lo^rithmic and other tables, by
the addition of successive orders of differences.
The principle of construction of the simpler forms of calculating
machines, of which the Arithmometer of M. Thomas, of Colmar,
. is a favourable example, may be apprehended
Fig. ITS. by reference to the diagram. Fig. 172. Let jl
be a pinion of 20 leaves, of which one, two,
three, &c., tenths of the entire length are suc-
cessively cut away, and let b be a wheel of 20
teeth, capable of being set at any noint on its
axis, and in gear wil^ the pinion. If now it be
required to multiply 6 by 4, the wheel b will be
brought by suitable means to the indicated
number 5, which will place it opposite the
middle of the pinion, where 6 out of 10 successive leaves will
have been cut away, and consequently each half revolution of the
pinion will carry the wheel on 6 teeth, and four half revolutions
will carry the wheel on 20 teeth, as the result of the o^ration,
and that result will be shown on fi^re wheels in connexion with
B. The carrying is effected by a pm in each figure wheel, which
carries on the next wheel one unit, when the former arrives at
the cypher, 0. The same contrivance is applied in the counting
apparatus of gas- and water-meters, &c.
These machines are, however, not of any great practical utility,
rince the multiplication and division of numbers, containing many
places of figures, is almost always effected in practice by tne aid
of a table of logarithms, in a shorter space of time than the result
can be elaborated by the machine.
233. The Difference Engine. — The first conception of this im-
portant machine is due to Mr. Babbage, who constructed one in
the year 1821, and a year or two afterwards commenced the con-
struction of a more elaborate and perfect engine of the same kind,
under the auspices of the British Government. In this provision
was made for stereotyping the calculated results, b^ impressing
corresponding figure punches in a soft plate. Dunn^ the con-
struction of Uiis, the idea of a much more comprehensive ennne
presented itself to the mind of Mr. Babbage, some notion of which
may be given by stating that it is to the difference eneine what
the Jacquard is to the simple loom. In consequence of this new
discovery, the completion of the machine in progress was abui-
iaaai. The fivdwd portion of tbe diftrsODe eneine hw be«n
dspodtod bf Um aa>«niin«iit b tha HoMom of Kidk'b Collegfl-,
and > deUQad deaaintioii of it will ba fbiuul in the '' Edinfaunh
deriTcd tlie omccptioD of cotiitmctiiig a aimplar form of maohine,
to KcompliBh tin msm object aa that of Mr. Bttbb*^, nunelji
that of calcnlatiiig and dmnltaneoiul; printing namencal tablea.
AR«r coDtending with man; difficnllieii, and more eapeciallr with
want of meaoa, he eomphited, with the aid of hit aon, Mr. &.
Schanta, a workiDg model of hia madujie in IMS ; bat from the
want of sulEi^nt iiinda, the machine waa not finallj complalsd
nntil Octt^r, ISAS. Thii nueUne waa exhibited for same weeka
tntiuacoDObrr; itwaambeeqiientlxidarod in the Paria UniTeraal
Exhibition of ISSS, and a edd medal waa then awarded to the
inrenloT ; it wM finallj niTonaaad bf an enterprinng and libera]
Bifaii ail merchant, and now balonga to tiw Diidlej Ubaerratorj
at Albanj in that eoontiy. Another maohina waa anbaequent^
eonatmcted bj Heaara. Donkin, fnm the deaigtu of Hr. Schauta,
br the naa (tf tha affioe of the B^^iitrar-Oeneral, where it baa
pecfinmed modi naefid work in the calcnlation of tablea.
^.173.
The machine oonaiita of two prinapal part* : —
1. The nalcnlatinB apparataa, compriatDg the TSrtical apindlec
and rowa ttH^an whaela occupTin^ the entire front of ¥Q. ITS,
■od the carTTmg oama and traverting carriers, or " traTBllerB,"
of which one, o, tniTatMa in finat of the Egnie wheela, and the
other behind them ; the latter being oat of sight.
3. lie joiDtii^ apjaratni, which ii placed at the npper and
back part of the machine, and conaiila of the horiiontal ipindlea,
with their oomaponding toothed and cam wheeli, pnlleya, 'racks,
and tjpe wheela, partJaDiy nen in the Egore,
V22
Tfaere is >Ibo a counting apporatns, or eaumenttor, not Men in
the figure, which indicBtes the number of reenlM completed.
The calculaCine apparatua containa fifteen vertical, and fire
hnnHmtsl rows of figure wheels ; it will therefore comput* fifleen
plnces of G^res, but of these only the eight first are in connexion
with the printing apparatus ; and as the last five places are neg-
lected, a consideralile time will elapse before an; sensihle eiror
arisiog from figures neglected bejond the fifteenth place con occur
in the tabulated reault. The first, or upper, of the five hotiEon-
tal rows shows the result of each complete action or "stroke" of
the machine: the four succeeding rona corrospond with the first
four orders of differences.
The figures on the first, third, and fifth rows read from left to
right, and their motion is [direct,* those on the aecond and fourth
rows rciui from ri(i;ht to left, and their motioD is r^rogradt; the
latter are in relation with the traveller behind.
In worliing (be machine, the handle is turned coutinnoual; in
one direction, and the inotion being transmitted to a mangle-
wheel. A (251 )j which b; means of a toothed aegment, s (periodi-
cally bmught into action by a projcctiog atud at the back of the
mangle-wheel), acts on a rack in gear nith a seriea of pinions on
the opper enda of the vertical epindlea, and canseB them to make
a complete revolution, firat in one direction, and then in the other;
and these two contrary revolutions constitute a complete "atroke"
or action. These spindles pass through the open centres of all the
figure-wheels, but without touching them, ac that without the
adding mechanism they would have no action on the wheels.
As each figure wheel must necessarily bn el rest while ita num-
ber Ih being Carrie^ or added to the wheel above. It followa that
the nitematc horizontal raws only can be in motion at one ^me ;
consequently, by the fl™t revolution of the spindles, or "half-
u iTi stroke," the numbers on the third and fifUl
J'lj. 17*. ^^g ^^ carried up to the second and fourth ;
but durinj:; the aecond or reverse half stroke,
3 theae remain at rest, while their numberi are
carried up to the first and third rows reapec-
tlvely, the adding mechanism being such as
to fct in one direction of rotation onlj.
The figure wheels are cylindrical rings,
resting and working in rebated bearinsB on
their respective ahelvas, and admit of being
reailily rotated cither by Che hand, or by the
action of the adding apparatus.
, The fewer part of each wheel it cot into
ten parallol sided teeth with sloping faces,
wliich correspond with the figures above
THB D17FSBBVCB SKOIHB. 123
tfaem, and tbe direction of the slopes corresponds with the sequence
of the figures. These teeth project below the shelves, as seen in
Fig. 174, and are in gear with the adding arms beneath, which
maj be thus described. In the interral between each two wheels in
a Tcrdcal row there is a small stage, b, attached to the spindle, to
which a rising piece, h, is jointed. From this riser a pin or finger,
c, projects forwards, and an arm, d, downwards. Tlie finger, when
oot of action, rests in a notch in the tumbler, k, the short arm of
which falls nnder this finger when raised, and keeps it up until
the tumbler is thrown back by coming in contact with the fixed
stud, L, when the finger again drops out of gear. The arm, d,
reaches nearly to the top of the figure wheel below ; and as the
spindle reTolves, it carries this arm, or grapnel, with it. From
toe Qpper edge of the wheel below there projects upwards a small
catch, E, which ia balanced at its centre, and as the spindle re-
volves the grapnel meets with an obstruction in the catch, the
effect of which contact is to raise the finger of the riser, and to
cause it to enter a notch in the undei^ part of the wheel above,
and carry that wheel round until the riser falls, and the finger
drops fiDm its hold on the wheel above.
it thus appears that the amount of action on any figure wheel,
or the number of unita it is carried forward, depends on the
bterval between the raising of the finger, c, in the space beneath
it, by the action of the ciftch e on d, and its falling by the action
of L on the tnmbler, k, and this corresponds with the figure shown
on the wheel beneath. It will be noticed that, with regard to
the bottom row of wheels, no provision is made such as has just
been described, and, therefore, whatever number is set on those
wheels remains unchanged throughout the operation.
When any figure wheel is set to 0, as nothing is then to be
carried to tne wheel above it, the little catch, e, Fig. 174, is
brought under a fixed arm, r, which depresses it below the reach
of the grapnel, and the position of the figure wheel above will
remain uniutered. Thus the action of the machine may be cur-
tailed, by setting one or more of the lower horizontal rows to zero ;
and if every wheel in the machine were set to zero, the spindles
would revolve ad infinitum without changing the position of any
of the wheels.
After each act of addition has been performed, whenever the
sum of the added digits exceeds 9, it is necessary that 1 should be
carried to the next figure-wheel in the same horizontal line, which
is thus ingeniously enected : — Each wheel has a small stud placed
on its figured surface in such a position that, as the 9 passes and
the 0 appears in front during the rotation of the wheel, it presses
against the short arm of a bent horizontal lever, a, which has a
cam at the end of the longer arm, standing in front of the next
wheel to the left, and throws the cam forwards. The cams in
front of the first and third wheels are shown in Fig. 174, and the
commencement of the arms in front of the adjacent wheels.
124 PBDrOIFUBB OF lOBCHAXnil.
Then, by means of an endless cbain geared to the mangle-
wheel, an upright ^t traverses in a groove in front of the figure-
wheels. ^ Tnis moving post, b, Fig. 173, — ^the " traveller," as it is
called — ^is fitted with arms working on pivots, and these arms
grope along the line of march for any obstmcting levers which
ma^ have been thrown forward during the calculation as an indi-
cation that a carriage needs to be made. Whenever the arm of
the "traveller** comes in contact with a projected lever, the arm
is raised, so as to brinf its finger against a tooth of the figure-
wheel in front of whicn it is passing, and carry that wheel round
one place: the traveller then replaces the lever in its position of
inaction. This process of carrying is independent, and oomea
into action between the rotations of the spindles.
The printing apparatus is at the back part of the machine, and is
brought into play by means of weights and pulleys acting upon a
series of horizontal spindles connected in the front with the calcu-
lating-wheels, and at the back vrith a set of racks geared to a cor-
responding set of type-wheels, e. Whilst the calculation is being
penormed, all the type-racks are kept fixed by a bar, p ; but as soon
as the line of figures represented on tne top row of calculating-wheels
is ready for printing, the bar is withdrawn, and the racks are left
to the action of their weights, which is regulated by the motion of
a snail, d, similar to that of a common cfock. The snail in this
instance drops down on to a notched spital cam, o, attached to the
top row of calculating- wheels, and the position of the type-racks,
therefore, depends upon the corresponding positions of tne snail
and the spiral cam. When the type-racks are thus regulated, the
type-wheels will have ranged themselves in proper order for
stamping the result of the calculation upon a plastic surface fixed
in the printing-rack, which, by a simple contrivance, is passed
along gradually underneath the type, and by the action of an
eccentric is raised up to receive the impression, and then pushed
forward so as to leave a clear space for the next line of figures.
The material used for receiving the impression of the steel dies is
papier mcichS, in slips of about eight mches lon^ by two inches
oroad, and of moderate thickness, the surface bein^ rubbed over
with black lead, so as to give greater facility in casting the stereo-
type plates. From this mould an ordinaiy stereotype plate is
cast, and thus the errors incidental to the calculator, the com-
positor, and the printer, are alike eliminated.
234. The principle of action of the difierenoe engine may be
thus explained : — Ut Ux be any quantity involving as, to which a
series of- numerical values may be assigned, by giving to x suo-
cessive values differing by unity; thus let ttx^xl^t ^^^ ^^ series
&c. It.,, ttoi «i» ««f «•» «4» Ac-
will be represented by the numbers
&o. 1, 0, 1, 16, 81, 266, &c
THB DIITBBXXCB XHGIRX. 125
being the biquadrates, or fonith powers of the natural numbers,
poadve and negative. Further, let
UkU^f Atf|, All,, All,, Att4, &c.
be the differenees resulting from successively subtracting each
term firom the following, and similarlj let
A*Up, A*tt„ a"«,, A"l*g, &C.
be the differences of these, or the $econd differenoeB^, and so on.
The following table ma^ now be constructed, in which the nume-
rical value, in the particular case that has been assumed, is Buh<
jotned to the general expression : —
&C. tt . «•«,«•«. u,, &c.
1
1 «•
0
1
16
81 25(
-1
1
\
Att,
15
Att, Att,
65 175
2
1 '
14
, tJr^ AS
50 110
a'u.,
1
aX
A«tt,
12
36
60
A*tt., A*«,
24 24
From the numerical values {pven in this table, the machine ma j
be set for the calculation of this particular series of numbers. This
is effiscted by setting the initial number of the series, 1, on the
last printing figure wheel, all the rest being 0 ; then the first and
second difierences of the preceding term (calculated backwards),
which in the given example will be 1 and 14, on the first and
iecond diffisrence wheels beneath it ; and then the third and fourth
difierences of the term before that, which in the present case will
be 12 and 24, on the third and fourth difference wheels beneath the
preceding ; and all the unoccupied figure and difierence wheels are
set to 0. The machine is now ready for work : by the first half-
stroke the even differences are added to the odd ones, giving the
tueeeedina odd differences (in the example, 15 and 36), as may be
seen by tne above table, and the numoer on the first horizontal
row is printed : and by the succeeding half-stroke, the last obtained
odd difference are added, one to the first teftn, giving the second
term 16» and the other to the second difference 14, giving a new
second difference, 50. This will probably be more readily com-
prehended by the aid of the following diagram : —
A»,
ti, = 1 1 J ... 16= tt, 16 J ,.. 81= tt,
»P=H... 15$ 15=Att^) ... 65} 65=AM,
,Mw=14) 14 ) ... 60 = A»u, ( 60) ...110 = A%
A,tt, = 12) ... 86 5 36 = aXI •• 60 5 60 = A>M,
A,i«.,=24 5 24 24=A*tt.5 24 24=A<iti
126 PRDrCIPLBS OF MBCHAVISM.
AHer the first complete action it will be seen that the second tenn
of the proposed senes is shown on the figure wheels, and that the
differences have all advanced one step. After completing a second
action, as indicated in the diagram, the third term, 81, of the series
appears above, with the first and second differences of the second
term, and the third and fourth differences of the first term beneath,
which again is another step in advance, according to the first
arrangement. From this it must be apparent that so long as we
continue to work the machine, successive terms of the proposed
series must be produced, until the number of places is limited by
the extent of the machine.
In the series given as an example, the fourth difference is con-
stant ; but in most series required in practice this is not strictlj
the case : the machine will however calculate correctly, until the
variation of the fourth difference exceeds unity, when it will
require to be reset. The determination of the actual differences
to which the machine is in any given instance to be set, as well as
the limits beyond which a table, calculated to any proposed number
of places, will cease to be strictly accurate, are matters of easy
determination, but they are beyond the scope of this work : it
must suffice to conclude with an explanation of the easy mode by
which subtraction is effected, namely, by the cMitum of the
arithmetical complement of the number required to be subtracted.
Thus if we wish to add —45 to, that is to subtract 45 from 6789,
we should set the machine to add 9955 to that number :
for 6789 and 6789
less 45 increased by 9955
gives 6744, gives 16744;
but as in the setting of the machine there is purposely no figure
wheel to which the left-hand 1 can be earned, that digit is lost
sight of, and the correct number 6744 is stereo-moulded.
It may here be observed that if the machine be stopped at any
term of the series, and readjusted with the odd difierences, the
first and third, taken negatively, if for example (according to the
last column of numbers m the preceding diagram) it were set to
81, -65, 110, -60, 24,
the terms already calculated would be reproduced backwards; and
thus, after any continuation of working, the machine may be made
to reproduce the numbers with which it was oiiginally set.
GLASS B. DxBRCTiOKAL Rblatiok conbtant, Velogitt-
RATIO VABYIKO.
The preceding elementary combinations include those which are
employed in all the largest and most important machines, for the
▼AKTnra yelocitt-katio nr whkbl-wobk. 127
parts of bea^y macliinery are made to move with uniform Telocity,
if possible : the combinations we have next to consider, are those
in which the yelocity-ratio is variable.
Dmsioif a. Communication of Motion hy BoOxng Chntact,
235. It has been already shown (165), that when a pair of curves
revolving in contact in the same plane, are of snch a form as to
roll t(«ether, the point of contact remains in the line of centres :
the radii of contact, therefore, must coincide with this line, and
make equal angles with the common tangent ; there are two well-
Imown curves that fnlfil these conditions.
L In the logarithmic spiral^* the tan^nt makes a constant
angle with the radius vector: consequently, if two equal logarithmic
spirals be placed in contact in reversed positions, and be made to
turn round their respective poles ; tiiey will fulfil the conditions of
rolling curves ; for the radii, making equal angles with the com-
mon tangent, will always be in the same straight line, the line of
centres.
n. Let A p X, a p m. Fig. 175, be two equal ellipses, of which
8^ H, «, A are the foci, and let them be placed in contact at any point,
p, situated at e(}ual distances, a p, a p, w- iw
from theeztremitiesoftheir major axes; ^'
and draw tp f, the common tangent at p.
By the property of the ellipse, the tan-
gent nuikes equal angles with the radii,
8 p, HP, and, because the ellipses are
^ual, and a p=a p, the tangent makes
the same angles with the radii, « p, A p ;
and hence ^psstph, and hp«, isa
straight line : also s p=:« p, and hence
«p-hPH = sp + PH= A M, aconstant dis-
tance, whatever may be the situation of
the point of contact, p. If, therefore, s
and hj OTB. snd «, be made centres of
motion, the curves will roll on each
other.
236. Let APM be the driver, and a p to the follower, and h, *,
the centres of motion, then it is clear that the velocity-ratio of the
driver and follower will be greatest when the points a and a are in
contact, and least when the points x and m are in contact, and
that there will be a gradual decrease of the velocity ratio, followed
• The aqnafeion of tUi enzre is
r=a , or log rs9. log a ;
m wliidi r is the radios vector, 9 its angnlar distance from its first position,
•ad a is a eonstant.
w daimg «acb complete reTolation. Alio, it !■
tlidODt that greatest Telodtj-ratio: leutdo. : :
Z37. If the yelocity-ratio were required to vary more than onco
penodicall; duniig each revolndoD, it ie ceceeearT to find a curve
of at man; lobti, or sacceBsiTe projectione and iDdentationa, aa the
unmher of changes required, i^ich ma; be done by the fotlowing;
conBtrncUon :• let a and b be the greatest and least ra^ii of the
reqaired cnrre ; describe an ellipse, wbow mtuor axis is a 4- b, anil
a — b the diitanoe between the foci. From odo of the foci draw
■traigbt liaee to the elliptic cirenmfereDce, making eqpal anglea
with each other. Diride the circalar base of each lobe into aa
man; eqnal parii as there are equal angle* rennd the Cocna of the
ellipee, then the distances from tbe centra to the seTera] points of
thole' ■ ~- • -
corves, aa, for example, of one, three, and fanr lobes re-
spectJTel;, draw a circle, ao, whose diameter is a— b,
and npon a tangent, ad, set off a c = ^/a 6, a b ^^ 3 a c,
AD = 4AC,and drawee, catting the circle in r ando,
and Di., E L cutting tbe cirole in k and l. The curve
of one lobe will be an ellipse, of which the greatest and
least radii are □ F and c o, end the major axis o p -t- c Q.
curve of 3 lobes, draw a aemi-ellipee, g, with focus e, and
diameter B ■ 4- ■ i^ and from « draw a suS-
cient number of radii, e 1, e S, &c., at equal
angular distances. To describe the 3-lobed
cnrve, describe a cirole round its centre, e, and
divide it into ail equal sectiena, each of which
will contain half a lobe. Divide ttis ioto as
manj equal angles as thoee of the semi-etlipse,
j^, i7g, q, and draw radii, upcn
which set off in oiiler
distances equal to tbe
tadii of the semi-el-
lipae, as indicated b/
oorresponding Gguree.
Througfa these points
draw tie curve of the
semi-bbe, k I, and tbis
repeated right and left
aJtematelr wiU com-
plele the ngare.
To describe the 4-Iobed curve, p, draw en ellipse, of which the
■ FortlieaB>lrtics]lsTertig*liaBoflliu,iHWmu, Prine,arit«&p.Ml.
VASTIBG YSLOCITT-RATIO IH WHKBD-WOBK.
129
7^.179.
greatest and least radii are d k, d l^ and major axi«, d k + i>U and
proceed in the same manner as before ; transferrine the radial dis-
tances to hi, the semi-lobe of p. It will be founa that any two
of these corves, or a pair of the same curves, will roll together.
238. When these rolling cnrves are employed in practice, they are
nsnidly supplied with teeth
like ordinary spnr-wheels, as
in fig. 179 : which represents
A ^r of wheels emploved in a
printing-machine b^ Messrs.
Saoon and Donkin. The
iorms of the teeth are epicy-
ciosdal corves, as in ordmaiy
spar-wheels (190), bat the
forms of consecutive teeth are
not precisely alike, because
the pitch-line is not a circle.
239. The following; is one of the simplest modes of efiecting a
varying velodty-ratio by common epnr-wheels alone : the centre
of a moveable ial&>wheel, c, Big. 180, is connected by links with
the centres of two wheels, a, d, of wluch d revolves on its central
axis, and a on an eccentric axis, b, which must be j>laced suffi-
ciently iar from d, that the distance of b from the ciroumference
of D may be greater than the larser segment of the diameter in
which B lies, as indicated by the dotted lines : it is clear that by
this arrangement a varying velocity-ratio will be communicated
horn B to i>.
JKff.190.
Fig. l«i.
^.nTWIx,
240. If the axes of the driver and follower be at right angles to
each other, a varying velocity-ratio may be obtained by an eccentric
crown-wheel, F, rotating on an axis, a b, the teeth of which are in
gear with a long pinion, o d ; the angular motion corresponding to
each tooth of the crown-wheel will be inversely as its distance from
the axis, a b. This arrangement has been employed by Huyghens.
180
FBUGXriiES or HSCHAVBIC.
CL.B: Diymiovft. CknnmuMcatianofMaUonhySUdmgChnU^
241. The Bimplest mode of obtaining a ▼ariable ▼elocitjr-ratio
between parallel aies is by tbe pm and
alit. In Fig. 182 a a, b (, are tlie axes,
placed with their ends opposite each
other ; ▲ a has an arm carrying a pin, d^
which works freely in a long slit in an
arm 6/ carried by b&; either axis will
commnnicate to the other a Tariable
velocity-ratio, for the pin in revolring is
oontinoally changing its distance from b.
242. An intermittent motion is con-
veniently produced by the Geneva stop,
a contrivance for preventing overwind-
ing in Geneva watches. The driver a, has a projecting tooth,
flanked by two hollows, r, «; the follower, b,
has a number of hollows (usually five) for re-
ceiving the tooth, tbe spaces between which
are hollowed out so as nearly to fit the plain
^ part of the rim of a, to prevent any motion in
^h B except when the tooth is passing the line of
centres, and having entered a notch, d, is car-
nring the notch along with it. A portion of
tne circumference of b is left convex, as off
against which the tooth strikes after passing
each notch across the line of centres, and fur-
ther motion is prevented. During tiie unwind-
ing of the spring the driver, a, moves the re-
verse way until again stopped by the convex
surface, /0.
A similar contrivance, except that a single
pin at the back of the driver is substituted for a tooth, is miule
use of in the registering wheel-work of gas-meters, as well as in
other registering machinery.
248. Any required variation of angular velocity-ratio may be
produced by an arm resting on a cam-plate. The requisite form
of the cam ma^ be traced out by taking any number of equidistant
radii,, and makmg their lengths proportional to the required changes
of velocity-ratio of the arm.
CL. B: BrviBioR c. Oammunieatum of Motion hy Wrapping
Ckmnectort.
244. A contrivance frequently employed under this head is that
of placing two equal conical frusta m reversed positions, but with
their axes psrallel, and connecting them by means of a flat band.
isL this airangement care is required to maintain an equable
ISt
of tbft btuid to
JV IM.
e of tb» cooi
tnTsl toward* the bue of »Ub cone (21 1)
thii diffica1t7 will, bowsTcr, be lent Mt
when the cones are pUced u near etch other m
U the nrjing leiuion of the wrapper will I
permit ' If a, fig. 164, be the driver, aod B P
the foIJower, it ia evident that the telocltj I
ratin trill be continiullj dlminiahed, *■ the I
band Inveli from A towjrda a.
It ie deairable that the conee ahoiiM n
hiTO ■ IsTge angle, since tbe Tariatioa in tl
teniion of the band then becomee conriderable
!45. The Jwte te the mott important contriTance under thii
bead. jkn. b&, Yig. 185, are parallel aiea, one of wbicb A.a,
carriea a solid conical pnlley, or fueea, apoa the aurface oS wbicb
ia tnced a apiral eroove lo receive a cotd or chain ; tbe aiit a h
eairiej a plain cf Hnder, and one end of the connecting band ii
attached to ihe fiuee at m, tbe other end to the barrel at n. The
extent o[ motioo will be limited onlj bj the iiamber of tnint b
the fhaee. It is clear that the
Telodt^nlio wiU graduallj ^- '»■
Tary during the whole extent
of motion ; and if a power be ,
■pptied to Bfr that thall be
alwaja iniereely as the nuliui ^^
of the fiiEee, the axis of the '
latter will rcToWe with ani-
form power ; this, in clock- I
work, ia eflected by a main-
tpring in Ihe barrel B, which la woand op b^ winding the cord
from the larger to the imaUer end of the fiuee.
346. iJjTKMinpfii&y.— Inwmsfonnaofinechaniani.eapeciallj
in spinning machinery, it is Tery important to posuaa some ready
and accarate meani
of varying the Telocity- 'V- !*•■
iBlio of a driver and ■
fuUower connected by a I
band ; aa, for example, I
for the purpose of van- 1
ing Ihe speed of thi' I
bobbina, in roving and I
■Inbbing frames. An in- 1
geniona contrivance fori
Baling ibia object has ■
been patented by Henrs. Combe, of Beirast; — an gipanding
pallej.* Tbe two aidea of tlua pulley, a, b, Pig. ISS, are iep«rate
and compcaed oF alternate riba and slots, which mutually pMi each
• 8« Finy-i Bigtuiinii PiUey : Bm^ Crdapwdia.
I 2
other, and an sppnuiniUsd hj meuia of righi- and left-handed
■cren (SOS) c, d, on an aiia that pasaei through them. The ap-
praximation of a and b, and conaequent expansion of the pulley,
or MM vtni, VI eSected b; meani of the wheel a. An in thu
arranKemenl the length of the connector andentlj nriei conii-
deraU;, iti tennon ii rendered uniform hj paesiiig it over a pnlley
attached to a loaded moreable ann.
CL. B: DiTiBioad, Ootmrninieation of Motion bg Liniyieorh.
DOlice il the Hooke'l Joint, a method of connecting two aiM, tbe
direccione of which meet in a point. Let
"»-^- .Ld, B& be the two aiea whieb, if pro-
'' dnoed, would meet in X. Two lemicircnUr
« attached to the aiea,
tne enoa oi theie to a rectangular
18, of which tbe anng ce, nd^ intenect
. .. A ball or rioK msij be SDbUitaled
for tbe croeg, the 011I7 neoeuarv conditioo
being that tbe pointi of connexion, c, c,
o.d, and the point of interaaction, x,
■bould be in tbe aame plane.
34S. Ibjndthe AngiJor Vthaly-raiio of tmi Axe* emnecUd
bga ffookt'4 Joint, — Letobetlieiiilereectianof theaxea, abd (he
J. circledeeoribedbytbeaniiB of the driTinft
^' axie, which is eupposed to be perpendicn-
lar to Ihe plana of the paper; let the plane
in which both axes lie intersect the paper
in BCL, and let the ellipse, a^d be the pro-
jection of the circle described b; the arma
of the follower. If 6 be the angle con-
tained between the axes piodaced, wo
Let F 0 o be any position of the arms or the cram connected wtth
the driving axis, then h c 1, the projection of tbe arms connected
with tbe rollowiofl: axis, will be perpendicular to r o. Now in the
projection, A i n, lines parallel to ad are not altered in length;
henoe am, perpendicnlar toBC, is the sine of the angle ^ thmogfa
which the follower has been mured, reckoning from a c, and draw-
ing A b fc through H, parallel to BC,Bch it that angle; am) tba
conespondina angle, a, throogh which the drirer haa been immd
i* A c F, whicD m B c H ; then
tanfi _ coto _ Bi _ 6£ _
B00KE*8 JOIHT. 133
If we trace the progress of the arms round the circle, it appears
tbat the angles dascrihed coincide at the points a, b, ]>, l ; and that
stsrtiag from n, the follower moves slower than the driver, but the
motion afterwards becoming accelerated, it overtakes the driver at
D ; bevond that point it precedes the driver, and then becoming
retarded, the? complete the second right angle simultaneonslv.
The motion through the third and fourth right angles corresponds
^th that through the first and second.
The amount of variation of the velocity-ratio will dej^nd on the
magnitude of the angle 0 ; and if two or more such jomts be em-
ployed, the axes being either in the same or in dinerent planes,
the amount of variation will depend on the product of the cosines
of the angles 9, provided the arms of each following axis be in the
plane in which it and its driving axis lie, at the commencement
of motion.
249. If, however, two Hooke's joints be employed, and the in-
termediate axis be equally inclincKl to the first and last axis, and
also the positions of the driving and following axes reversed, as
regards the common plane, then the second variation of the velocity-
ratio will exactly counteract the first, and on the whole a uniform
motion will be transmitted ; this is the purpose for which a double
Hooke's jcttnt is frequently employed. Uniform motion may eri-
dently be communicated between parallel axes by means of this
irrangement.
250. If a Hookers joint be made use of as an universal joint of
flexure, the nature of the motion it would permit may be readily
nnderrtood, by stating that if the arm were attached to the shoulder
by a Hooke's joint, and the forearm had no rotatory movement
from the elbow, the arm might be moved in all directions, but the
palm of the hand would always be turned the same wav. In order
that the direction of the palmar aspoct of the hani might be
changed at will, as weU as the direction of the arm, it woiild be
necessary to have a third motion round an axis not in the pUne
of the other two, and which would be most advantageous, if the
third axis of motion were at right angles to the pkne of the other
two.
The joints bv which the limbs of cmstaceous animals and insects
are moved, and which are all formed between consecutive portions
of the external skeleton, are from the nature of their formation,
essentially different from the joints formed between the adjacent
pieces of the internal skeleton of the higher animals; they furnish
many beautiful examples of the principle of the Hooke's joint. For
each separate joint in these animals is a hinge-joint very curiously
constructed, but possessing but a single axis of fiexnre ; these
joints are, however, so grouped as to form universal joints, in the
manner just expUined.
The front claw of the common crab may be taken as a good
example. This consists of five separate pieces a, d, g, d, e, beodes
134
PBIHGIPLES OF MSCHAHlBir.
F, the moveable jaw of the claw itself; of these, c and e may be
considered the principal members, a, b, and d being the interme-
diate pieces of Hookers joints. The piece ▲ is jointed to the bodj
by an axis 1, 1 ; and to a second piece b, by an axis 2, 2, which is
nearly at right angles to
^.189. If 1- The piece b is
jointed to c by an axis
3, 3, nearly perpendicular
to the other two, which
is vertical in the plan,
and consequently appears
only as a point. By
combined motions round
these three axes, the limb
may be turned in any
required direction. The
powerful members c and
•^•4 Bare likewise connected
by an intermediate piece
D, the axes of motion
between which and the
former are 4, 4, 5, 5, which cross at k and are nearly perpendicular
to each other ; by the combined action of these two joints, every
requisite variety of motion is provided for.
CLASS C. DutSCTIONAL BbLATIOH CHANQIHO.
DiYisioir a. CommuniccUion of Motion hy Boiling Contact,
251. If a spur-wheel revolving constantly in the same direction
drive another spur-wheel, the axis of the latter will revolve in a
^. 100. contrary direction ; but if the follower be
an annular wheel, its axis will revolve in
the same direction as the driver (176). If
then the follower be a pin wheel, and the
pins be so arranged that the driver may be
in gear with them alternately inside and
outside, the axis of the follower will move
periodically in opposite directions: this
combination is called a mangle-wheel^ from
the machine in which it was first employed^
but it is now of frequent use in macninery.
In this arrangement a groove is cut in the surface of the disc,
concentric with its axis, except where the inner and outer portions
are connected b^ a short curve ; the axis of the driving pinion
prolonged rests in, and is guided by this groove.
ir tbs ivlocitj-ratio vera raqnirad
r, tba ptck-liiie of tlie nuuielo-whsel
in *UT required i
In R(
Ig. 191, vbich re-
oier it* H ^ .
pTHuli ft mangle-wlKel emplojed i
amtb'e eelfaoiiiig mole, lb« groote Ic I ii
uKtl; ndial, and tberefbre the pinioD
tQI not oeounaiiicftle any motion to the
■heel, when prooeading in thii portion of
it! path. In thii wheel teeth are em-
plojed in the place of pins, u the luoe
wt cannot ftct on both eide*.
253. If the reciprocating piece moie backwards and fbrwarde ia
I right liDe, which ia freqoentlj the case, it is called a manph-
rati; thii admits of ao airangement b; whicb the inconTsnieat
■biftiDg motion of the pinion maj be obnated. The man^e-iack
nprewnted in Fig. IBS, is G^m Cowper'e cylinder printing ma-
chine. In thiH the reciprocating piece b6 ia guided between
roUen; the aiia ofthe driving -^ ,„
niuoB * ia fixed, and the tit- va.
• aeepanle i © __l°1
U,' '- ■ I —
e teeth o
•f the prajecting
the gnide-g]
Dde-gToore within the
of teeth. The ^
lack i« connected with
bj two gnide rods of eqnal ^-' ^~'
feneth, OMrhioh the ends k, i, are jointed to b i, and Uie ends c, t,
lathe rack; the middle points of the two rods are abo connected
hj a monaUe croM-pieoe km. Bt these means the rack will, at
Mch aitnmity <rf its comae, be ahifled up or down nearly parcel
to itself throagh the small apace between the gtoorea.
CLC: Di*iBio«(. Oommiaueatio* of MolioH by Sliding Contaa.
254. By meana of a properly ehaped reTolTing cam-plate, a mi-
pmcating motion may be given tu a follower, which will Taiy
periodiculy according to any required law.
136
PBnrCIPLBS OF KSGHAHIBM.
I^.IH.
If oniform acceleration and retardation be reqnired, the form of
tlie cam is given hj two spirals drawn in contrary directions, pro-
ducing a heart- sliaped cnrve, b.
If several alternate moyements be required during each revolu-
tion of the cam, a curve with as many lobee or projections, as o,
will be reauired.
If the follower be required to advance graduallv, and recede
suddenly, portions of the curve, as in d, must coincide with radii.
If intervals of rest be required, corresponding portions of iho
curv^ as a &, e d^ must be concentric circular arcs.
When the cam is employed to lift a vertical bar or gtofty^t <^ ^i
the projections become separate teeth, and are termed wipen or
Uyppets.
In these instances the follower is supposed
to rest upon the cam, either by its own
weight, or the pressure of a spring; i( how-
ever, the cam be required to act on the fol-
lower in both directions, then a parallel-sided
groove of the reauired form is cut on the sur-
face of a cam-plate, a. Fig. 194, which will
guide a friction roller at the extremity of the
follower, both in advancing and in receding
action.
255. When the required alternating motion is in
the direction of the axis of the driver, the requibite
curve must be formed by corresponding elevations
from the surface of the cam-plate. The simplest
form of this arrangement is a flat disc f g, Fig. 195,
placed obliquely at the extremity of tne driving
axis Be, against which rests a bar o^, with a
friction roUer at its extremity, capable of sliding
in the direction of its leneth ; the^ effect of this
arrangement, called a swaMrptaUj is to communi-
cate to the follower the same motion as it would
derive from a crank (218, II.).
2^.195.
S80APE1IEST8.
256. When the tooth of a driving wheel, after having conmiuni-
cated motion to a projecting piece of a reciprocating follower, slips
off or ueapes from it, and eitner the same or some other tooth im-
mediately fiJls on to another projecting piece of the follower, and
communicates to it a motion in the contrary direction to the
former ; such an arrangement is called an efcopanent, which is
constantly employed in clock and watch-work. A vast variety of
eacapements have been devised, for an account of which onr readers
must be referred to the standard works on horology ; two of the
simplest may be described by way of illustration.
B8CAPKMSRT8. PROFEUICIiTB.
137
Fig. 196.
267. When the driiriiig and foUowing axes are at right aogles
to each other, the verge or crown'Wheel escapement, Fig. 196| is
commonlj employed. ▲ is the driying axis, to the extremity of
which is fixed a crown-wheel, c<2, with large saw-shaped teeth.
The reciprocating axis cc, carries two cofle**,
a, 6, set in pUines at ri^t angles to each other,
to aHow of the escaping action. When the
wheel revoWes in the direction of the arrow, a
tooth dj pressing against the pallet a, will tnm
the verge in the same direction, until its ex-
tremity is lifted snfficiently to allow d to escane
from onder it ; hnt the pallet h is then brought
into nearly a Tertical direction, and^ the^ tooth
e fislling against it produces a motion in the
axis ce, in a cUrection contraiy to the former prodaced bv the
action of a and d. When e escapes from 6, another tooth falls on
o, and the same moyements are repeated. This escapement is
now chiefly employed in bottle-jacks, and in watches of the com-
monest kind only.
256. The most simple escapement adapted to parallel axes is
the anch(fr or lever escapement. Fig. 197, in which the revolving
wheel has pins, 1, 2, 3, &c., and re-
▼oWes in the direction of the arrow.
The Tihrating axis b has a two-
armed piece carrying pallets at its
extremities, somewhat resembling
an anchor, whence Ihe name. The
pin 1, is shown in the act of driving
the pallet a 6 by sliding towards 6,
and thereby turning the axis b in
the same direction as the driving
axis ; when the nin 1 escapes from
bf the pin 3 will fall on edt and
sliding towards d, wiU drive the
axis B in a direction contrary to the
former, and these actions will be repeated alternately.
Another simple form is seen at d, in which the pallets m, n, are
attached to the same arm, and are acted on alternately by the
same or consecutive pins. This e8cai)ement has assumed a great
variety of forms : the 9ocq>e^heelf as it is called, is now usually a
spur-wheel, with slender pointed teeth. If the vibrating piece
OBC move on an axis distinct from that of the balance ^eel of
a watch, which is universally the case in practice, it is called a
detached escapement.
1^.197.
FBOPELMEKTS.
259. In all escapements, properly so called, the wheel is em-
ployed to drive, and its teetn successively escape from, the reci-
procating fbUower ; bQt in Bome formi of mecbaninn, m in alecMcal
clocki and dial-telegraphs, tha nciptocating piece may be employed
to drire the vlieal, aDd to this modiGcation the term prop^meiU
has been applied. Several arrange men tg have been devtaed for
thia purpose, but it vitl suffice for the present to describe an in-
genious maohuiiam applied by Profeasor Wheatatone, in his very
elegant and efleotiTa dial- telegraph. In this a reciprocating ai
Flt.lM.
noTingoi
natcd
by an electro-magnet (Ch.XV.J, carries
at ils extremity a spur-whee!, a,
which ia driven by pallets at the ex-
tremity of two fixed springB, d, b.
The acrowB indicate the directions of
the alternate motion ofB,duringirhich
thej respectivelj act ; and the direc-
tion of the motion of the wbeel is
also shown by an arrow. Tha piece,
B, IB repraMnted near tha middle of ils
motinn from Tight to left, and (he tooth,
r, ia abont to be held by the pallet, D,
while the onward motion of B canies
the wheel round, and the tooth o slips over the pallet, s, to be
held by it daring the reverse movement of ■ from left to right,
during wbidi the next tooth, h, slips under the pallet, n, and the
same actions are repeated. In all these contrivancei some means
must be osed to prevent the backward action of the wheel, and
to ensure its progioeaive, though interrupted, motion ; this is evi-
dently provided for, in tha propalment here described.
CL. C : DiTisioM d. — OotnmvmcatUm of Motion by LitJMBork.
S60. If it be required to oommnnioate ■ reciprocating motion
bj link-work, the reault is moat frequenll; obtained by means ofa
crank (21B), or eccentric (354), the reciprocating point q. Fig. 199,
fff^ l^ being either constrainad t
n the line a
Let
A F be the radius of the circle
described by iha point r
round the fixed centi^ a, and
FQ the link; take nn, nd
eacb = PQ, then Odin tha dis-
tance through which the ptnnt
Q oscillates. The distance a r
is called the ihraa of the
crank, or eccentric, and in aod
« are the dead points of the nstem.
2S1. In the ecoanttio, which is the most common form, the
link is t«muaated by t, hoop ab, Fig. 119, which embrdcss
TABT1M TmUXWT-miTtO W I,t«-»<I«K.
■ wctMigoUr irrocrrt in tba cjr- **■ "•■'
CDDifereDOe of the ectgntric diK ;
tlie hoop >■ made in two balvei,
ooDDCCted by scram »t o, 6, in
order to enable it to enter tb«
rman. The thro* of Uw eccen-
trie )* endentlj tbe diitance
betwMn ihe centre of the disc
and ibe centre of the uie on
which it ie fiied- . - j i_
a6! A »«ri»tion in (he Tflocitj.nitio may be obtained bj 'mj'
mg Ih'e pomtion of ihe link with raspect to the mtu portion of
the drinng mm : a mpidly retarded Telocity maj bo Uiiw ptodoeod.
centre of motion offtBc^and oofod,
and let the anna ji(i,B&,aiidBCDdbo
(ODDected by Unka 06, ed. I*t io,,
1 a- « a« be three eqnidiatant pootiona
of iIm ann io; a, midway between a,
aado,; and i„fcpt,; e,. 61, "Jj ; ^i. <*!.
be oonMponding poations of the pomts
fr c, (i, and let the poaition of ■ be inch
that the line 6, 6, produced nearly bi»act«
a. a Then it 11 evident that the ipaoe
b, fc'oorreBpondioR to a, a, will be mncb
ereater Iban 6,6, which correeponda with
0,0,; and if the arm od hew placed aa
to be parallel to e, e- when in the poai-
tion n iL, it ie also evident it wiU remain
ment from e, 10 e^ that ii, doring the moiement of tba ann *«
A.^^imtemhe,ed, da maybe connected with afc, 10 thai
the arm d d would reat during tbe motion from 0, to o, ; thH u
tbe principle of oonatrnction in Erarde donblftacbMi harp.
for »4jn»ting the qoantily of mo-
from 0 np to w
commnmeated bj ■ aimple
hnk connecting b and 0,
which we may call unity, or 1. For thii puipoae tet *B
•BHwit>a lure for thaos ' "'-
.^. A »ery ingenione pli
tion transmitted by link-
work baa been deiised ,
by Prof. WtUis. For ex-
ample, let it be required
to ttinsmit any given
emonnt of motion from the
crank 4B, Fig. »01, to
ooDDSctpd with » fixed piece, f a, b; > link, b ■, and an urn I r,
a little longer than Sad. Then let a triansnlar liok h e L be
joined at k to aa arm ems, moreable ronnilii, and capable of
leing flxed in ita pmition (h by a screw and Dut noving in a
groove o f), and let tbe pomt L be oonueeted at the point >
wilb the arm e r by a link l >, iDoh that the distanco between
tbe centres he, kl,lh, eh, fB,LR, may each be equal to S a b.
When by the moTement of h towards o, Uie pranti f, e, and coo-
•eqaeDtfy the poinla u, b, are made to omndde, the motion ia
the same as if tbe points h and b were connected. If h ba moved in
the opposite direction towanli r, nntil the pointa r and i. coincide,
then the arm ■ f will oommanicate no motion to the point i. and
coueqaentlytotheanncD. Ai the paints advances from P too, the
EnotioD of the point c will increass nearly imifonnly from 0 to I.
The ^ore represents this mechanism in ita mean position, in
which the distance between ths extreme positioa* o( the point c
«». SM. 'il •* nearly equal to * b.
364. If it be required to moltiplj oacillatifnis
bylink-woik, letoAcFig. 303, be an OKnllating
arm, moviug ronnd the centre a, and let a, a Cj,
a,AC„ ba its extreme and a, a e^ its mean posi-
tion, and let the aim a 6 ba so placed that, in ila
extreme positions, tbe points h„ i, may be in the
line a, e,, and let the two arms be Munected by •
link b e, then, when e reaches c„ h will reach b^
and will retnm to £, when e arriTes at e^ so that
(, and b, will cobcide ; thus a doable ntoiement
of B & will result from a sinsle movement of a a.
The motion transmitted by link-woTk may be vari-
onslj mndified by oomtnnations of this and the
preceding MnanRemsnts.
S65. The ratclutvihea and tSei is an anangement frequently
employed for oonunnnicating an intermittent nkotion from an
•ItematinE driter to a revolving follower. The driver is an
arm of wMch ths centte of motion is a, Ft«. 303 ; the follower ia
J^, ]0g_ the ratchet-wheel, having teeth fortited
like those of a aaw. lie piece b c ii
jointed to tbe d[iving«nn at B, and
resta on the wheel by its weight. If
tbe arm be moved into the poution a h,
tbe extremity of the piece c will puah
the radial side of the tooth against
which it rest*, so as to turn the wheel
in tbe direction of the arrow ; when tbe
arm retunu to it* former positicii, tbe
piece BC will slipover the slanting side
of the tooth, and fall into another epaoe.
To inaors the wheel agwnat any aod-
denUl backwATd motioD, uiotheT •im, dw, or ke, actiDg by it«
wei^t, or n**, actiog bf ft iprtDg, » (idded to detain tbe vlteol;
this U cklled > deleM, uid ths piece b n j^ j^
■ cUei. In order that tbe click maj hold
the whe«I efbctnall}', it ia necMaar^ that ■
itt piw»niu on tbe looth, i««oli«d in the I
direclioa of ita inrfac* (71), maj act I
tmtdfxb the centre of tba wfaesl. I
366. The dririDK-piece maj be made I
to act on the ratchet-wheel dimnBits I
motion in both dirsctiona, il, aa in Ft((. I
304, a click ab be attached to the drir- I
ing-arm at a, and another, e ^ at a point
c, equidistant with a from A.
367. With doe attention to the reqninie dit«clion of the line of
action, a click or detent may
be applied to a common spur- rig. Mf.
wiiect aa at A, Hg. 305, or
to a pio-wheel, aa at B. A
detent i* mnetimea required
lu retain the wbeel in an
exact position : for this pnr-
poae it rbonld reat on two
cooaecntiTe teeth aa at c. Or
if the detent be not required
to witlutand much resiatance,
it may be fomiahed with a
roller, aa at d, which wilt
render ita action mora easy.
The aereral doited lines show
the nonnala of contact.
268- In click-wurk, the slipping and ci
ntcbet prodnced much noiae and wear ; theae incooreniencei may
be cbriated br tbe employment ot
the tiUai didi. one of the aimpteat
fbnna of which is repreaentea in
Fig. S06, in which the wbeel d is
concentric with ths arm d, which
cairies the click ph, joined to it
atjr; itiesnu Ao la also conoentric
with tbe wheel, and ia connected
with the click by a link e/, jointed
at « and /. Tbe motion of the
arm ac on the piece n, ia limited
In- two pina, a, 6. When the end
of the arm c i* raored downwards,
the click will carry the wheel in
the dinectioD of the arraw; but
of tbe click and
Fig.iM.
142 pRnroiPLBS or mbchakibm.
when it ii nised into the poaition a c, proTided the frictinn at the
points A. e,/, j7, be less than the resistance to the motion of the
wheel, tne click will be raised out of the space between two teeth,
and then the piece b will be carried round the axis a, leaving the
wheel at rest. When the arm c is again depressed, the click h
will be brought into another space ; the motion of the arm will
then be communicated to the wheel ; thus the first part of each
alternate action of the arm a c is employed in silently engaging
or disengaging the click.
269. An intermittent motion may be ]^roduoed hj link- work, by
makingf a slit at either end of the link, m which case the motion
will be intermitted, while a pin working in the slit is passing from
one end of it to the other.
Having now investigated the mora simple oombinations of ele>
mentary mechanism, our readers must be referred to the treatises
on machines for a detailed account of those more complex arrange*
ments by which either an aggregate velocity ratio may be ob-
tained; such as the complex trains of wheelwork for lunar,
sidereal, or e(}uation clooks, and orreries ; or an aggregate motion
in space, as in the parallel motion of the piston-rods of steam
engmes ; or a combination of both, as in the various motions
required in planing, boring, slotting, shaping, screw-cuttingi and
various other kinds of machinery.
148
CHAPTEB VI.
BTHIMICS; THB SELATI0V8 Or BODIES UT MOTION.
270. Th£ preceding chapter on Statics bas been devoted to tbo
ooosideration of matter in a state of rest ; the properties of matter
in motion, constitnting the science of Dynamics, nave nert to be
inTOStigated.
Bj motion is miderstood the act by which a body changes its
posilioii. It has been divided into several species ; thns a body is
said to be in abtolttte motion when it is actaally movinf from one
part of space to another, instanced in the moTcments of the planets;
and to be in a state of relatwe motion, when its position is con-
sidered only in relation to some other body : thus a man standing
in a sailing vessel is in motion with relation to the shore, and at
rest in relation to the several parts of the ship ; in this case also
his motion is said to be eommom, with that of the vessel. Besides
these, there are some other divisions of motion which it is im-
portant to understand ; thus the motion of a body is vnifarm,
when it passes over equal portions of space in equal times ; it is
acoderaJted, when the successive portions of space passed over
are increased ; when diminished, it is said to be retarded; and
when this increase or decrease of motion is constant, the motion
is said to be unifomdy accelerated, or retardedm The motion of
any body is wwifter or «2otoer, in proportion as the space passed
over in a given time is greater or le«s. The degree of rapidity
with which a body moves is termed its velocity^ and is measured
by the space uniformly passed over in a given time.
The number of feet traversed in one second is the usual measure
of velocity ; if this number be called v, and « the space described
in (^ (t seconds), then evidently «» t x v.
271. If the velocity of a moving body be variable, its measure
at any given time is the number of feet that would be uniformly
described in 1', if the velocity remained the same. This is what
is meant by the term rate in common parlance ; as, " the train was
going at the rate of fifty miles in an nour," "the winning horse
came in at the rate of a mile in a minute ;'' meaning, not that a
mile was, or could be, actually passed over by the horse in one
minute, but that it would have been so, if the speed had continued
uniform for that period of time.
272. The releiive velocUy of two bodies is the rate at which
144 DTVAMICS.
they recede from, or approach, each other; if they move in the
same straight line, the reUtive velocity will be the snm, or dif-
ference, of the absolute velocities, accoraingly as they move in an
opposite, or in the same direction.
273. In the chapter on Statics, the effects of pressures producing
equilibrium, or rest, have been investigated ; it remains to consider
in the present chapter, the effects of pressures producing motion.
Accelerating force is measured by the velocity generated in 1"
by the continuous action of an uniform pressure ; and if the pressure
be not uniform, by the velocity which would be generatea by the
given pressure acting uniformly for 1'. If we call ihe accelerating
force ^ then /will be the velocity generated in 1", and if v is the
velocity generated in <•, then v =/x *.
274. The nuue of a body, or the quantity of matter it contains,
is measured by its weight, at any given place ; and generally, it
is measured by the weight divided bv tne accelerating force of
gravity, which is not uniform at dirorent points of the earth's
surface (63).
275. The votume of a body is its bnU^ or the space which it
occupies ; the unit of volume is one cubic inch. The denaity is
the quantity of matter contained in an unit of volume. If we call
u the mass of a body, d its density, and v its volume, that is, the
number of units of volume that it contains, then h =y x d :
and if w be the weight, and g the accelerating force of gravity, then
WaspXMsspXy XD.
276. The momeiUwn of a moving body is its mass x its velocity ;
but the moving force is ihe momentum generated in 1", that is, tne
product of the mass and the velocity acquired in 1* : it is neces-
sary that the distinction between the terms momentum and moving
force should be clearly recognised, as they have been sometimes
confounded together.
277. It follows as a conseauenoe of this definition, that when
the momenta of two moving bodies are equal, their masses must
be inversely proportional to their velocities *, and conversely, if the
masses of two bodies are inversely proportional to their velocities,
their momenta will be equal. Also, a light body will, by having
its velocity, and therefore its momentum, increased, possess as
much momentum as a heavier one, having a proportionably slower
motion. A cannon ball, of 3 pounds weignt, possessing a velocity
of 300 feet in a second, will possess as much momentum, as
one of 30 pounds moving at tbe rate of 30 feet per second, for
300 X 3 = 30 X 30. The existence of momentum explains why the
large masses of loaded ships or icebergs, althougn moving but
slowly, are capable of exerting such enormous force upon bodies
with which they come in contact. The term momentum has been
applied indifferently to the quantity of motion existing in a body,
and to its striking force, or power of overcoming resistance : the
latter, hereafter explained as the vis viva of a moving body (338),
unsTiA. 145
depends on the 9qujaTt of the Telocity, conseqnently the striking
force of the small cannon hall above-men tioned is 10 times that of
the larger one, and not simply equal to it, as it sometimes is re-
presented to he.
278. All forms of matter, whether in the atom, or in the mass,
are alike inert, and incapaUe, hy the exertion of any spontaneous
force, of changing their state or position : loAereoer a body is
placed hy any external cause, there it must remain forever, unless
acted upon by some disturhing force. This property of matter is
termed its Inertia, or passiye resistance to a change of position.
JHe amomd of tnertia increasei with the qttantity of matter.
The resistance experienced on first settine any body in motion,
and the difScnlty experienced in stopping it wnen moving, arise
equally from this cause ; for being absolutely inert, it follows that
matter must retain its state of motion, as well as of rest, for ever,
unless acted on by opposing forces.
279. The ' following are examples of the effects of inertia: in
turning a fly-wheel, hy means of^a winch, a decided reaiatance is
at first experienced to our attempts ; this is gradually overcome,
and then the wheel continues to move rapidly by the continued
application of a force, just sufficient to overcome the resistance
oiBered by the medium in which it moves, and the friction at the
points of support. In a team of horses attempting to move a
neavily-laden waggon an immense exertion of muscular force is
required to overcome its inertia, but this once effected, the horses
continue to draw that weight with facility, which at first they
were scarcely able, hy the utmost exertion of their physical force,
to move. A traveller sitting in a coach, on the horses starting, is
thrown backwards: his inertia opposing a resistance to his body
acquiring at once the movement of l£e vehicle, and therefore
tends to leave him hehind-; and on the coach stopping suddenly,
he is thrown violently onwards, from the inertia of his body
tending to retain the mo-
tion previously acquired. J^. 207.
A hoDet thrown at a pane
of glass hreaks it in the
direction of numerous lines
radiating from the point of
impact, but fired from a
rifle at the glass, it merely
pierces a circular hole, the
tnertia of the glass pre-
venting the surrounmng
portion from yielding to
the rapid motion of the
bullet, and oonsequentlj
that portion only which is
opposed to the impact is carried onwards, and participates in the
I.
rapid motion of the ball. Similarly, a. common (allow candte, when
fired out of a musket, will pierce a bole in a deal board; and aatick,
of whicb tbe enilii rest on two wine-glasees, Fig. 207, mnj be broken
b;n smart blow with & poker in its ceotro withoDt injuriog its brittle
aupportB. The eiistenoe of inertia maj likawiae be demonetrated
b; a more familiar experiment. Let a card witli a coin, of not too
Boiull a aize, laid upon it, be poised on the point of the finger ;
tiien by a dexterous Sip of a finger of the other hand against the
edge of the card, it may be displaced, teaving behind it the coin
ntill poised on the finger. This remll is due to ^S inertia of Uie
coin, which is, however, to a small extent displaced by the friction
of the card against it,
230. The Conaation Fuxe, — An admirable illustration of inertik
utilized is met with in Sir W. Armatrong'g oonousaion fuM. The
object of this ingenious deiice is to insure the explosion of ■ shell
the ingtant that it strikes any object against which it is fired, bnt
by such means aa will not cause explosion at any slight concoa-
KioD, such as a fall on the ground from (h'- band, or tlie upsetting
of an ammmiition waggon. Many different kinds hare been con-
g^ 2(i§ Btrucled to meet special pnrpoeasi that repre-
sented in_ Fig. 208, is intended to be enclosed in
the interior of a shell supplied also with a time-
' , in order to ensure explosion, in case the
>r fails, or has been adjusted for too lone a
J ; and is so placed aa to move, on projection,
in the direction of the arrow. It consists of an
iron cylinder closed b^ a plate at both ends ; in-
side this is a brass cylinder, a, capable of sliding
in the former, and anneil with a steel polut, c,
which, when It slides forward, strikes and ignites
a small portion of percussion powder, in a plug of
wood, d. This in ita tnm ignites the p-iwdcr, #,
in the top of the fuze, whicb again, through a hole in the plate
covered with gaoze, ignites the charge of the shell. Bat the
firing pin and cylinder is fixed in a safe position by a sling of
severaTsmall vires, b. b, which is soldered to the bottom of a, and
to the oatude of the fuze. The ilin^ is sufficienllj strong to hold
the pin in its place during any accidental concussion, but when
fired from a gon the velocity of projection is so great, that the
inertia of a breaks the sling, and the cylinder retiree to the bottom
of the fnze ; but it is now free to move, and the instant the motion
.J efi'ected.
Moreover, the seecient- shell of the same ingenious inventor, the
most destractive misBile for field parposee that has hitherto been
devised, is a further illostration of the application of inertia. Thi«
shell coniislt of ■ large number of rkdial eegmenta of flat rings
SBWTOIi'8 FISST LAW OF MOTION. 147
packed close tc^ther in a thin cast-iron case, and sarroonding a
sdulII cyUndrical carity in the centre, which contains just enough
powder to burst the outer iron skin, but without scattering to anjr
great extent its fragments, or the contained segments: these
pursue a slightl? diyergent course, by their own inertia, and are
probably as fatal in their effects as they would be if fired from a
six-poundeT blunderbuss at the point where the shell bursts.
281. In consequence of the inertia of all bodies, force must be
applied to cause them to assume motion. If it be merely intended
to caose the body to move in the same horizontal pl&ne which it
preriously occupied, the applied force must be sumcient to oyer-
c<Hne the tTiertia of the body, together with the friction of the
supporting body. But if it be intended to place the body on a
higher horizontal plane than it previously occupied, the appUed
force must also be sufficient to overcome the attraction of the earth,
or force of gravity (58).
THE BSWTOlflAH LAWS OF MOTION.
The simplest principles to which the phenomena of motion
can be reduced nave been arranged by Newton in the form of
three axioms or laws; well known as the Newtonian laws of
motion. These laws will be found to be variously expressed by
difierent authors, but their import is in all cases the same,
FIRST LAW OF MOTION.
A body at rest w3l continue at rest : and if in motion^ it will
continue to move in a right Une with uniform velocUyt unless
acted upon by some external force.
282. This law is a necessary consequence of the inertia of
matter (278). The second part, however, referring to a moving
body never resuming a state of rest until acted upon by extemid
force, might at first be doubted; but a little reflection on the
commonest phenomena presented by moving bodies will expel this
doubt, and demonstrate the truth of the Newtonian axiom. The
veiy inertia which a body^ when at rest opposes to any applied
force which tends to put it in motion, equally opposes its return to
a state of rest, when once in motion. Therefore, whenever a
body in motion comes to a state of rest, we may safely infer that
some external force has been exerted to check its pro^ss through
space, llie chief external causes checking the motion of bodies
are, 1. Friction. If a ball be thrown alon^ a common road, its
motion becomes obstructed from its encountering so many obstacles,
and it soon stops ; on a smooth bowling-green or level pavement,
there being less friction, the ball moves to a longer distance, and
still further on a smooth sheet of ice, from the great diminution
of friction : and an accurately balanced wheel running on smooth
L 3
148 DTVAMICS.
piTotfl, well oiledf will continue its rotation for some time ; but
for a much longer time, if the piyots rest on inction-rollers.
2. Benstance of the medium in tokieh it ie moving. The re-
sistance of the atmosphere has been already referred to as a
powerful cause in checking motion ; it may be very satisfactorily
proved by causing the wheel already mentioned to rotate in air, and
then in tne vacuum of an air-pump ; it will be found to continue
in motion for a much longer time when the resistance of the air is
removed.
BECOITD LAW OF MOTIOV.
The ejfeet of a given force aetina on a hody^ ia the same in mag-
nitude and direction^ whether tne body be in motion^ or at rest^
or aimuUaneouely acted on by any other forcee.
283. It may be observed in illustration of this law, that a clock
pendulum vibrates in the same time in any vertical plane ; that
IS, whether its motion in common with the earth's surface is, or
is not, in the plane in which it oscillates. A heavy body dropped
from the mast-head of a ship in motion will fall at the foot of the
mast, for the horizontal motion which it has in common with the
ship will not interfere with the motion resulting from gravitation :
and if the vessel were pitching or rolling at the moment the falling
body is released, it would reach the deck at some distance from
the mast, and in the direction in which the mast-head was moving
at the time when the body was released. Here two, or even three,
distinct motions may be communicated to the body, without at all
interfering with each other's effects. The two following experi-
ments will further illustrate this law. A ball is placed in a small
carriage on wheels, containing a spring, which when released, will
project the ball vertically, that is, at right angles to the surface on
which the carriage rests. If the carriage be placed on an inclined
plane furnished with some means of releasing the spring at a
ffiven point, during the descent of the carriage down the jplane,
it will be found that the ball will drop into its place in the
carriage, just as it would do if the carriage were at rest On this
principle equestrian performers, who leap over ropes, or through
noops, from the back of a galloping horse, and regain their foot-
ing, find it necessary to leap only umoards and not forwarder
HiiB practical knowledge is probably obtained by such individuals
at no small personal risk, which
^* *•• a little acquaintance with theory
•B would obviate.
284. If a ball, a, be placed at
the comer of a smooth table, a g,
in the form of a parallelogram.
Fig. 209, resting against two
springs, E, F, which, ^ when re-
leased, would respectively drive
11 1 . ■ 1 ■■ _— — -•IP J.iB « J ^^■•WBV. .^.•J.^^.IW
VBWTOH*B THBD LAW OV VOTIOV. 149
the ball alonp the adjacent sides of the table i. d, i b, in the same
time, then, if both springs be released at once, the ball will be
found to describe the diagonal of the table a c. It may here be
remarked, that all the conditions that have been hitherto deter-
mined with regard to the resoltants of Btatical preswres (70, 71),
applj similarly, mtUatU mutandiSf to the resultants of motions
produced bv tfynamdcal forces. The proposition in this form is
commonly known as " the paraDeloeram of velocities."
285. lUnstrHtions of the action of one force on a body are too
£uniliar to require notice ; of two forces, we bave an example- in a
beat tending to be carried westward by the tide, whilst the boat-
man, by the aid of his oars, attempts to (Urect its course north-
ward ; supposing both these forces to be equal in intensity, the
boat proceeds in the direction of the diagonal of a parallelogram,
of which the two adjacent sides represent the direction of these
forces, or north-west. A steam vessel, of which the paddles propel
the vessel northward, whilst the wind blows eastward, and the tide
running in a third direction, illustrates the application of three
forces ; for the vessel obeying all three forces simultaneously, sails
on her way in the direction of a resultant of the whole.
THIRD LAW OF MOTION.
When a preuure produces moLion^ the momentum generated in a
given time is proportional to the pressure.
286. The truth of this law may be satisfactorily established by
the following experiment. Let a quantity of matter, Q, consisting
partly of several equal small weights, p, be placed in a wheel-
carriage, on a horizontal table ; and let a stnng attached to q,
and parallel to the surface of the table, pass over a pulley fixed to
the edg^ of the table. A weight, p, is attached to Uie end of the
string, and the space through which the carriage is moved on the
table, in a given time, by the descent of p, is observed. If we
now remove a weight, p, from the carriage, and attach it to the
end of the string, we find that q will describe the same space in
half the time ; and if a second weight, p, be removed from q to
the end of the strine, in one-third of the time, and so on. Now,
in all these cases, the quantity of matter put in motion is the
same, and, therefore, the momentum is proportional to the velocity
(277), but the experiment shows the velocities generated by the
pressures r, 2 p, 3 p, &c., to be as the numbers 1, 2, 3, &c., and,
therefore, also, the momentum generated is proportional to the
pressure.
This third law has sometimes been expressed by the terms
" action and reaction are equal, and in opposite directions ;" which
have been abandoned, from the difficulty of assigning any definite
meaning to the terms '* action and reaction." Thus some of the
£acts which we find adduced as illustrative of the law thus stated,
150 DTlTAiaGS.
are nothing more than examples of the eqnalitj In amomit, and
oppositeneBs in direction, of two mntually counteracting statical
pressures, as the '* action" of a weight supported is equal to the
" reaction" of the support. An experiment frequently adduced in
evidence of the equality of " action" and " reaction" is that of float-
ing separately a magnet, and a piece of soft iron of equal weight,
on the surface of water : when placed at a distance from each o&er,
and then released, they will move towards each other, and meet
midway between their firat position ; here, then, the " reaction' ' of
the iron on the magnet is said to be equal to the " action" of the
magnet on the iron. But this experiment does in fact illostrate
the third law as expressed aboye, for the attraction is a mutual
foree, by which equal momenta are generated in the two bodies ;
and this may be farther proved by making the soft iron double,
treble, &c., tne weight of the magnet, when it will be foimd that
the space it has passed over at the point of meeting will be i, ^,
&c., of that traversed by the magnet, and hence in aU cases the
momentum generated in one body will equal that generated in the
other in the same time, and by the action of the same force.
The recoil of an elastic body from another body, on which it
impinges, has also been adduced in support of the equality of
'* action" and " reaction ;" but this is evidently nothing more than
an expression of the property of elasticity (18).
287. It follows as a consequence of the third law of motion, that
when forces of eqnal intensities act on bodies free to move, they
cause the bodies to move with velocities which are in the inverse
ratio of their masses, because in all these cases eoual momenta
are generated (277). So that if equal charges of exploding powder
he made to act upon bullets, of which the masses are as 1, 2, 3,
4, 5, 6, &o.f it will cause them to move with velocities as the
numbers, 1, h, 4, 1, {-, i, &c.,^ so that the bullet of the mass 6
will be propellea with a velocity one-fifth of that with which one
of the mass 1 is projected. Hence, for eoual forces, the masses
of the projectiles multiplied by their velocities give the same
number, and this is termed the ^ptaniity of motion; and a force
double or triple of another will produce two or three times the
quantity of motion. From the same law, the following conditions
have also been deduced, as corollaries :
(A.) Forces are to each other as the momenta they produce, or
as the masses multiplied by the velocities.
(B.) For equal masses, the forces are to each other as the velo-
cities they produce.
(C.) For e<|ual velocities, the forces are to each other as the
masses on which they act.
It may be here remarked with regard to the laws of motion,
that they are not susceptible of any general demonstration, nor
even can a result accurately true be obtained by experiment, be-
cause the interference of prejudicial resistances, sucn as friction,
OOLUBIOa AHD IMPACT. 151
•nd the resistaiice of the atmonphere, cannot be entirfliy obviated ;
but it has been shown that in proportion as we remove obviooaly
interfeiiBg cauaee, in the same degree the. practical approximates
to the iheoretical result. Again, the correct, solution of the most
complicated problems of physical astronomv, as for instance the
occurrence of a lunar eclipse, true to a second to its lon^-predicted
epoch, based as it is upon the assumption of these very laws, must
be Bufficieut to convince the most sceptical mind of the entire and
abstract tmth of those laws, from which such all-important de-
ductions flow as a neoessaiy consequence.
OOLUSIOM AHD IKPAOT.
288. When two bodies come into colb'sion, their opposing sur-
fiu^es are mutually compressed, until their velocities become the
same ; and during the time that this equalizatioa of velocity is
taking place, velocity is generated in one of the bodies, and de-
stroyed in the other. When their velocities have become the
same, the bodies will, if perfectly inelastic, move on together ; if
elastic, their elasticity brings new mutual pressures into play, by
which the bodies are separated. This impulsive action that takes
place between the bodies, and which generally occupies an inap-
preciably small period of time, is called Impacts The impact of
two moving bodies is said to be directy when their centres of
gravity move in a straight line passing through the point of
Impact ; under other circumstances the impact is said to be obUque*
289. In treating of the theoretical effects of impact, many
authors have ascribed to bodies the hypothetical property of per-
fect hardness or incompressibility, a property to which not the
slightest approximation exists amongst natural objects ; and the
hypothesis is purely gratuitous, as the absence of elasticity is the
property actually required.*
All kinds of matter mav, therefore, in their mechanical action
be considered as either elastic or inelastic. The Elagtidty of a
body is the ratio that the foitoe of restitution (19) bears to the com-
pressing force, or
_ force of restitution ^
^roe of compression
The quantity c has by some authors been confounded with the
Modtdut of JSiasticUy^ a tenn that has been applied to the nume-
rical value of the force which would be required to elongate a
prismatic bar of any given material to double, or to compress it to
one half its length ; that is, provided the elastic limits of the sub-
stance permitted so great a coange of form.
* The absurdity of the bypotliMis ii rendered manifest by etating thet
tbe nirf»?»^«*^ oonaequeneee otptrftel hatrdneu are firequenUy pQatrafeed in
leotnres by the impaot of hunpa.of patty, or moist day I
152 DYVAMICS.
Table of the Valuee of tin some SvhHaneee.
Glass 0-94
Ivory 0*81
Hard steel . . . 0*79
Cast iron . . . 0*73
Bell-metal . . • 0*67
Cork 0-65
Brass 0*41
Lead 0*20
290. In many highly elastic bodies it is found that the amoant
of mo]ecnlar displacement is proportional to the disturbing force,
within considerably wide limits. Of this, glass threads, spiral
springs, and vulcanized caoutchouc are conspicuous examples.
The elasticity of glass threads is manifested by torsion, and the
angular deflexion is found to be proportional to the deflecting
force : this is the principle of Coulomb's torsion balance. The
linear extension or compression of spiral spring obeys the same
law, namely, that the lengthening or shortening of the spiral is
proportional to the pressure. Several varieties of weighmg ma-
chines are thus constructed, and by the same means, the pressure
on the safety-valves of locomotive engines is regulated. Precisely
the same kind of molecular disturbance takes place in the exten-
sion or compression of spiral si>rings, as that which occurs in the
torsion of a single filament, as in the first example ; the torsion of
a spiral sprine correspODds with the flexion or bending of a fila-
ment or thin lamina. The same law is observed in the extension
and compression of caoutchouc ; this is most readily shown in the
extension of a thread, or narrow strip of a lamina, of this substance.
291. The third law of motion having been established without
reference either to the amount of a force, or the period of time
during which it acts, must hold with respect to impact; conse-
quently in the direct impact of two bodies, whatever momentum
IS lost by one of them, the same is gained by the other. Let the
masses of the two bodies be m and m', and their velocities before
impact V and v', and first let them be supposed inelastic, in which
case they must move together, after impact, as there is no force
tending to separate them. Let x be tne momentum communi-
cated m>m m to m', then
m,v — a; = the momentum of m after impact,
m'.t/+x= „ „ ml „ „
and "" = the common velocity after impacts — '—. —
whence a:=^^,(t;-tO (a);
therefore the velocity lost by m= — = — ; — > (v— t/) . . (6 :
fn fn+m
* . . . gained bym'=—/=s —- — -,(»—»') . . (c):
coLusioM OF nrxLAsnc bodixb.
153
and the common Telocity after impact
w-
If the masses are equal, m^ml^ and (d) becomes \ (v+v'). If
the Telocities are also equal, and in opposite directions, t/a -v,
tnd (<2) disappears altogether ; that is to sa^, two equal inelastic
bodies meeting each other with equal yelocities will, after direct
impact, remain at rest
f»
If t/=o, (d\ becomes ,0, and if also me m' U[\ becomes h v.
392. The troth of these and many other corollaries may be thus
shown experimentally: — Let two hollow Fig,tiO,
vooden cylinders, of equal weight, one of
which is furnished with a conical pin
capable of entering a hole in the other, be
suspended by equal strings from a nori- ^
lODtal frame, gd, Fig. 201, the quadruple
Rtapeosion being designed to ensure steadi-
ness in direct impact. If one of these, a,
be raised by the hand, and allowed to im-
pinge on B at rest, the two will remain in
contact after impact, and will describe an
arc very nearly naif that through which a descended:* and if
either of the cylinders be loaded so as to make it double, treble,
&C., the weight of the other, then corresponding spaces, according
to the formuu, will be found to be described. If both cylinders
be equally raised, they will remain at rest after impact, if equal ;
or if unequal, they will moye through a corresponding space, in
the direction of the motion of the heavier weight.
293. Let us now suppose the bodies to be elastic, and c to be
their elasticity ; then, m consequence of the force of restitution,
111 will lose and nti will gain an addit^nal quantity of momentum,
which will be c X, and the whole momentum lost bj m and gained
by m' will be
but (291, a)
a: =5
x-htjXj or (l+f)a?.
X IK
therefore TeL lost by «»==»(l+«)-=»(l + e)r-ri-i(*^— «0 •
X
m
and yeL gained by m'=(l+e)^=(l+e)^^;^,(»-«^)- • to)
and if « and tf' are the yelocities of m and m' after impact, then
* BtffetlT speakiBf , the yarsed sineB of the ana desoribed will be as 1 : 2 ;
and gMMnulyf as m : •+■•'.
154
DTHAMICB.
(»)
tt=« — (1 +g)^:rr^(p— ty^)= ^ . ^/ —«rr;;p (t>— t;') .
wi+w^ ' fn+m 111+ 111
If e be made^o, then the second terms of {h) and {k) disappear,
and their values both coincide with (d). '
If the elasticity were perfect, e=l, and (/), (g\ become
2m'
(»— rO, and
2tn
■dv'V);
Fig. 211.
«l + OT V - " fll + m'
therefore when the bodies are perfectly elastic, the velocities
respectively lost by m and gained by w» , are double what they
would be, when the bodies are whoUy inelasttc (291, &, e).
If flisfn', i/so, and € = 1, the quantities (/) and (^) become
each = v; that is, if the bodies are equal and perfectly elastic, and
one impinges on the other at resl^ the impinging Dody will re-
main at rest, and its whole velocity will be transferred to the
other body.
294. These and other deductions from the general expressions
(/)» (9)t (^). (^)i may be thus shown by experiment: — Let
two equal ivory balls, a, b, be suspended bpr equal threads from
the hooks, o, d : if one of these, as a, be raised by the hand, and
allowed to impinge on b at rest, it will be found that ▲ will re-
main very nearly at rest in b*8
place, and b will swing into a
position b', nearly equidistant
with that from which ▲ de-
scended; B will then descend,
and impinging on a, will occupy
its place, and a will be driven
back nearly to its former position.
If the balls were perfectly
elastic, the same motion would
continue, until gradually ex-
hausted by the resistance of
the atmosphere; but owing to
their imperfect elasticity, uiey
begin to oscillate in the same direction, after three or four suc-
cessive impacts.
295. If instead of a single ball, b, a row of equal balls be suspended
from equidistant hooks in the frame o d, b will receive nearly the
whole momentum of a, and will transfer nearly the whole of what
it has received to o, and so on ; and the last ball in the series will
recede from the preceding one, with the momentum transmitted
frt>m A through ine entire series. On the return of the last ball,
its momentum will be transmitted through the series back again to
A, and the same movements will be repeated. It may be
remarked, that the transfer of the momentum through the entire
O
wtieioflnUaUkea place in u tlmost iiufipTeoiabl; miKll space
If a Domber of iioir balla, iiutead of being BUipended, be placed
OD a table, bo that tbeii centree lie
in the »anie right lino, or (as Ibe '*■ '"■
ui obtow angle, anil one of them, a, Fi^. 312, being eeparated
ftno the icet, be propelled towards a with a certain degree of
ntulico a, the iiit«rmediale ipheraB being nnaffected, except in
the impennptible manner jnit deuribed.
296. IneuletkX ontJ JUfleetion. — If an elastic bodj impinga
oUiqaelj on a plane aBrfaoe, the
lorca of raatltation will, after impact, ^- *•*-
cairy it awaj from the plane. Let ( p
b« ikg elasticity of the body and plane,
ud lei CD represent in magnitude I
ud diiectioD the Tclacil; with which I
Ibe bodj strikes the pUne at D. I
Draw D ■ perpendicular, and c B pa- I
nUeltothepWejtakeDr-ixDC, I
ibranghp, draw Ffl parallel and eqoal I
lo CB, and join De; tben will ds *
represent in magnitude and directirai the Telocity of the bodj after
Let c D ■ 1:3 6, which is called the 01^ o^ ineidewM,
andEra = 0', „ „ ibe angle of r^/ketion ;
now c D, rep reseating the Telocity of incideace, may be resolved
(284) Into the two TelocitieB, c a, B D, of which c i beine in the
direclirai of the plane, will be unaltered hy impact, and mtty be
n^irasented by an eqnal and pantliel line, r a. Bat the Telocity
I □, will be eipended in compreaeion, aad a corresponding Telocity,
nr, will be generated by reatitntioa ; consequently, do, the re-
nllant of ■> r and r u, will represent the Telocity and direction of
tb« body after impact.*
•od t=^„!
fhRB vUcli two BqnatioK tt thm of the qnaotMei r, t*. 0, ff', « ba j^Tn
(ineofwliHliuiiKbeatdfnhj) tlw other tao ni>j be deUnnined .
166 DTHAMIOS.
297. Impact has been termed "a prefsnre of aliort duration,'* bat
practicallj there is a great difference between the effects of pressnre
and impact : thus, for instance, a very lai^ weight will be reaujred
to press a nail into a block of wood, which may be readiljr oriven
into it bj a small hammer ; the reason of this is, that a longitndioal
compression of the nail towards the head is the immediate effect
of the blow ; this is followed by restitution, and these actions
follow each other towards the point, consequently the friction of
the nail against the portions of matter in contact with it is oTer-
come at successive pomts, and not over the whole surface at once,
as when pressure is emploved. Thus the nail eirters by a kind of
vermieuiar action, precisely analogous in its kind, although Tery
different in its duration, from that by which an earth-worm is
observed to progress. The great power of the wedge in splitting
the touffhest materials depends solely upon impact, and may be
similar^ explained. The Pile Engine and the Steam Hammer are
conspicuous examples of the impuJsiye application of great forces.
Hardness in the impinging oody is obviously advantageous in
avoiding loss of force by compression ; thus a hammer with a hard
steel face will do its work much better than one of soft iron.
Also, when impact is employed to communicate motion to one
body relatively to another, tne amount of effect produced depends
greatly on the immobility of the latter ; thus, many more dIows
will be reonired to drive a given nail into a loose board, than what
would suffice if it were resting against an immovable support;
mach of the force in the former case being spent in communicating
motion to the board itself. There are, however, certain cases in
which the effect of impact is diminished by a firm support ; thus,
hard and tough mineral substances may be more readily broken
by a hammer if resting on a cushion, than if placed on an anvil, or
other heavy mass of solid matter ; probably m the latter case the
effect of momentum on the successive particles is interfered with
by a contrary momentum generated by restitution.
The effect commonly known as " deadening*' the force of impact
is due to the gradual exhaustion of the momentum of a moving
body ; thus, a bullet may be arrested in its fatal course by a son
cushion, or even by a loosely-suspended silk handkerchief. The
well-known art of catching a stone or other hard substance without
inconvenience, by withdrawing the hand at the instant of contact,
may be similarly explained.
▲CnOH OF 17HIF0BM AGCKLEKATIVO F0B0B8.
298. OrapUation. — Among the forces which are the most ener-
getic in producing motion on the sur&oe of our globe is the at-
traction of gravitation <59) ; this force, whilst acting on bodies
under its infloence and approaching the earth, is a uniformly ac-
celerating force, becoming as uniformly retarding on bodies receding
ACnOH OF UHIFOBM AOGBLESATIBO FOKOBS, 157
froa Uie earth. So tliat a body acted upon hy it» passes through
dtflfereiit poriionB of space in different times, and vniLst approach*
mg the earth, would m each instant pass through a greater space
than th*t which it traversed in the preceding instant of time. If
a ball be let &11 from the hand, it can readilj be caught during
the first few inches of its path, but its Telocity afterwards so rapidly
increases, that it cannot be intercepted by the most aeile arm
without difficulty. Even if the descending body fall obliquely,
stin the same rapid increase of velocity is perceived ; this is well
illustrated by the falling of bodies down steep descents, or long
inclined planes: for the nrst few yards the mass appears to move
slowly ; gradually, however, it increases in velocity, and, as for
example in the fall of a eranite block from an afpine ridge of
ToA, or of the more terrific avalanche, acted upon by the con«
Btantiy accelerating force of gravity, it acquires such an accumu-
lated energy (2^3). as to enable it to overcome the resistance of
almost any obstacle it encounters.
299. If # be the space which a body describes from rest in t
aeconda by the action of a uniform accelerating force,/, then
Let A be the point from which the body be^ns to move, a b
the space s described in t",* and v the velocity acquired at b.
Let us now suppose the body to be projected from b Fig. 314.
towards a with a velocity v, and to be also subject to the a
accelerating force,^, in the contrary direction, a b, then
since the accelerating force is uniform, the velocity that is
being subtracted at any point of the line c, is equal to
that which was being added at the same point on the
farmer supposition ; hence the velocities being the same
at B, they will be the same at every other jxnnt of the line
ab: in the time <", therefore, all the initial velocity, o,
with which the body left b will have been exhausted, and
the space described in that time will be s. But by the second law
(Amotion (283), a constant pressure produces the same effect in a
given time, wneiher the body on which it acts were previouslv
m motion or at rest ; wherefore s will be the space through which
the accelerating force,/, will prevent the body from moving in t",
when projected with a velocity, v, in a direction contrary to that
of the force ; and the space actually described bv the body in <",
on being projected from b with a velocity v, will be v.i—S'f but
it has been shown that this space is $ ; therefore «=<. v— s,
whence §=z^v.i; {a)
but (273) »=/.«, therefore »=i/.<*. (6)
* The diaraeten I™, <*, are now oommonly aaed to denote t minutes or
Moonds (^Umtf while If, f, denote t muratas or Beconds of at^, or angular
It sppean turn (a) that Ihe naee daeribed, roabmiu^-oM the
beginning of tnolion, it half Inat wkieh leovld be dttcrioed w the
tame time, miik llie latt aequirtd vdodty continued vniform.
300. If s body be mOTBcTfrom a stala of rest _bj the action of a
aniform acceleratiiig force, Ibe apaoes described in equal HacceHiTe
portioDB of time, reckoning from the benaoing of motion, will be
as the odd numbers, 1, 3, 6, T, &a. For the ipace dewribed ia
r being I/, t*, tbat described n r— I'wiUbe !/((-■)■; conse-
qaentlj the Bpac« described in iJie f second is,
i/C-i/('-l)' = i/(2<-l),
and writing for t, 1, 3,-S, &c. BQCcoMiTel;, tbe spaces described in
SDCcesBiTe seconds are \f, |/.3, 4/.fi, &c., that is, they an as the
numbers 1, 3, S, &a.
301. Mcrin'i Apparattii. — Ths aboTe relation (b) between
time and space nu; be verifled bj the appantof of H. Morin;
this consists of a vertical cjlinder, covered with paper, and made
to rotate with uniform velocity. A falling weight, with a pencil
attached to it, which rests on the paper, is
liept' parallel to the cjlinder by wire guides,
and IS released from its support while the
cylinder is rotating. On subsequently temoving
the paper fmm the cylinder, a curve, a p, Fig. 215,
willbe fbandlrHced upooit, enda straight line, oi,
drawn before the weight descended. Draw o i
perpendiculAr to o x (or draw it on the cylinder,
by allowing the weight to fall, while the cylinder
in at reet). Take any point r in the curre, and
draw Ti, rr, parallel to OT, OT, rvspectiTtlj,
and call them x, y ; then g represents the space
fallen through in the time represented by t,
-.nd it will be foand that x*= A y, and tbe carve
p ia a parabola, as in projecrilee.
SOS. A bodj leFt free to more, and acted upon direetlr by the
force of gT^ritation, all opposing forces being eicladed. will, in the
latitude of Greenwich, descend throngh 16'0954 feet in a second
of time, acquiring by this motion a velocity of 331908 feet, or
3S6'iB96 inches per second. This velocity, expressed in nnmbers,
is termed the ybriw o/'prority, and is represented bj 3. Th« space
traver>ied by a fallinz body in a second is hence v«iy nearly equal
to 16 feet 1 inch; which is sufficiently correct for ordinary calca-
lations, and to enable os to avoid decimals, which are very incon-
venient, unless we nse logarithms to lessen the number of Ggat«i:
and the space described in t* it (^,t*.
303. AttiMod't MaMae. — It is difBcult to submit the resnlts ,
here obtained to tbe test of eiperiment, by means of bodies Ealling
freely by the action of gravity, both from the space reqoiute, and I
Ibe mMrtaintj of obHiriiig rapid motiom ; but meana hsTS been
ikrued bj which the force of graritj mtj be so fnr diluted, u U>
U snecepcible of euy obMrratian. If two bodicB, r uid q, oT
wbicb e jg the grester, be cODDeoted br > itrine pesfiing OTor •
bed pnllej, r by ita greeter weiebt viu deieeLd and draw op q.
Lei tfae moring force (ST6) be f{r + n), neglecting (he inertia ol
,\. „ii ^ jhe rigidiljof the string (13), bnt "■• -*-^--
>f gravitj i»y (p— Q); coMequBntly,
/{l-+Q)-ff(F-«), Md/=^-=^ff.
If two equal seighia of 7) oi. be taken, and a weight of 1 oz.
ba added to p, then —" will be ^, and the force,/, will be ^ p
nrj ftearij ; f will conaeqoentlj descend from net one foot in a
ncgod, and will aci^uire a Telocity of two feet per Mwnd.
In order that prejudicial reeiituiceB may be ■• far M pOMlble re-
mored in Attwoiid ■ machine, Fig. !16, the
pnUej i* made to run nerj lightly on friction- '*'■ *•*
ipilen, F (57) ; the ptiUey thua fnmiahed ii
nppnted fay a ■tandud, or pillar, and a rod,
B.gndnated in feel and inches to indicate the
•paceBdeacribodbytbedeacendiagweigbt. At-
tached to the atem ia a aeconda penduhun a,
■hich when drawn totratdi a, and released, at
the moment it passee its central poeil loo, re-
\ateM Ihe weight, p, by meam of a lever, n ;
tlw veight r in ita descent drawing ap q. The
tnoat conveiiient arrangement ia to make the
■eighia r and q equal, and to place on f an
•cceJemting weight, h, which may, when
reqnired, be detached from f in the coarse of
If the weighta be bo adjnated that the de-
fMndiiu; weight will deacribe exactly one foot
in the Gnt second, marked by the beat of the
•ecoodi peadulum, it will be found to descend
tluoogh three feet In the next, and fire feet
>a the third second.
It may aleo be ahown by this machine, that
Ibe apace deacribed from rest, by the action of
a unirorm accelerating force, is half that which
■ould be unifomly described in the same
time, by a body moving uniformly with the
Ism acqaired Telocity (2»9) : for if, at the end
"f one second, the accelerating weight, n, be arreated by a per-
Cunted atage, b, through which the descending weight pasaea, it
160 DTHAinOB.
will DOW be found to descend uniformly throngb two feet during
each BQCceeding Becond : and if the weight be detached after the
end of two seconds, four feet will be described daring die third
second. This may be shown by placing a second stage, c, on
the graduated rod, o, so as to be struck by the descending weight
at the end of the required period.
304. The practical application of the preceding formulsB will be
best understood by an example : thuSj if the space be required
through which a heavy bodv will descend by gravity in 23', re>
ferring to the expression B—^g^ (301), we have i^=16'095, and
<-i23; consequently,
«-16*0954x [23'»] 529«8511'4666 feet.
The height of any lofty building, or the depth of a well or shaft
may thus be roughly estimated ; for by lettmg fall a pebble from
the top of the one, or into the mouth of the other, ana noting the
number of seconds which elapse before the sound of its striking
the ground or water is heara ; then, on sqnaring this number of
seconds, and multiplying the product by 16^^ feet, or, more accu-
rately, by 16*0954 feet, the height of the biuMing or distance of
the water from the mouth of the well may be approximately dis>
covered. This process is of course open to the error arising from
the time requireid for the sound produced by the pebble striking
against the ground or water to reach the ear ; and consequently
the calculate length of the path of the pebble will be somewhat
greater than the truth.
805. Also, knowing the time required for the fall of any bodv
through a given space, we can readily discover the velocity witn
which it moves ; and by knowing its velocity, we can of course
ascertain the time required for its fall through any g^ven space.
The following formulas will be sufficient to answer every question
connected with this subject : — v being the velocity of the falling
body, t the time of its descent, a the velocity acquired by the
body after moving for a second of time, and $ the space passed
through in the time <, then (270, 273, 299),
9
«-»-L*= /!J.
806. When a body is acted upon by any projectile force inde-.
pendently of the attraction of gravitation, the motion it assumes
18 a compound one, produced by the combined influence of the im-
pulsive force, which is momentary, and the gravitative force, which
IS continuous. If a body, instead of being acted upon by ^vita-
tion idone, be proje^ed downwards wiUi a given velocity per
MOTIOH OV PBOJBCriLES. 161
Mcond, tluB 18 to be taken into account, and being expiewed in
feet, and multiplied by the nnmber of secondB, the product is to be
added to the space, also expressed in feet, which the body would
bare^ tcaTeraed in the same time, if acted upon by the force of
gravitj alone. Jfj on the contrary, the body be projected yertically
ppwaids, its coarse being opposed to the attraction of gravitation,
instead of being added, the effect of the latter is to be iubtraeUd
from the space passed through by the projectile, if acted upon by
the force of prqiection only. The following examples will illustrate
these remans :
(A.) To what height will a body rise in three seconds if pro-
jectea upwards with a Telocity of 100 feet per second ?
The space described by force of projection
alone wiU be 100x3=300
Space through which the body would fall, if
acted upon by gravitation alone daring
that time will be 16095 x 9=14485
The diffsrenoe of these quantities is . 155*15
and conwquently the height attained will be but 155*15 feet.
(B.) Where will a hodj, projected perpendicularly upwards,
with a Telocity of 80 feet per second, be in 6 seconds ?
By the force of projection alone, 80 x 6=480,
.... graTitation alone, 160954x36=579*4344,
and 480-579*4344 = -99*4344.
The body will therefore be nearly 99^ feet lower at the end of
6 seconds, than the spot from whence it was first projected ; pro-
vided no mechanical obstacle be present to preTent this taking
pUoo.
(C.) What space will a body pass through in 4 seconds, if pro-
jected Terticafly downwards with a Telocity of 30 feet per
wcond?
Space due to projection alone = 30 x 4 = 120,
.... graTitation alone = 1 6095 x 1 6 = 257*52
The answer is 377*52.
The body will consequently pass through rather more than 37 7}
feet in four seconds.
MOnOV OV PHOJECTILBB.
307. The only motion hitherto considered has been that of a body
acted on either by an accelerating force alone, or bjjr an impulsiTe
force conjointly, but in the same straight line with the former.
Bat if a body be projected in any other direction than Tertically
npwards or downwards, and consequently in a course obliaue to
tnat of graTitation, it will not follow the direction of eitner of
these foroes, but its path will be determined by the joint action
of both the foroes. Thos let a body placed at ▲ be projected in
162 DTVAMIOB.
the direction ▲ b, with a Telocity v, Dnw ▲ b perpendicnlar to
the horison ; then let a e be the space
^' ^"' over which the velocit j of projection
X will carry the body in a given period
of time, <*, and ab the distance it
would traverse in the same time when
acted upon by gravitation alone : now
• draw BC paraUel to ae, and bo to
V ▲ B, completing the parallelognun ▲ c.
\ Then, in consequence of the united
^ action of these two forces, the body
-^ will be found at the end of the given
time at c instead of e, having de-
scribed the curve ac by the com-
"^ bined action of the two forces of
projection, A b, and of gravitation, a b. By this, which may be
called the " parallelogram offerees," the place of the moving body
at the end of the given time may be determined, but neither the
diagonal, nor the curve AG can be called a "resultant*' in the
sense in which that term has been previously applied (70, 284).
The line ae, representing the direction in which the force of
projection alone would have carried the body, is a tangent to this
curve at the point a.
308. But A e has been taken to represent t. v, and ab^J^. t*
(302) ; also b o » a e ; therefore,
Bc*=t»xi;«=g.<« xif7«=?^xAB;
2 o <7
whence (Hymers* Conic Sections, Art. 93^ the path of the body is
a Parabola, of which the Axis, bein^ parallel to a b, is vertical.
309. If A be the space through which a body must fall freely from
rest under the accelerating force of gravity, m order to acquire the
velocity with which the body is projected from a, then (805) A = ^- '
and the preceding equation becomes
bc'=4A'xab;
and 4 h being thus the parameter at a, h is the distance of ▲ from
the directrix of the parabola ; hence,
The vdocUy of Projection i» that which a body would acquire
hy faUing fredy from the directrix to the point o/ projection,
310. Let a be the inclination of the line of projection to a hori-
Bontal plane ; then a is called the elevation of the projectile. As
the velocity of the projectile in a horizontal direction is not
a£fected by the accelerating force of gravity, it will remain con-
stant ; let, therefore, the imtial velocity be resolved into two velo-
cities (284) in the horizontal and vertical directions ; thus, v. cos a
is the conetarU horizontal velocity, and v. sin a is the iniUal
vertical velocity; in consequence of which the body will oon*
KA.NOB OP A PBOJEOTXLB. 163
tmiie to rise above the horizontal plane, until the efiect of the
aooelerotxng force of gnmiy has destroyed the vertical Yelocitj ;
coBseqaentlj, the greatest height to which the body will ascend
is equal to the space throagh which it most fall to acquire that
Telocity, and this, since <=»- (305), is
— (sin a)', or h (sin a)*.
311. Daring the time of flighty or the interval between the
period of projection and the reium of the projectile to the same
norizontaf piane, the initial vertical velocity will have been de-
stroyed, and, since the body will have descended through the
same vertical space throagh which it ascended, a vertical velocity
downwards, equal to the initial vertical velocity upwards, will have
been generated by gravitation ; therefore, the time offUght of a pro-
jectile U twice the time of acquiring the initial vertical velocity by
the action of gravity.
Let this time be represented by T, then since generally < = -7. (2 73)
m 2v .
T= — sm a.
y
312. The horizontal range of a projectile is the distance from
the point of projection to the point where the projectile returns to
the horizontal plane, and this is the space described daring the
time of flight with the constant horizontal velocity (310); but
generally e^t.v (270) and if in this case the range be repre-
seoted by B, then,
r» 2t7 . v*^ . «» . «
£= — 8inaxi7.coea=:— 2 Binaxcosa = 2^. 8in2 a.
ff . ^ .
313. The value of B just obtained will evidently be greatest,
fin- the same value of A, when sin 2a is greatest, that is, when
2aB90®, and consequently, 0 = 45**; therefore, with a given velo-
city of projection, the greatest horizontal range is obtiuned at an
elevation of 46*. Also, any ranre ^^
less than the greatest may be ob- '^'
tained at two difierent elevations, * A,ff . . 0
as in Fig. 218; for sin (90*'+2/3) ** a' 4.5
=a sin (90* — 2j8) ; consequently,
whether we make the elevation
46'+ A or 45' -ft {B being any
angle less than 45*) the range will
be the same. This property is fre- ___^____.._
quently important m the art of c ^
gunnery, in enabling an olject, a,
to be struck at the higher elevation, which would be protected
from the lower, by an obitacle b o.
H 2
164 DTXAMIC8.
Tliese poinU may b« illustrated ezperimentallj by dischai^ging
a bullet trom a tuoe furniBhed with a spiral spring, and trigger,
and capable of being ac^usted to any angle of elevation, bj means
of a vertical graduated arc.
314. In all tbese observations, the resistance of the medium in
which the body under consideration moves, as well as the inter-
ference produced by friction, have been neglected ; they fnmish,
however, very important sources of opposition to the regularity of
motion. Cceteris paribus^ the denser tne medium, the greater the
opposition to the passage of the body moving through it ; and in
the same medium the resistance opposed to the movement of the
body is proportioned to the square of its velocity. It has been
demonstrated by Newton, that when a spherical body moves
iu a medium at^ rest, of equal density to itself, it loses half
its motion before it has descrioed a space equal in length to twice
its diameter. This resistance is a consequence of the molecular
inertia of the medium, preventing the particles opposed to the
moving body acquiring instantaneously a degree of movement cor-
responding to that of the body. The atmospheric resistance is
Bufncient to prevent projectiles describing a strictly accurate
{)arabolic curve, as required by the theoretical considerations, and
imits the ranj^e of the projectile in a remarkable degree. Accord-
ing to Vega, it appears that a cannon-ball weighing four pounds,
and which in vacuo would traverse 23,226 feet, will, when passing
through the air, travel over only 6437 feet ; also a 24-pound shot
discharged at an elevation of 45** with a velocity of 2000 feet per
¥ig, 219. second, would in vacuo reach (the hori-
zontal distance of 125,000 feet, but the
resistance of the air limits its range to
7300 feet.
The curve actually described by a ijro-
jectile in the atmospnere, when its resist-
ance is taken into account, is not the
parabolic curve, ▲ b, but a curve, a c i>,
which continually approaches to a ver-
tical asymptotet ef.*
316. Principle of the Rifie. — The resistance of the atmosphere
not only retards the progress of a projectile, bnt also frequently
causes it to deviate laterally from its course. If a ball be fired
from a common gun, a rotation will probnbly be communicated to
it, the plane of which will be determined
by the last point of resistance to the exit
of the ball. Let the ball, c d, be projected
in the direction ▲ b, and let the plane of
rotation coincide with that of the paper;
" also, let the rotation of the ball be in the
direction of the arrows, c, d. Then it is
• See Wbewell's Dynamics, p. 180.
^ft^^'^m'^^r^rmi . , - m. • ■ — — ■ •* - — ■ ■■ ■ -jixj.
BOTATIOV AND TBANBLATIOK. 1^
dear that the actual velocity of the side, o, of the ball will be
greater than that of the side d, the former being the sum of the
▼elocities of projection and rotation, and the latter, their difference ;
coneeouently, the ball will experience more resistance at c than at
D, ana will therefore be deflected in its course towards n : and as
both the direction and yelocitr of rotation are purely accidental,
the direction and amonot of deflection of the ball are not deter-
minable. To remedy this uncertainty, the barrel is rijledf that is,
spiral grooTes are cut on its interior surface, by which a definite
rotation is communicated to the ball in a plane perpendicular to
the tine of projection ; this rotation has no effect in altering the
comparative velocities of any two opposite points of the projectile,
and consequently does not cause deflection.
316. The form of a body is found to influence considerably the
amount of resistance ; thus, a projectile of given weight, with a
S'ven velocity of projection, will have a much greater range if it
i of a conical, or still belter, a parabolic, form, than if it be sphe-
rical : this &ct has been applied in the construe- «^ ^,
tion of the Mini^ ball, Fig. 221, which has also a
canty in the wide end, as in the figure. The ad-
vantage of the cavity is twofold ; first, the centre
of gravity, g, is brought more forward, and the
lighter portion of the mill acts like the shaft of the
arrow, or the stick of the rocket, in steadying its motion ; secondly,
the tlnn tubular pordon is more readily pressed into the grooves
by the expansive force of the charge.
317. The flight of a rocket is a practical example of the resultant
of two accelerating forces ; the path of the projectile is nearly
straight, instead of being the parabolic curve resulting firom an
impulsive projection. In point of fact, a Gongreve rocket is com-
monlpr observed to deviate upwards in the latter part of its course ;
this 10 due to the altered position of the centre of gravity (85), in
eonsoquence of the exhaustion of the chaige, the stick then pre-
ponderating more and more.
318. If a spherical body, a b, receive an impulse in the direction
of a tine, dcb, passing thraugh its
centre of gravity, c, all its parts Fig. 222.
will move with equal velocity in a
straight line. But if the force ap-
plied do not act in the direction of
a tine passing through the centre
of gravity, uie particles of the
body will possess unequal velocities,
and* the whole mass wiU acquire a
revolving or a rotatory motion, at
the same time that it moves on- '^
wards under the influence of the
apptied force. Thus, the earth is a body which revolves on its
166 DyxAHics.
own axis, at the same time tbat it moves thron^h space ; and if
these motions have been ac(|uired from a single impulse, it must
have been exerted upon a point situated about 25 miles from a line
passing through the geometric centre of the earth.
The communication of a rotatoiy as well as a j^ro^ssive
motion to a billiard-ball, hy means of a slightly eccentric impulse
from the cue, is an artifice well known to billiard-plaverB.
319. As in the instance of the musket-ball already mentioned
(315), so, whenever the progress of any portion of the surface of
a moving body is more impeded than that of other parts of the
surface, a rotatory move-
"^' ^^' ment will ensue. This
may be shown by placing
a watch-glass, or convex
lens, on * a smooth in-
clined plane, Fig. 223, as
a pane of glass ; having
previously dipped the
convexity of the watch-
glass in water. Thus arranged, the glass, on sliding down the
plane, will rapidly revolve around a vertical axis ; whereas, if the
plane and glass be perfectly dry, it will slide down and reach the
bottom of the inchned plane without revolving. This rotatory
motion is explained by the adhesion produced by the drop of
water, not being exactly the same on Ojpposite sides of the point
on which the carved surmce rests. The side on which the adhesion
is greatest is more retarded in its progress than the opposite side,
and thus commefices the rotation round an axis passing through
the point of contact, which continues, from the same cause, until
the moving body reaches the bottom of the inclined plane.
CEMTEIFDOAL FOBCB.
320. In consequence of the inertia of bodies causing them to
persevere in rectilinear motion, it is found that when revolving in
a circle they constantly endeavour to recede from the centre. This
is termed the centrifugal or centre-flying
force. If a ball affixed to a cord, c, be
made to revolve rapidly in a circle, from
a fixed point, s, as a centre, it will de-
scribe the circle ▲ b d. If whilst rapidly
moving the cord o be cut with a sharp
knife, the inertia of the ball will cause
it to continue in motion, not, however,
in a circle, but in a right line corre-
sponding to a tangent tc the circular
path it described whilst the line o was
entire. The force which caused ▲ to fly
off in the direction of a tangent is the
CBHTBIFUOAL TOKCB.
167
cadrifugpl or centre-flying force : and the cord c, represents the
direction of a centripetal or centre-seeking force. Thus, consider-
ing the circle to be composed of an infinite dumber of lines, the
ball will tend to follow the direction of one of these lines, and msh
off at a tangent to the curre. This circumstance, taking place the
instant the force which binds a to the centre is overcome, shows
that the centrifugal motion is the result of the tendency which
bodies possess to move in rectilinear paths (282), and is not owing
to the deTelopmeot of any new force.
If a body moTO with a velocity, o, in a circle of which the
radius is r, the general expression for the value of the force is,
centrifugal force =— •
The expression — may be put into a more convenient fonn if
we call the anffular Telocity of the rotating body oi, then its
linear velocity in its orbit will be <■» r ; the expression for the cen-
w
»r»
trifngal force will therefore be , or t^r.
321. The truth of this proposition may be shown experimentally
hy an apparatus called the whirling-table, Fig. 225. This con-
FSg.226.
sxsis of a horizontal frame, a b, to which are attached a wheel o d,
and two small tables, b, f, each fixed parallel to the frame by
means of three legs. Two rotating vertical axes pass up through
* Eanitfaaw's Dymmies, p. 80. Koteley's Mechanical PrinoipleB, p. 123.
168 DTHJLiaOB.
the centres of theie tables, underneath which pulleys are fixed on
the axes, and to these rotation is communicated by a cord, bod,
passing round them Imd the wheel. Two pulleys, o, h, are usually
fixed on each axis, the diameter of one being aouble that of the
other, and the diameter of the corresponding pulleys on the two
axes being exactly equal. By means of these the axes may be
made to rotate either with equal angular yelocities, or with Kuch
that one angular velocity is exactly double the other. A frame is
attached to each of the axes, so constructed that a weight, k,
sliding on wire guides, may, wlien it has acquired sufficient
centrifugal force, draw up by means of a string a weight, l, en*
closed in a circular cage, concentric with the axis of revolution.
One only of these frames is represented in the figure, the screw at
the end of the axis to which the other may be attached being
visible at e. If now equal weights be placed at k and x', and at
L and l' (k', l' being the corresponding weights in the other frame),
and the distances of k, k' from the centres of motion be exactly
equal, it will be found that, when put in rotation, they will at the
same instant acquire sufficient centrifugal force to draw up l and
l'. It* now the weight k be placed at double its former cUstanoe
from the axis of motion, and the weight l be doubled, or, more
generally, if the distance of k from the axis and the weight l be
either increased or diminished in the same proportion^ it will be
found that the two weights, l, l', will still be raised at the same
instant : thus showing that when the angular velocity remains
constant, the centrifugal force of a given weight will vaiy as its
radial distance from the axis of motion. Again, let the cord be
placed over the smaller pulley at h, so that the angular velocity of
x may be double that of x', and let the distances of x and x' from
their respective axes of motion be eqnal. It will now be found that
when the weight l is Quadrupled, it will be raised at the same time
as l' ; that is, when tne angular velocities are as 2 : 1, the centri-
fugal forces are as 4, or 2^:1, the radius remaining constant ; and
if the an^lar velocity of x were increased or diminished in any
other ratio, l being at the same time increased or diminished in
the square of that ratio, the weights l and l' would be raised
simultaneously: hence it appears that the centrifugal force of a
mass m will be ,
822. It may likewise be shown, by means of the whirling-table,
J. 2«ii ^^^^ ^^^ connected bodies will
'^' rotate round each other, if the
C A B D axis of rotation pass through
f>— O- - QJ f) their common centre of gravity.
/ \ / \ Let two spheres, i, b, be connected
JL> g (jjv bjr a small tube, through which a
> r< wire, c, passes, which is kept
^ ^ tense and parallel to a bar of
tbs oommoa oenira of gnntj of a and b ba marked oa the
Alwlrt the bftT, bf, be atUcbed to ana of the n*oInag ■___ ...
ill middle point, lo that the «Hrs, o d, maj reTolte in ■ tunixontiJ
plane. It willnowbefboadthatifthepoiDt abenudetoaoiDdde
vfth the aiia of rotstioD, tbe cantrifog^ forceii of the two Iradiea
will be sqiia], uid the; vill cootiiiae to rotate rouiid each other ;
bat if the point o be displaced on either aide of the axis of roU-
tion, the nyalem vill, vhen rotated, Sj off in the ume direction.
This would foUow as an immediate dedoction from the expns-
ROD giTen above for the centrifagsl force ; fbr if m and m' are the
tDssaes of the two bodieg, and rand f' the distances of their centres
of grsTitj &oni tbe axis of rotation, then, their angular Telocitiea
baing the sanH, we have, if thair centrifugal forces are equal,
or, m.r = TO'.r';
therefore (85) the centre of graTiCr lies in the axia of rotation.
333. We see magniScent examples of this force in the revolution
of the spheres of nor nniierse. The earth and ntber planpta re-
nrire roand the sun as a centre, with sDonaoDa velocity, eTerywbere
tending to rush off into infinite space in the direction of a tAUgent
to their elliptic orbits, and prevented onlv by an equsUj powerful
oantripetat force, the grantative attraction of the sun. Equally
balanoed between theaa opposing forces, the elenMnts of our uiu-
verae faave rarolTed for myriads of sgei around the great centre of
SOT sjatem, presenting a wonderfuT spectacle of infinite wiadom
and urmooy.
324. The form of our own globe presents a remarkable instance
of the e&eots of this force, from its revolving on its own axis at the
Tate of 135 miles in a minute at the equator. An ener^tic cen-
trifugal force is generated at the eqoatorial parts, by which, at an
FIg.lX!.
:s 17
miles Krester than its polar diameter.
This alteration in Bgore admit* of anesay
■Qoatration, by rapidly revolving two elastic
iron hoope placed transvaraelj. These are
fixed to the iron axis at i, and are loose at
B ; OD tomiag the handle c, so as to rotate
them rapidly, the moveable ends of the j
hoopa wQt nse up the axis to d, and will
bulge oat at the sides. Thus repreeentiDK
tbe figure of a holtow Satteneu spheroiiC
■0 long as the rapid motion continues;
wlien thia ceuei, the looae psripheriea oT tho hoopi will d«K«ad
and regsin their oripnal fiRnra. On ftccoont of the eiceu of the
eqaatonsl aboTe the polar diameUr of the earth, bodies wei^ Ion
at the eqaalorlhanat thapolea: JOOOpoands at tho latter corra-
■pondinff to 996 at the equator, from the diminiahed fotw ot
grantjTSS).
The projection of a atone by a aling ; the iparka from a grinder't
wheel ; the aoattaring of dropa of water from tlie wet reTdvii^
camage wheel, or honaemajd ■ mop ; are so awar fatailiar eiaiii~
plea of the action of centrifugal force.
S35. A lorther illiutratioD of this force will be roond in the
Xa,^. fiwmaMon of the pluiet Saturn's ring. The
formation of aa annaliu by the acUon <^
oentrifngal force may lie thm oonfeiiteDtl<r
iiloatrated. Let a wide-montbed bottle be
nearly filled with a miztnie of alcohol and
water of the mme dentUg u oli™ tril, of
which a deaaert-Bpoonful should be ponred
into it Let a hent wire, a, paaa throng
the cork, and have a imatl circular disc of
metal attacbed to the bottom of it. If the
diio, preriooely oiled, be introdnced into tha
miied liquid, the moaa of oil will adbete to
-it. On gently rotating the disc by meani
' of tbe cranh, t, the maia of oil will be
flattened out hy its Bcqaired centrauMl
nlnD tka r^iit:.... >k. „;i — II ■-offfromtha
OHCIU^nON OF £ PAMICUI.
sphere, the velooity acquired ia tha sl_.
as that which woold be acquired by the
I body falling freely through tbe hwght ot
I the plaoe.
Let A B, Fig. 229, be an inclined plane,
no its base, and AC its height, draw OD
I ptrpendicnlar to ab. Let the vertical
line, A c, be taken to represent tbe force of
graTi^ ; this may be resolred into two, a d in the directiaD of
the plane, and d c perpendicular to it, of which d a can have no
eflect in moviog the Iwdy along the plane i ai> therefore repre.
ssnta the efiective portion of the force of gravity. But bf ainulai
triangle*,
therefore a d cb a o x
▲ B'
and die effective force of gravity on the inclined plane is ^ x
MOnOS OV CDBYE8. 171
▲ 0
L tne inclined plane is ^ x
Bat since generally v^=2g^ (305),
A C
(velocity)* acqnireddown abs2^x — xab^
A B
«2^XAC
= (velocity)' acquired down Ao;
or, as it Lb commonly exprossed, the vdodJty acquired hff a body in
daeending an ineUnea plane is thait due to the height of the
pUme,
^ 327. When a body nnoppoeed hv friction, or resutance of the
air, descends a series of superposed inclined planes, the velocity
acquired by it is equal to that which would be acquired in fiJIing
through tlie vertical height of the series, as in the case of a sinrie
niane (326) ; supposing that no motion be lost by concussion of the
tiody in passing from one plane to another.
Let ABCO represent tne planes, and let do and ob be pro*
dnoed until thev meet ab in o and e. The velocity acquired by
a body falling from a to b is equal to that which it would acquire
in Calling from g to b, for the planes, «. ^3^
AB, o b, nave the same perpendicular ^*
height ; and when this is the case with
any two planes^ the velocities acquired
in falling down their whole lengths
aze eqnal, as the acquired velocity has
been shown to depend on the height
of the plane alone. The body bavmg
reached b,- will descend b o with the
same velocitv^ whether it &11 down
A B or o B ; toen the velocity acquired
at o will be the same, whether the
body fall down gbc or Bc; and finally, it will pass down to d
with the same velocity as if it had descended directly from b.
The same reasoning will apply to bodies falling down curves, for
their figures may be considered as made up of an infinite series of
planes : hence,
Jf a body descend hy gravity along a smooth continuous curve
situated in a vertical plane, its velocity at any point will be the
sasne as if it hadfaUenfredu through the same vertical space.
328. All bodies free from oostaoles will »^ 231
have their motion as much accelerated,
whilst descending, as retarded, whilst as-
cending a curve. Let gab be a curve,
and a ball be placed at c, the attraction
of gravitation will cause it to descend to
IT!
a; U thU motiaii it will uqnira ui unoimt of monentum (STG)
ntkient to c»nj it onwardi to a point, b, at tho auae height
(Mgtecting friction uid resiatuice) aa a abore the horitonlai
pUn^ from which point it will descend by graritj to a, and the
m thus guiented will carry it onwaids to c ; it will aguD
~. w ., «nd ao on, oscillaling from o to b, nntil opposing caoaoa
tang It to a aUle of rest. The whole time of awwol to b or c will
be eqaal to the time of descent to A, tu the velocitiea at oao&l
ahituds" wiU be aqnal. ^
829. For the pnrpoee of caoBing the bodj to move in a curved
path with aa little reiiatance aa poasible, it may be altioW lo ■ au»-
pended wire or rtriog, and then pormittad lo oacillate ; u ingtm-
ment thus conalraoted ia tenned Aptndulum. Thia, theoraticatlv
considered, consist* of a heavy particle suapended bj a tHramd,
unacted upon by anj opporingor reaiating foroes. If ihe Wl t-
Fiff, 233, be raised to i and allowed
^-'^ toM, itpaaaea through c to BJD the
manner alreail; described, and the
/ '\ whole movement afthe bill lirom a to
B, oi B to a, ia termed an 04eiUatiim ;
from * to c, its moremeut ia termed
the deecendinr, and Ironi c lo b ita
ascending Hmt-osctUoJ ion. The diir
tance t a, measured in degrees, is
termed the amplUwU of an oadtla-
tion; and the auralion of an oscilla-
tion is the time required lo effect thia
~ moTement from A to B, or rics serafi .
33(1. 7%e Cydmd.—Tbia oune is described by a tracing point
in the circumfereaoe of ■ circular diec, which la rolled along ■
straight edge laid on a sheet of paper. Let c a e, Fig. 233, bo a
Ki ea. cycloid, described by rolling • circle, of
^' which the diametena A B along the line
Ice: it has this remarkable properly,
that if we produce a b to ■, making
BB-AB, and through ■ draw omd
equal and parallel to ce, and on db,
Ei{ describe two semi -cycloids, bc, bc,
equal to c a, a c, then a tracing point
at the end of a string unwound from b c
will describe 0 A ; and simiUrir, wiU de-
scribe AC as it is wound ufon ic; consequentlj, if tbe cnnre*
c^ Be be placed in a vertical plane, and a heavy panicle, r,
occupy the place of tbe tracing point, it will oacillate in the
Cjcloid, C AC*
Anouier important property of the i^oloid is that if we take
* For ■ noof of thli, ind ottanr sooimoii prapcrtlBi at Ib« crgtotd, •••
Bjtata; lodhBDlwX or *>>r •taodird treufaeoathe DiSknatiil Cslsalaa.
Lrtc
the !« , .
Tcrtical, and iW bme, BC, iori... .
Let 1. bo the point froni whicli the body
b^ns to descend ; then ihe time
complete OKilUtioD, vluch ii the bu
tbo time« of the deBcendingond &Bcend-
ing MtDi-occillationi |339), ii equal to I
■ ■ » the time of dencandinc ihroagh
Divide L A into Tary Bmall prntBiOfi
Di»w the borizoDtal lines, L
Kmidrcle, calling th, un ia «, n; jom An, bid, tuid Ant, sn,
cnttiDg eacfa olber in o.
Then since v, the Telocltj at ii, ii the nme u that soquired in
Uliog throagh »t (327), t= yjpxat:
•nd if X ■ be supposed so ■mall tbat the Telodtj acquired in poss-
mg tbroDgh H n may be neglected, compared vith the Telodty at
K, or, in other ward*, that &e velocity may be considered onifono
throng KH, then since generally (~ — (270),
the time of moving throorii m a =—, — •
. 3, the chords
... .a iolenect the geeerating circle de-
scribed on the diameter, a b, ore respectively eqaal to
%/abxat, and ^ a b >f a u ;
and as the an» A K, an, are respectivelydonble the length of these
chorda, m « moat be doable their diSbrence, or
H s = 2 {v^iT.ri^- y nrr^= 2 yA B c /* T - ^'A o) i
hence the time o[ moving throngh h ■
_VAB(yAT-y>p)
2AB yAT-^AI
174 DTHAMICH.
2a»x!!L£ ultimately,.
~V 9
g Bw
because as m h is diminisbed, a o n approaches a rigbt angle, and
tbe yalne of ▲ m— ▲ n approaches m o.
Bnt^^=sin f»&o,=arc subtending the angle msn, ulti-
Rfn
natelj, because as the angle is diminished, the arc and sine
approach equality ; consequently the time of moving from v to h
X arc subtending the angle m b n : and as the same
9
may be prored for the time of moving through each of the other
small portions into which l A is divided, the sum of the times, or
the time of moving through the arc l a
^(the sum of tbe arcs subtending the small angles m b n),
9
2ab t.
9 2
= / -^ (arc subtending the right angle a b l), = / ■
/2ab
Hence the time of making a complete oscillation =ir./ —- ,
and as the point l is arbitrary, it follows that the times of descent
from all pomts of the curve to A are equal, and consequently that
all oscillations in a cycloid are uochronous^ or performed in equal
times.
Since a b = b b, Fig. 233, a e the length of the pendulum = 2 a b ;
calling this l, and the time of a complete oscillation t, we obtain
•=v^
L and g being numerically expressed in parts of a foot.
332. Oscillations in a true cycloidal arc are practically unattain-
able, but a small arc of the cycloid, of which a is the middle point,
coincides very nearly with a circular arc described round the
centre e with* the radius e a (Fig. 233) : and therefore the time of
an oscillation in a small circular arc coincides with the above
formula for the time in a cprcloidal arc. As, however, the cycloid
c A c, must evidently lie within the circle described by the radius
E A, because straignt lines joining the points e p, e c, must be
shorter than the curved lines e f p, e f c, it follows that the time
of an oscillation in a lai^ circular arc, will be greater than that
in a corresponding cycloidal arc, that is, greater than that in a
small circular arc. If we take unity ap the time of an oscillation
APFUOXTIOHS OF THI PBXD0LUM. 176
in an indefinitely small circular arc, then the times of osdllationa
in Uoger arcs will be as follows :*
In an arc of 2* the time is 1*00003
„ 6" „ 100012
„ 10» „ 100190
„ 15' „ 100426
„ 36' „ 101676
833. Since t=w^-- -y-x v^l, and x=531416 nearly, an4
^ in the latitude of Greenwich « 32*19 feet nearlj, therefore ;
-^=0-66372;
hence in order to determine the time required for a pendnlam of
an J given length to complete an oscillation in the latitude of
Greenwich, it is only necessary to take the square root of the
length of the pendmnm computed in feet, and to multiply this
result hj the decimal 0*65372. Thus, if the pendulum were nine
feet in length, it would perform an oscillation in 1*66 seconds, for
V^9x 0-66372 =1*66116. .
Also T*=ir*x-, consequently l=^,xt*, but •^=8*2616 in
the latitade of Greenwich, therefore, in order to ascertain the
length of a pendulum which is required to perform a vibration in
a given time, in this latitude, we have only to multiply the square
of the number of seconds bjr the number 3*2616, and the jproduct
will be the required length in feet. Thus, if it be required to find
the length of a pendulum beating double seconds,
Ls2*x 3*261 6s 130464 feet=:13 ft. ) in. nearly.
^ 834. Let L and l' be the lengths of two pendulums, and t,'!' the
times of their oscillations in equal arcs, then
TIT':: * X yL: ' X v^L', : : ^l : -/l';
that is, in a given latitude, the time of oidUation of a pendulum
ii propcrtionid to the square root of tie len^h.
And as the number of oscillations in a given time is inversely
as the duration of each, it follows that in a given latitude the
number of oscillations of a pendulum in a given time is inversely
proportional to the square root of its length.
336. As the movements of the pendulum depend upon gravita-
• -v| { ^ -(i) ^( j-;j)*(A)'H^.)'(k)'-*- }.
I fa the vctsed >iae of the mad^ito of mcmation. -Baimliaw's Dj-
p. 127.
176
DTVAMICS.
tion, and as this force decreases in intensitj as we recede from
the earth*B centre in the proportion of the inverse square of the
distance from that point (31), this instrument forms a most valuable
mode of determining the intensitj of gravity, and consequently, the
distance of the surface of the earth from the centre, m different
parts of the globe. This is done either by ascertaining the time
required to complete the oscillation of a standard pendulum : or,
the length of a pendulum requisite to complete an oscillation in
a given time. The length of a pendulum required to vibrate
seconds in the latitude of Greenwich is 39'1393 inches » 32616 feet.
ir«L
Since T=ir\/—,^ = ^ — j-, consequently in the latitude of
Greenwich, ^s (31416)' x 32616 £9et=32'19 feet, nearly.
336. Let T and t' be the times of oscillation of a given pendu-
lum, and g, ^ the accelerating forces of gravity at two given points
of the earth's surface, then
_. wVl irv'L 1 1
Let B, b' be the radii of the earth, at the two given points, then
smce
we obtain
. • »___ • * It ' VT
therefor© t:t'::e:r';
hence the radial dUtancea of any two poitUs of the earih\B surface
from its centre are to each other as the times cfvihrattcn of an
invariable pendulum at those points. Having then determined the
radius at any one point by actual measurement of a known arc of
the meridian, any other radius may be found by a simple pro-
portion. . ^ . ^ .,
337. In practice, however, it is more easy to measure the length
of a pendulum vibrating in a given time, in one second, for example,
than to maintain the length of a pendulum absolutely invanable.
Since then g=v*J. generally, when t=1, the values of g may b©
readily found when the values of l are known.
The following are the resulte of some measurements of the
seconds pendulum, at different parts of the world : —
FUoe. LatAtnde.
iValne of l.
Obterren.
Spitsbergen .
Leith . . .
Loudon . .
Jamaica . .
Ascension .
Sierra Leone
76' 49* 6S" N.
66" 68' 41" N.
5V 31' 08" N.
17* 66' 07" N.
7* 66' 48" S.
8' 29' 28" N.
39*21464
3916640
3913908
3903608
3902406
39*01964
Gen. Sabine.
Capt. Kater.
Do.
Gen. Sabine.
Do.
Do.
A* «n example of the Die of Ihia table, mppoM that the force
of grsTity at Sierra Leone U reonired ; at thii place Oeoeral
Sabine hse detenoiiieil the Talus of l to be S9'019M iochei, con-
Mqnently, by logarithnu,
Slog)r.0OT430
hg L- 1-59136
tbair Bom = 2-56663 - log SS6-1,
ccrreapondiiig coDBeqaeDtlr to 3851 iDcboB, which will be the
tetodt; acquired by a bodj falliug freely doling one wcond at
Sierra Leone.
findtht -_
Let B be the particle nupended Iram A., and let
a B ^ a, and B the angle conluned between b a.
and the Teriical AC; m the maw of b, and ( the
teniiion of the euEpennoa A a ; then the tanlion t
wouM prodnce dd m an accelerating Ibne = — in
g force. ThiB
foite may be reaolTod into two, - sia 9 in the
direction a c, and — coa S, acting vertically iipwardi ; bnt unce by
3=icc«9. (I)
Tha oentrifngal fbrce of a body moring vith a nlocity v
circle whose raJioaiirii ~(320}, andiatliiecaaeiB — ,or — ,
But from (I)
Laae
therefore
>^,
whence
«•
,''-I'(""^',and
Batdnct
ithe
motion of B ie tini
motion,!
--
(210), hera
'"v coee"
lifomi, and geuerally in uniform
time of rsTolutJQn
drcomftrence S ir
.a /^.
V "-9
- - -J a COB S-
Since tbii remit depends <
t of a revohdwa of '
•rUth
vikaXeBerbt Oie length of the lutpetuioH, provided the aUitiuie of
tie cone Ttmaat the tame.
This m*f be ihown eiperimentallT bj ittiching three or faar
htiXi t)y wireg of different lengths to the top of i veiiickl rotatiDg
uia : when the vhole ia mule to ralste wilh ■ufBcwnt ra)Hdit7, it
will bo ab*enr«d that the balls will be all in th« wme horiioDlal
SS0. Thii i« one of the important principles tfaat are freqnentlj
fonadto be of great aerrice in the iiolution of d.vnamical queslioaa ;
it may be thui enunciated. If the forces impreutd on the Mrersl
paita of a ijiitem, howaoever related, be each reaolred into two
others, ono of which is effective, uid the other, fiom the given
condilioni, wholl; ineffective, then the ineffective reaolved ^irvea
would, if acting on the avetem alone, pradace equilibrium. Thii
will perhapa be better understood hj a aimple
"t- **■ illuitration._ Let * D be a parallelognun ;
Ithco (TO) if two preasarea mpreaenled in
magnitade and direction hj A B, ac, actoD a
lodj at A, their resultant will bia represented
in magnitude and direction b? ad; coune.
qaenll;, the effective parts of both preainres
maat act in that direction. Draw be, cr,
perpendicular to A D, then the resolved puis
AP, AC, of the preaaurcB ab, Ao, are wholly effective, ajid their
sum is AD, since a ■'"E d; also, the rcaolved parts, s b, pc, are
wholly ineffective, being perpendicular to the direction of th« re.
auttant ; thej are also equal, and in opposite directions, and would
thpreforo, if acting alone, produce equilibrium.
n't ^--iplemay othenriiO be atated thus : — If theefleo-
ceiofthese ' - '
tive accelerating force* of the several parts of a connected aysteiu
b« applied to them in direciione contrary to those in whic' "
act, tbey will, lopether with the impressed accelerating
satisfy die statical coodiiiona of equilibrium. This mode
prMsion is eridsntly the aatne in effect aa tha fonner.
179
PRINCIPLE OP THE GOKSEItVATlOX OP VIS VIVA.
340. The vis vi?a of a body in motion is the product of its mass
and the square of its velocitjr ; or since (275) w - ^ x m,
VIS viva = — v*.
9
If the force of a body in motion be measared by the whole effect
which it will produce before the velocity is destroyed, or by the
whole e£Ebrt that has been employed in generating that velocity,
without regard to .the time, it must be measured by the product
of the mass and the velocity*. Thus, balls of the same sisse, pro-
jected into a dense resisting medium, as a bank of moist clay, will
penetrate to the same depth, provided the product of their weights
and the squares of their velocities is the same. Force, thus mea-
sured, is called vis viva, in contradistinction to force measured by
miimentum, which is proportional to the pressure, or dead puu^
producing it.
The principle in question is this: — If the jparticUs componng a
9^tkm m any measure connected or constrained in their motions^
or otherwUe, he acted on by arCy continuous forceSf then the change
of vis viva of the system, in passing from one given position to
(mother^ is the same as if the connexions of the system had been
ditsoHvedf and each part of iJie system had been suffered to move
frtdyfrom its former to Us latter position,
Ml. When a system of bodies iu motion passes through a posi-
tion of stable equilibrium, the vis viva will be a maximum at that
point; and if tnrough a position of unstable equilibrium, the vis
viva will then be a minimum. This is evident, since it appears
(98) that the centre of gravity occupies the lowest point of its
patD in a position of stable equilibrium ; the centre of gravity must
therefore descend in approaching this point, and the vis viva will
be aagmented by the accelerating force of gravity; but after
passing it, gravitation will tend to diminish the vis viva, which
must therefore have had a maximum value at the point of stable
equilibrium. Similarly, in a position of unstable equilibrium, the
centre of gravity occupies the highest point of its path; conse-
quently, gravitation will tend to diminish the vis viva before
reaching, and to augment it after the system has passed the posi-
tion of unstable equilibrium ; the vis viva will therefore have a
minimum value in that position. .Thus, for example, the vis viva
of a moving pendulum is greatest at the lowest point of its oscil-
lation ; and if a pyramid resting on its base were overturned by a
horizontal thrust against the upper part, the vis viva of the moving
pyramid would be a minimum, when, its centre of gravity reaches
a vertical plane passing through the edge on which the pyramid
id turning.
In respect to rotating bodies it may be shown that the vis viva
N 2
180 DTHAMICB.
due to rotation U equal to the moment of inertia (846) of <A« body
dbouJt its centre ofgravity, mvUiplied by the $queare o/the angular
velocUy,*
342. As in Statics it has been fonnd necessair to establish some
standard of comparison, or unit of pressure (67), so in DTnamics
there must be some unit of action, or unit of work^ as it is called ;
work being defined to be a continned motion, accom^nied by a
continuous pressure. The unit of work adopted in this country is
that which is necessary to overcome the pressure of one pound
through the space of one foot : thus, 10 units of work will be re-
quired to raise 1 pound 10 feet, or 2 pounds 5 feet, or 20 pounds
6 inches. The compound term foot-jxmnd is now generally em-
ployed in this country for the unit of work.
843. Accumulated Work. — A certain amount of power is accu-
mulated in every moving body by the action of tne forces that
originated the motion, which the bodv reproduces upon any resist-
ance opposed to its motion, and which is estimated oy the amount
of resistance overcome. This cumulative power of working, mea-
sured by the amount of resistance it is capable of overcoming, is
termed aecumulaled work. Thus, in the water of a mill-stream is
accumulated the work which it vields up to an undershot wheel ;
and in a carriage that is allowed to descend a hill rapidly, work is
accumulated that will carrv it a considerable distance up the suc-
ceeding ascent Work is thus accumulated only when the effective
forces in action exceed the resistances opposed to them ; and it
may be taken as an almost self-evident proposition, that the accu-
mulated work is precisely equal to the excess of work done upon
a system of bodies, over and above what is necessary to overcome
opposingresistances.
844. The amount of work thus accumulated in a body moving
with a given velocity is evidently the same, whatever may have
been the circumstances under which its velocity has been acquired.
Thus, whether the velocity of a ball has been communicated to it
bv the sudden expansion of compressed air or vapour, or by the
efasticitv of a spnng, or by falling freely through a corresponding
height, it matters not as to the result ; provided the same velocity
be communicated to the ball in all three cases, the accumulated work
will be the same. And similarly, the whole amount of work done
upon any opposed resistance is the same, whatever may be the
nature of the resisting force.
Let w be the weight of the moving body in pounds, and v its
velocitv in feet; and suppose the body to be projected vertically
upwards with the velocity v, then, by the second law of motion
^283) it will ascend to the height A, from which it must have fallen
treely to acquire the velocity v. There must then have been at the
instant of projection an amount of work accumulated in the body
sufficient to raise it to the height h^ and this will consist of w . A
* Bamihftw'» DToaimof, p. 169.
MOMXlfT OF IHEBTIA. 181
nnitaof work (342); but A = 4— i whence it follows that if u re-
presents the nnmber of units of work accnmnlatedy then
u=w.A = 4- r";
9
hence it appears that the aeeumulated ujorh it one-half the vis viva
of any moving body, or syetem of bodies.
345. The principle of ris viva, in its relation to acciunulated
work, may perhaps be more readily comprehended by applying it
to the case of a machine, considered as a system of vanonsly con-
nected bodies. The entire amount of work done by the moving
power, whatever it may be, upon the machine, is partly exhausted
at its working points, in overcoming the resistances opposed to the
motion of those points, that is, in doing useful work ; it is partly
expended in overcoming the friction and other prejudicial resist-
ances that are opposed to the transmission of motion through the
various parts of the machine to its working points ; and the rest
18 aeewnukUed in the moving parts of the machine, ready to be
yielded up under any deficiency of the moving power, or to con-
tinue the action of the machine for a time, should the operation of
that povrer be withdrawn * Thus, for instance, a fly-wheel will,
by means of its accumulated voork, continue the action of a machine
lor some time after it has ceased to be driven by the steam-
ensine.
Vis viva may be further illustrated b^ reference to the cireum-
stances of a railway-train in motion : smce the vis viva is propor-
tional to the $quare of the velocity, it follows that a train going
fifty miles an hour will possess more than tix times the vis viva
that it would have when going at twenty miles, and, therefore, will
possess more than six times the power of dealing destruction,
either to an obstacle, or to itself, at the former than at the latter
rate; and thus, the too-well-known relation between speed and
amount of damage, in cases of accident, is readily accounted for.
346. Moment of Inertia. — The statical moment of a material
muiicle, m, Fig. 237, about any given point, c, has been explained
(77) to be the product of the mass and its distance from that point,
orm .cm : but in relation to an axis paKsins through o, and per-
pendicular to the plane in which m moves, the moment of m acts
with the IsTorage of the distance cm; the effect, therefore, of m
on the other parts of the system with which it is rigidly connected
is representea by m . c m', which is consequently called the moment
of inertia of m in relation to an axis passing perpendicularly
vmmgh. c. Similarly, if any number of particles, m, are rigidly
connected with an axis, c, and the lines c m are the perpendiculiur
distances of the particles m from the axis, then the moment of
* lloselty's Eoginearing snd AreUteotare, p. 133.
182
DTXAMIOS.
J^. 237.
inertia of the Byntem is the sum of the same moments of (he several
particles, or as it is generally expressed
2 (m . c m').
I'he next point is to determine
the accelerating force on each ele-
ment of a system of particles acted
on by gravity, and moveable about
a horizontal axis with w'hich they
are rigidly connected. Lot m, n, p,
be the particles, and c the point in
which the axis intersects perpen-
dicularly the plane of the paper.
Let a bo the centre of gravity of
the system, which we may suppose
to be so placed that c and o may
be in the same horizontal line. Through o draw c <f, and from
m, ft, /), draw m<f, n«, oA, perpendicular to cd^ and therefore
vertical lines. Let/ be tne effective accelerating force on m, then
the effective accelerating forces on n, j9, are as their distances from
c, that is, they are
/.en /.cj>,
cm ' cm
and the effective moving forces are these quantities multiplied by
the masses of the corresponding particles, or
f.m, /•"•'"', fP'^P;
cm cm
also c m, c ft, c^, are perpendicular to the directions of these forces,
and therefore their moments round c are the products of these
moving forces and corresponding perpendiculars, namely,
f.m.cm, /•»•'"'*, fP-^J^;
cm cm
but the impressed moving forces are the weights of the particles, or
flr.wi, g.Uy g,py
and the moments of these round c are (77),
g.m.cmy ^.n.cn, g.p.Gp\
but by D*Alembert*s principle (339), the sum of the moments of
the impressed forces = the sum of the moments of the effective
forces, therefore
f ,m,cm-t^- +*—^ *- ^g,m.cm+g,n,cn+g,p,cp.
cm cm*' " if f jTi
whence .cm,g(mcm-^ncn^p.cp)
m . c m' + n . c n' + j> . c j?"
and the form of the expression would be the same, whatever number
CBXTBE OF OSCILLATIOX. 183
ef particles we assume. Bat m.Cfn+n.ci» + &c. has been repre-
sented by S (m .cm) and it has been shown (86) that
S (m . cm) B c o . 2 (m),
and m.cm' + ».cn' + &c.= S(m.cm') ;
therefore y^cm. cg .y. 2 W (1)
•^ 2(m.cm«) ^ '
347. The Centre of OeciUation. — Let it be now required to find
a point in the system, that will be accelerated exactly as much as
if the system consisted of that point alone. Let o, a point in C6
produced, be the required point ; then substituting o mr m in (1),
we shall have
1 1- r co.co.fl.S (m) .
accelerating force on o= — =-; — — i\ »
£ (m . c m )
but if the system consisted of the point o alone, then
accelerating force ono = g:
therefore equating these values according to the supposition,
co.oo.^.2(m)
^ S(m.cm«) '
whence S (m.cm')=co.oo.S(m),
^ _- r(m.cm*) /ON
c o . 2 \tn)
The point o is called the eeiUre of oscillation of the system,
because the accelerating force on that point being the same as if
the whole mass of the system were collected there, it follows that
an oscillation of the system will take place in the same time as
that of a simple pendulum of the lengtn c o.
348. Join Qm,aiiif op, Fig. 237, then by the property of the
triangle, we have cm* = co' + om' + 2co.od,
similarly on' = co*+om*— 2co.oe, &c.
multiplying these equations hj m, n, &c. respectively, and then
adding them together, we obtam
S(m . c m*) = c o*.S(m) + 2 (m . o to') + 2 c o (m . o d - « . o e + &c.);
but since o is the centre of gravity, therefore (86)
m . o c2— n . o e + &c. s o,
whence it follows that
2(m.cm')=2(m.om*)+co*.2(m), (3)
that is, the moment of inertia of a system about any point c is
equal to its moment of inertia about the centre of gravity added
to the moment of inertia t^tout c of the whole system coUeeted at
its centre of gravity,
OoroQary. Since co*. 2(m) must always have a positive
value, it follows tliat the value of 2 (m . c m') in (3) will be a
184
DTHAlflGB.
immmiMK, when cosO, henco the moment of inertia of a tyHem
about an axie paseing through the centre of gravity ^ is lew than
it would he about any other parallel axis.
349. If we call 2 (m) or tne aggrogato masa of the flystem, m,
then a distance h may be found (depending on the form of the
system] such that m . A;* =» S (m . o m*),
then ib is called the radiue of gyration of the system : for the rota-
tion of the system is precisely the same as it would be if all
its material particles were collected in the circumference of a'
circle of whicn h is the radios.
If these ralues of S (m .urn'), and 2 (m) be substituted in (3),
this equation becomes
2 (m . 0 m') = Jfc* . M + c G* . M ;
and if this value of 2 (m . o m*) be substituted in the equation (2),
we shall obtain
^^ M^ + M.CO*
C O OB _ -
M.CG
therefore
CO
co»co=sGO=s — , or CQ.oosAr:
CO
from which it appears that the jwinte c and o may he interchanged,
that is, if o be the centre of oscillation when o is the point of sus-
pension, then 0 will be the centre of oscillation when o is the
point of suspension.
350. The above relation of reciprocity between the points c and o
is the principle of construction of a very ingenious instrument,
jl^ 2j^ Kater'e Pendulum, which has been employed in deter-
'^* ' mining the absolute length of a seconds pendulum. It
follows from this property of reciprocity, that if a pen-
dulum have two fixed centres of suspension, and a weight
or weights be so adjusted upon it, that it will oscillate
in exactly the same time on either centre, then the
centres must corren>ond to the points c and o, and the
distance between them will be exactly the eouivalent
length of a simple pendulum, oscillating in we same
^ ^ time as the compound pendulum actually oscillates.
(Zj9 ' The following is the construction of Eater's Pendu-
]l lum. A brass bar, c n, Fig. 238, is.'fumished with two
■ transverse axes passing perpendicularly through it at
the points c, o; these consist of triangular stoel bars,
or, as they are commonly called, hnife-edgee, similar to
those on which a balance of fpood construction is usually
supported. Besides the principal weight, d, there are
two aclJUBtable sliding weights, b p, of which the lai^;er,
By is near to c, and the smaller, f, in an intermediate
CBKTSB OP PBSCtJOSIOH. 185
podtion. In aoing this instrament, it is made to oficillate Alter-
nately on the edges c and o, and the times of oscillation r3ndered
nearW eqaalbr utering thepoBition of the heavier sliding weight,
B. omail £flerences of the times of oscillation are then corrected
bj acynating the smaller vreight, f, which is to he moved a little
towardt ^e axis about which the number of oscillations is gretUest
in a given tune. The number of oscillations was d*itermined by
placing the pendalum in front of a clock pendulum oscillating in
nearly the same time, and obserying the intervals at which the
motion of the two pendulums coincided.*
It is not essential to the correctness of the result thus obtained,
that the centres of motion should present very fine knife-edges ;
for if the axes of motion, c, o, were cylinders with equal radii,
the distance between their opposed sur&ces would still correctly
lepreeent the length of the ^mchronous simple pendulum.'f
351. Tke Centre of Terevaeion. — If a body, moveable about a
fixed axis, be struck in a direction perpendicular to the plane passing
through the axis of motion, and its centre of gravity, then the centre
of percossioo is that point at which the impact mnst take place, in
oroer that it may produce no pressure on the axis. This point is
identical with the centre of oscillation (347), for as the acceleration
of this point is the same as that of the entire mass of the body, it
IS dear that the vis viva of the body, when moving, will be the
same as if the mass were collected at the centre of oscillation : the
vis viva, or dynamical force of the body, may be therefore sup-
posed to be concentrated at the centre of oscillation, just as the
statical pressure is in effect concentrated at the centre of gravity,
ukl can be counteracted only by a force applied at the same point
in an opposite direction. If a force acting in the same direction
be applied at any other point, it will be expended partly in oppos-
ing me vis viva of the moing body, and partly in producing pres-
sure on the axis. And the same will be true with regard to any
impressed force, as a blow, if the body be at rest.
AJso^ if a body moveable about a centre strike another body at
the centre of percussion, the whole accumulated work (343) is
effective upon the body struck, but if the impact take place at any
other point, the accumulated work is partly expended in producing
pressure on the axis, and the blow is therefore less/om^Ie. The
centre of oscillation of a rod suspended by one extremity is two-
thirds of its length from the point of suspension ; consequently, if
one end of a bar of wood of uniform thickness be held in the hand,
it will strike the hardest blow at two-thirds of its length — pro-
vided it is moved from the wrist, not from the shoulder, in which
case the point will be differently situated ; and if a fixed obstacle
be struck by the rod at any other point, pressure on the axis will
• For (brther information on this sabjeot, see Capt. Eater's aoooont of
tbe prooeM,- la Pbil. Trans, for 1818, p. 88.
t irbmU, Dynamin, p. 888. Pratt^ Meeh. Philosophy, p. 408.
166
be rendered STident b; an nnpIeuiaTit jar upon the hand, wbich
will JDcreatw with the dintance of the point of impact rrom the
centre of pcrousBion. Tliia fact i« well known to cricks tplajers,
wlieoETOr the bail ii not iitnick by the bat at the ri^t poiui.
352. The centre of oecillation or percnssion of a circDlar disc of
uniform thicknesB, oacillaCin); in its own plane abont ■ point in the
circumrerencc, ii a point in the diameter distant j of the diameler
from the point of BuspesBioD ; and if the disc oKillate about ■
tdngent, the diBtance u | of the diameter. Tbia may be readily
ohown by BuBpending a small heavy ball by a string of the len^h
above nUted, in front of the disc in the former case, and b; the
aide of it in the latter, wheu the disc and Btriog will be obierved
to OBcillite in tlie name tima.
S&3. Tlie centre of oscillation of any imgnlar body nay be
determined experimental]; b; ascertaimng the length of a simple
pendulum that will oacilhite in the sBine time ; it being remembered
that the centres of en>pens!on, of gravity, and of Caciliation, or
percnssion, vill always be in the same straight line.
354. As all bodies are acted upon by changes of lemperatnre, so
that their length btrcomes altered, it is of extreme importance to
have a pendulum constructed in such a manner, as to be unaffected
by such changea. Several modes have been proposed lo effect ao
desirable bq olg'ect ; of theae, the plan formerly most uauallj
adopted was the well-known gridiron-pendulum, composed of two
metalu, bo armnged that the eipimsion of the one counterbalance!
that nf the other. In ita Bimplest fonn, this contrivance cooaists
of a parallelogram of steel, ABoa, fixed to the
Fig. 136. rod, K, by which the whole pendulum ia suapeuded.
The braai ivd, F a, bent twice at right angles, is
fixed by its lower ends to the trwuvene oieoe,
C D ; and to the upper part of r O, the stonl rod
supporting the ball of the pendolnln is affixed,
wbich pasaes through a bole in the transveraa piece
o D. It is obvioui, that as a brass rod expands
much more in length than a steel rod by equal eleva-
tions of temperature, in the proportion of 0-00193
to O'OOlltl, if the length of the sleel and brass ban
be properly a^osled, when any elevation of tem-
perature takes place, the increase in length of the
steel ban, together with the snspendiDg rods, will
be completely counteracted by the excess of the
expansion of the braaa bars in the opposite direc-
tion. The Importance of an arrangement ot this
kind is snfBciently obvious, as an alteratioD of
30* of temperature, trould, by affecting the length
of a simple pendulnm with an iron rod, inlro-
dnce an error of eight seconds in twenty-foni
oT stcet mnd bna> rodi, the fbllmrmg plaD has met with much
ftTonr uiongst Continentml cbcknukera. The coDipenuted pen-
dalatn, Fig. 340, coasislsora Keel rod,e, cUiuped FI^.UO.
at the upper end to two bnw rodi, b>, all of
which paai throoRh the upper port of the drcalar
weight, or bob, which hu a caiitj at its cent
The gtael rod, s, has a bent cnMs-piece at its Ion
end, to which two ebort Ibtbh are jointed at
ud c. The lower ends of the brow rode, b, b,
rcK on the iiuer eitremitiea of these lever
while the bob reste on the outer, by meaua of t*
pioi, D, m. It ii endent that as the inner eD<
of the leien are depreued hj the exceu of
eipuiaion of the brara rods, the pointa, d, ■ will
he railed; and the amount of elenUdon of the bob, required to
compensate the eipaneion of the iteel rod, ma; be obtained b; B
Rotable adJuBtmeut of the emu of the leven.
In the preienl construction of English aatronomical clocka and
i^nlatora, the mercuria! pendulum is uniTersally adopted. This
couiita of a iteel itid, connected at ita lower end with an iron or
glsM TSBsel ooDtsiniDE mercnry, a abort colamn of which, hj the
great excess of its hnesr eipanajon abore that of sl«el, com-
pensates for the whole length of the steel rod.
356. A correction of the errors of a pendulum anMug Iron that
duage of length which is due lo change of temperatup' ■- — '
hownref, all that ii i^---—' '— ■' - - = ' •
ancB of an astranomic
that the time of oacillation in a circular arc ii
inoeaees with the amplitude of the arc, which depends upon the
amount of accelerating force transmitted U> the pendulum. Thia
fince is subject to minute Tariations, owing to theraryins amoont
of friction between the contact surfacrs of the icape-wbeel and
pallets (22S}, &nm dust, naciditr of oil, &c. ; and as these sources
of error are quite indefinite, a satinfacCorj method of equalising
the amplitade of the arcs of libratlon has long been an acknow-
ledged derideratom in horologj. This, however, has been in^
nieoal; snpplied by Mr. Loseby. A loop of balance-spring wire,
tbout three inches in diameter, ia Eied in front of the penanlum,
the plane of the loop betni( parallel to the plane of oscillatioD. A
pb attached to the pendnluni elongates this loop towards the
eibeme of its eicnraion, and it Is endeat, that the more the loop
is elongated, the more its elasticity will accelerate the recoil of
the {nndalam, and therefore tend to diminish the time of oscilla-
tion in huger arcs. The magnitude and podtioo of the loop must
ise
of conrM beniiitleniof oiperiment; bnt, wh on carefully »4)'i"*e<'.
it hat been found, that evea doubling the clock-weight produced
a TBTj amsll vsriation of the rate.
367. While diiciiHiag the aubject of OHcilUtioni, it rDa|r not be
nnintoreating to our renders to iDTeallgale a, piublem which com-
inftDdad n much laipir ahtre of public attention Ibiin, perhaps, it
iras entitled to, nnmely, the rotalion of the plaae of OKiUation of
the pendnlom. . If a heavj ball be anspended from a conaidenble
altitude, ao RB to oscillate Teij ilovly, and cnnaequentlj, to con-
tinue itg oacillntlntin for a conaiderable length nf time, it ii obserred
that the plane, io which the pandiilum iscilbitei, will graduall;
rotate on a vertical line drawn throuefa the P"int of inspeniion aa
an axis. This hai hfta anpposed to afford an independent
proDf of (he rotation of the earth, and it will preaentljbe ahown
that the time of a complete rotation of the plane of the pendalaia
will varr from the exact length of a dav at the pole, to a period
of indefinite dnratioD at the equator. The eipprimenl ie, however,
an unialiHfaetoiy one. The diflicnlties of mechanical adjaitment
are vet7 great. Jt ia indiepenaable that while the pendulan '
force! would immediately be brought into play, which would
the apparent plane, or in thi« caae the principal plane (359) ot ine
conical anrface generated hy the pendulum ilowly to rotate. Thia
motion, which may be called an apiidal motion, is analoKou* to
the motion of the itptida of thn orbite of tbe planeta, uidwhich
arisea from the action of forcea eomevhat analngoua.
In Fig. S4I, let H, H, be the polea of the earlh, o in centre, and
■ q the equator; and let A be a point in the earth's surface at
ffg_ }()_ which a pendulum in oacillating in the plane
of the meridian, which here coincides wilh
the plane of tbe paper ; let l be the latitude
of A. and let b be another point in the same
parallel of latitude, at which the pendolnm
make* its second awing. Draw tangents to
the meridians A K, bh at (ha poinla, A,B,
) which will therefore meet each other, and the
aiis of the earth, s a, produced, in the a&me
I point, t: join ao, and draw at perpen-
I dic'ilar to ot. Let a cone, of which the
iqiex ia T, be auppoxed to envelope the earth
— a sphere — along the drcle of latitude where
haTJng aa many ai ^
twenty-four houn. Let ua suppose the n
wcillation in the meridian av, which ooincides with at at a; it
will make ita second awing in a line parallel to a t, the inclins-
tiuD of which to IT ia equal to atb. But the direction of the
n
BOTATION OF ▲ RIOJD BODY OB BTSTSM. 189
meridian daring the second swing is the line bt. Hence the
angle atb is the deviation from the direction of the meridian at
the end of the second swing. Therefore, in twenty-four hours the
BQi&ce of the oone ronnd the vertex t will he the measure of the
whole deviation in that time. But if the cone were opened ont
on a plane, the anenlar space round t would he measured hy an
arc of a circle equju in length to the circumference of the parallel
of latitude divided b j its radius a t.
But the circle of latitude = 2«-.AC = 2ir.AO.oo8i^
and A T s A o . cot L,
hence the deviation in 24*» =s '- '— 1
AO.COtL
= 2ir.sinL.
ConBequently, the plane of oscillation will make a complete
rotation at the pole, and half a rotation in lat. 30** during twentj-
four hours, and will remain stationary at the equator.
358. The rotation of the pendulum has been experimentally
illustrated in a very ingenious manner by Prof. Wheatstone. A
horizontal piece of wMd, a c. Fig. 242, ^^ 242.
toming round freely on a heavy foot, e,
BQpports a vertical graduated semicircle,
ABC, furnished with a sliding clamp, b.
A piece of spiral wire spring is attached
to b and to d, the centre of the semi-
circle. If this elastic wire be made to
oacillate, by drawing its middle point a ^
nnall distance from its position of rest CZ—jV.
by the finger and thumb, and then releasing it, it will be found to
oscillate slowly and visibly. If the semicircle be rotated round a
vertical axis passing through its centre, the plane of vibration will
be constrained by tne attachment of the wire to pass through the
centre, in the same manner as the plane of oscillation of the pen-
dnlmn by the force of gravity, and the conditions of vibration
^11 be precisely similar to those of the pendulum.
If, for example, the slider be placed at 30°, and the wire made
to vibrate in the plane of the semicircle, then on turning the semi-
circle half round, or through 180", the wire will be found to
vibrate at right angles to the semicircle, the plane of vibration
having made a quarter of a rotation, which agrees with the pre-
ceding formula.
KOTATIOH OF A BIOID BODT OB SYSTEM.
359. The complete and general investigation of the rotatory
motion of a rigid system re(][uires a higher range of analysis than
that which is compatible with the scope of this treatise; for this
the more advanced reader must be referred to one of the standard
sDilTlkal treitisea on djnamici: but by id apprcpriale M:kcIion
of tnu lypical form uf ■ rotnling bodj, ihe impaitanl principlei
inTolTcd in this drparlmcnt tf ciyniiniicB maj be luRicicnll; tiuci-
ciated. Let us then auppose <ho rotating body lo bo of 1701016-
Irical form, and unequal m its tbree dinieuDionBof Impb, breadth,
■ird t1ii(^kaeaB ; a*, Tor inatance, an OTal or elliptic {late of melal
of anifumi ihickuesa and denaity.
]f tbia body le euppoaed to rotate round an :
itfl centre of ftravity, and coinciding with
mtniir axia of the ellipar, it ia manifcbt that in
ia ajDimetrical witli rraptt to the axil of n
ccnirlfucnl force (320) of eaib particle on 01
counlcTBCted by an equal and oppoaite force
aiJe ; and tbe fsmo renmrks may be applied lo rotation raDnd
axia paaaini; perpendicularly through Ihe centre of the plate: in
eilber of theaa casea, then, rolaliun will proJucu no prutart <n>
the aru. It may alto be remarked that Ibvae three aiei are per-
pendicular to each other, and are called ^inripal arti. And it
nmy be ahown gi^nerally* i]\M everii rtgid ig4ten hat three prin-
cipal 03-et 0/ rotation, perpendiailar to eath other; rotaliau
•bout either of ichich prodncei no preeture oa Ihe axit.
AImi a plane, in which any tvo of the principal aiei lie, ia
called a prindfal plane; conaequfntly, rotation in a principal
plane prouuees no pr«a«ure on ths "'- -' -■-■---
[ia paaKing through
tber (be Dii.jur or
:ilh:r case thu body
alien, and iliat ihe
I aide of the axia ia
be contrary
360. ir however, the body rotate round an aiia pamltel to eilbrr
[ra, aa loy, Fig. 343, paralfel to a a, it U
evident that Ibe ceutrifugHl force of each particle
of tho principal
Kff.W.
ibove B b ia counteracted by an equal fon
tbit line, Bod CDDBequently the rotation of (he
' ' '" ' " ' " produce angular
irtot
ist, the <
iaof n
But
the centrifugal force ut t bi/ r.ol being ci
balanced by Ysy, there will be a preaaure on the
axia of rotation, which may be rapreiented by a
single roenllant acting in iho diraclion o b. And
the aame propui-IHoQ ia genendly true, namely,
that if a rigid body rolatt abend an axi* parat-
Ul lo either of the jirincipat oxet, tha raaltanl
prtMtiTt on the axit maiibe repreievteil bg a tine
perpendicular to it, and pa$ting through Ihe centre of gravity of
Ihtbodg.
361. If the body b« now auppoaed to ratale alout an aiia
paaaing ibraugh the centre of Kravit^ , but not coinciding wiih
eilberof tbe principal aiea, let the ana cf rotation, ly, Fig. !4i,
<t the e
endiculai
ly, tbrougH «, cut the ellipse in the pointa r,
meota, fod, uoo, being maDifeatly unequal, the centrifugal force
* £atubaw'i Djaanuea, p. IBJ.
r
of the Bemi-ellipae, r d o, will prodDCD a
pramm on ibe Biia in tbe direction of the
arrow at i ; and for tlie name reason, tbe
centririi^ force of teo will produce an
equal and oppoeit^ preflsurs in too direction
of tbe arTDH at jf. These equal and np-
poeite parallel pressurescoasLituteacouple
(81), which cannot be represented bjanj
■iDKii resultant, and their leudency is to
twisl, or change the angular poeition of,
the axil of rotation. And the lime will
be true if tbe aiia of nitation be suppased not to cotQcide with
the plane of the ellipse; for in that case the plsne of rota-
tiuQ thrODgh o will intersect the ellipse in some diameter, and
the centriTugal forces or tbe eemi^ellipses on opposite aides of that
diameter will lend whollj and equally in opposite directions.
And the general propnaition is equuU; true, that if a rigid body
rotaU Ttniiul any aat patting throvgh the cenlrc of gravity, but
not toineiding aith a prindpal ocfi, the reiuUant pretiure on the
axis mau ht repraenied by a aruj^.
And 6om the preceding propositions it maj be inferred gene-
nllj that in any rotatituj body, the prettvre on the axil of
TiMtum may h« repramted by a tingle rtevltant, and a couple,
either or both of which matj — 0.
362, It is evident, from a considemtion of Hg, 244, that if the
bod; be supposed free to move in its own plane about its centre c,
so long as i coincides with t there will be no tendency to displace-
ment, out if A do not coincide with t, nil in the figure, the cen-
trifngal force will tend to make ao coincide with %x, or in other
worcu, to make the axis of ratatron to coincide with B b. And if
the body be supposed free to more about a a, or b b, when either of
these lines comcides with the plane of rotation (which is perpen*
dicuUr to ly), then if either a or b (as tbe case may be] ci '
to dii^a
ith y u, there will be no pressure on that aiis, or tendency
lacethe ! ■ ' ■■ ' -
e axis of rotation ; but if a or ■ be displaced from
o the aiis of n ' ""
lie bro^ht to ccincide with tbe plane of rotaliaii. that is, until
tbe axis of rotation be paipendiciiLar to \!»t plane of tbo
bjdy.
* ~ ''"'slbre, that when the axis of tnta^on coincides
axis Ao, it is in a position of wiettMe squili-
when the ails of rotation ocnncidss with the
9psl aiisBb, it is in a positjon of mixed equilibriniD (99),
It amea;^ then
with the prtBcipal
bnam; riao Aat
Caaps
ng s
I tbe direction of one principal plane passing
'"■"'-'"' ■■ '■' ■' and lastly,
tbwB)^ si, and nBstaWe in the direction of the other; and lastly,
when the axis of rotation coincides with the principal sxis per-
pendicular to the plane of the body, it is in a position of (foils
equilibrium.
36B. It mftj be tliowQ* that the momint of inertk ^346) oT •
Tot&tiDg bodj with reipcct to one of the priacipal kxci u greater,
and with respect to another of them ii len, than with reapeot lo
anj other line passing through (he centre of gr*»ily ; and it may
likewite be eliown that the position of stable equilibriom ooincidea
with that of greatest, tuiiii of anstable eqailibnuin, with that of
Ie«Bt moment of inertia. His is manifestlj tnie in regard to the
rigid body that has been here coDsidered.
Preciulythe same course ofargumentmighthave been adopted,
if *D ellipsoid had been taken as the type of a symmetrical li>-
tating body ; the form of this solid may M iinderglood ai that of
a long e^ flattened sideways.
In any solid of revolntion, that is, a solid of which the sarfaoe
is generated bj the reiolution of a geometrical figure abont a
straight line, snch as a cone, cylinder, spheroid, or paraboloid, one
of the principal aiea is the geometrical aiis of the solid, tiio
other two are any two lines at right angles to each other in the
plane of reTolotion which passes tbrongh the centre of grarity of
the solid. The geometrical axis may be either that of graateat or
least moment of inertia, and conseqnenlly of stable or onstable
eqnilibrinm of rotation ; but the slaiHll^ of rotation will always
be equal about Ibe other two princiral axes.
364. It may now be desirable to illustrate by some examples the
priiiciplei that haie been invest is'ated. The most ounrenient appa-
ratus for this purpose is an npright standard with a foot to it, to
which a horiiontal ana is atlacbed, carrying at its extremity a
vertical mandrel, with a small grooved pulley on it, lo which rapid
rotation may be communicaled by a band possinj; over a wheel
attached to the upright piece, TarioDS bodies may be inspeuded
b}; a cord (or still better, by a bundle of Ihraads) from the extre-
mity of tho mandrel; and when the rotation is sufGcientty rapid
for the centrifugal force lo overoome that of gravitation, the bodiea
Bt.Ui.
will all be found, afier a litlle time, to assume that poeition
which the axis of rotation coiacidea with the principal axis
»'■ Djaaniof, p. ISl.
193
moment of inertia, vhich ii slso tW of stable eqniU-
In Fig. 245, tbe dotted line in eich esse showg ths poii-
of the centre of ^n\itj Tertic&lly under the point of sapport.
Tbo body a is an ellipeoid ; in the poBitlon which this anumes,
the two greater aiea lie in a harizontd plane, and the leait ia
TorticaL B i» a cone, the centre of graritj, o, of nhich liel in itt
aiit, at a diataoce of ane-foorth of ita length torn the baae. Tbe
ftiis of the cone vill be found to asanme a hariaontal podtion, the
axis of rotation paniog tbrongh o. In Ihig caae nn; two aiei
pcTpeodicnlar to each other, in the plane of rotation throngh o,
will be eqoal axes of least moment of inertia (35fi) ; if the oone
had an elliptic baae, the nuyor axis of the ellipse wonld become
TOntsl position, the a
beads, or ballets, with holes throagb them. These, afier a varietj
of QDcertain moTementa which it is not Decessar]^ to investinte,
will aasnme tbe poaidon of a boriioatal circnlar ring, similat^ to
the ring c.
If these bodies be Strang on an elastio cord, as the velocity of
ntation increases, the drcle will be fonnd to expand nniformlj,
leaving nearly eqnal spaces between the bodies : this eiperimenl
afibrds farther evidence of the existence of centrifagal force.
365. The phenomena of rotatei^ motion have been aptlj illus-
trated by tbe gyroKope, an instmment long since G0li*tmctad,juul
.,_ ^ ,.. ._._ _..-._ ,._ ., „. jj^, -f|^j^
more recently bronght into notice by H. Foucault.*
instmment consists of a heary ring of metal, o. Fig. *•«),
attached centrally by a tbin plate to an axis, the pivots of which
work in two centres, k, r, paasina; thtocgh the
circamfetetice of a ring, n, waich ib attached by 'V- IM.
pivota at right sngles to i, r, to a semicircnlar "
BDpport, 0. This IB fixed on the top of a ojlin-
dncol stem that moves roond freely in the tubalar I
support, B, which itself rests on a heavy foot, j(. It ■
is uecessaij that a line joining the pivots of the I
rotating axis should accarataly pass thionffh the I
centre of gravity of o; ^eo that a line joiomg th" •
pivots that sapport D sbonld likewise puss tbrooj
the same pmnt ; tbe wheel when at rest will tht
be in tbe condition of indi9erent eqnilibriam (97). I
Tlie axis of a is fbmisbed with a snail projectiDg I
pin, to bold the loop at the end of a piece of string |
to be wound roand the axis, which is thea made to
rotate r^iidly by fordbly onwinding the string, as in apinniog a
ehild't top. As the vis viva of a rotating body, that ii, its energy
t or tUi Und m bron^t from Italr In 1817 bf l£r.
-'"-' — noTlJiuliiatniniSBtinsBHdbjlIr.Alrjinhh
fixed on the aiia, vhicb is capable of being placed in gear with
the lut of a train of mulliplving vbrnU, the first of which ia
tnmed br a winch. By theae means, especiallT if the diac b«
large and beary, a much higher Telocity of rotatTon maj be com-
mnnioaled to the diac than that otpnble of being prodnced by
meana of tbe itring. NumeroDa illuntration] of rotatiDn may u
thni giTen, but the foUowinK are tlie more importaLt.
If a imsll weight, h, of two or three ooncea (the diac being
foar inches in diameter), be aaepended at one extiemity of the
axis, u P, the ring, u, being honwntal, the point, f, wiU be im-
mediately drawn down by the wciglit, ai represented in the figure ;
bnt if the disc, a, be in rapid rotation, the weigbtwill prodnce no
Tiiibie deflpiion of the point, r, bat only a luw hotiaontal mic-
tion ; and if the weight, h, be remoTed &om r, and samended at
K. or if H be allowed to remain at f, and a heavier weight be ma-
^nded at a, tlie horizontal rotation will lake place in tbe oppoaita
The eiplanalion of tbia fact will be beat underatood bj re-
Xg. M7. ference to the diagram, Fig. S47. Let tlic
motion of rolaiion of a particle at a be repre-
■enled in magnitude and direction by the
arc A B, and let the motion of the piant «,
dne to (he weight, h, Buapended at r, be re-
presented in magnitude and directtnn br
A c, an arc of a great circle paiatng thnnwh
Ibo axis ; then completing on ihs sphere t^e
paTallelogram, ca, and drawinj; the great
circle, adk, through its opposite angle i>,
the particle at A will, in obedience to tbe
two modona, move in the plane a d e, and consequently Ihe pirat at
r will more boHiontally m the direction rr". It is evident that
if the accelerating force, a c, bad acted in (be contraij diredian,
tbe weight being inspended at I (Fig. S46), the movement of the
point r wonid also have been in Ihe contrary diiectioD to f r*. If^
when tbe horizontal motion of the axis bf is proceeding, it be
arrested br bolding the arc, c, by the bands, do mvltoiif motion
can take place, and tbe point r will be deflected by tbe weight a,
aa it would be, when the disc is not in rotation.
The preceding experiment may be made in another fbnn hj
detscbing the ring, d, from tbe stand, and haviDg produced a rapid
rotation of the disc, suspending the ring by a stnng attached at
B, or F, as in Fig. S4tl ; in this case tbe weight of the machine
{(•slf will act in the same manner aa the weight, h, in ths fanner
CMS, mnd tbe rin? vill roUto round the ati
in m horizontal pla^e, in apparent vioUtiat
tlte law ofgraiiUlioD. From those eiperimeola
it appean that a force applied to more the rots-
ting bodj prodnces a motian of i(e aiis a right
onjrie in advance of that wbich it wonld prodoce
<m tbe body at nat.
If the detached ring, o, be held bj the h
(the dice rotating rapidlr) the apparent (tm
of the ring to raiist any change of the poaitioo
of !ta plane of rolatloD, bear» a itriking reeem-
blaace to an act of ralition. If the ring be re
pisoed in the frame, c, in a horizont*! position
and (tbe disc rotating) the frame o be rotated oi
ita Mem, the ring, D, vill Immediately become inclined, and will
aasome a vertical position: the direction of ita inclination will
icadil/ be diKOTered bj a diagram aimilar lo Fig. 347.
It may here be observed that the tendency of the rotating dilo
is to eet ita aiia of rotation panllel to that of its aupport : and
nmilarly, if all aies of motion paaa accurately throngh the centre
of grsTity of tbe diac, acd the energy of rotation be considerable,
the inMrument wiU be infinenced by the rotation of the earth, and
the axil of the disc will become parallel to the earth's axis ; and
thus the tatitode of the place of onerration may be approximately
indicated.
367. Lastly, if an impnlsive force be impressed on the point b,
or p, in a direction perpendicnlar to the plane of tbe ring, na by a
alight blow with the fingers, the material axis of tho disc will de-
ecnba a apiral conical surface about the tanporari/, or, as it ie
commonly called, iatlaiUaaeini* axU of rotation, until the cguple-
pressure exerted on the latter has brought it into coincidence with
the aais of stable eq^uilibrinm(36':!), that is, with the material axis
of the disc. Expenmente might be indefiiiitely multiplied, but it
ia coDirideied that enough have been given to elucidate all Ihe
more important bearings of thissabject
366. In illnstration of their practical importance it may be etated,
that tbe phenomena of planetary motion known to astronomers as
freeation and nutation, are direct conseqaences of Ihe principle*
ofrotatoTj morion here detailed. Tbe precession of the equinoie*
arises from a displacement of the earth % axis, which sLowa itaelf
by a slow change in the position of the pole among the stars.
3liiB point appears to move in a small circle parallel to the plane
of the earths orbit, instaa the extremity of the axis of tbe gyra-
•cope mores parallel to the horizontal plane. The attraction of
the son, at the summer and winter solstice, on the protuberant
equatorial matter nearest to it will be greater, and that on tbe
portion most remote will be less than the mean altracrion. Theae
foi«ea then, besides carrying the earth ronnd in its orbit, would
196 DTVAMIC8.
have the effect of raising the North Pole, in the direction in which
the earth is moying in its orbit.*
An ingeniooB and important practical application of the
gyrascope has been made bj Prof. Piazzi Smyth ; it consists of a
series of laree gyrasiv^pes, rotating in yertical and horinmtal
planes, attached to a platform hang in gimbals on board a shi^.
The energy of the eyraBCopes will maintain the platform in a hori-
zontal position, ana in the same azimuth, in spite of any amount
of "liveliness*' in the ship ; and thus enable astronomical obserra'
tions to be made, when it would otherwise be impracticable to
do so.
VIBBATOBT MOTION.
369. The constituent molecules of bodies when they are at rest
are in a state of eouilibrium. This state will be disturbed by
means of any appliea force ; and if the disturbing force be not so
intense as to produce disruption, the molecules soon recover their
natural porition. This restoration of eouilibrium does not, how-
ever, occur at once and suddenly, but oy a series of alternating
movements, by which the atoms are approximated and separated
repeatedly, imtil at length they attam a state of rest in their
normal position. Such motions are of high importance, and are
known by the names of vibrations, waves, undmations, or oscilla-
tions, according to the particular circumstances under wluch they
are produced.
Id every complete vibration, or entire wave, Fig. 249, the follow-
ing parts are recognised : —
A E B D c, the whole vibration or wave ;
ABC, the length of the wave ;
ABB, the phase of elevation of the wave ;
B D c, the ^hase of depression of the wave ;
E F, the height, and o d, the depth of the wave.
370. The effects of these molecular movements are readily ob-
served by fixing an elastic piece of steel to a support at one end,
leaving the other free. The rod shown at rest is perfectly vertical ;
on applying force to draw it on one side, and tnen removing the
hand, it will fly back, not, however, to remain in its original
* For IB aaalytioal inTMtigation of the phenonMna exhibited by the syrt.
■oope, the reader it referred to » pH>er by Mr. Bridge, in the PhUoeoiSacal
Mftfuiae for Kovcoiber, 1857.
reetiUnew potitiint, bot it will go beyond thu con- ^- MO-
■iderablj, becomiDg comd is the opposite direotioo,
End diiu s seiiea of vibraltoM, each deCT«uing in \^\^ Wtf
magnitode, u ihown by the dotted correa io tlie y; [j./
fiKore, will continue for same time ; bat at length V L /
l£jy iriU op«ae, and tho Kid onra more be left in its * |'
origiiully Tsrtics] positioD. A little reflection will F
(hotr that dnring tbii aerie* of movemenCa, the i~ M.
cnnatitQent atomi of the rod moii h»Ta been alter- [\ ^~\
nstelf wpanted and appraxinuited, acoording as \] (
one or other of the ddw M the rteel becMoe convex
or coDcare [13].
Some bodies vill, in consequenoe of their natural eluticitr,
readily assume these motions; others are made suffidentl/ elastic
b; artiGdatlj hardening tbeA, as in the tempering of iron and steel;
or by tension, a« by sttetching cords anil membraDSB, as in the
Aiings of a barp, or the bead of a drum.
371. Vibratai7 motion may be snccessively Sig, iSV.
cODUDnmcaled to every part of a body, or be
partidiuled in by every part at once. To illnj-
trate the fonner of these conditions, attach one
and of a rope to a sapport, a, Fig. 2&1, grasping
the other end, k, in the hand ; on ^ving the end
a sharp jerk, or Btill better, on drawing aside the
rope at the poiut a by the finger and thumb of
the other hand, and releasing it, the primary
defieiioD prodaced at a will be obtervod to travel
•long the rope successively asanming the posi-
tioiia h, c, he., and after reaching the Sied end
will be propagated back again in the tome plane,
bot on tA< eovirary side nf the position of rest,
d; wheitce tho wave will travel back to the
These phenomena are best demonstrated by
«mployiDg a piece of vulcanised india-nibber
toM about hall an inch in diameter, filled with
•and, a* on account of its weight, the undulation
travels slowly ; and if the tube be about ten feet
long, and be slightly stretched, and attached to
two blacks fiied at the enda of a vertical board^
the undolalion will be seen to travel up .
down from end to end several times in e
Tlua is termed a prcgruitve nndnlalion, and
if B second impulse be communicated at a when
the first reaches b, and a third when it reaches
198
DTHAMIOS.
372. Vibrations are tenned stationary when every part of the
body assumes motion at the same time, as when a rope is fixed at
A and B, Fig. 253, and being drawn at its middle from the recti-
linear position, ultimately recovers it, after performing a series of
vibrations in which every portion of the rope simoltaneoosly
participates.
373. When a body is made to assume a series of stationary
vibrations, the points where the phases of elevation and depression
intersect are always at
A B, which has been made
to assume a series of sta-
tionaiy vibrations, the
parts marked n will be
in a state of rest, and
pieces of paper resting
upon them wul be undis-
turbed; whilst, if placed on the intermediate portions, thev would
be thrown off immediately. These points are called nodtU paints.
When a plate is made to vibrate, these nodal points of rest always
exist, ana may be easily detected (see Acoustics).
A ready mode of rendering the nodal ]>oints visible is by means
of a piece of elastic spring wire, five or six feet long, one end of
which is fixed, and the other held in the hand. On stretching
the wire slightly, and communicating to it a number of equal
successive impuues by a vertical movement of the hand, at such
intervals that the advancing and reflected waves ma^ coincide, the
nodal points will be rendered distinctly visible. This may readily
be accomplished by a little practice. The same thing may lie
aocomplisned by means of the loaded india-rubber tube already
mentioned.
374. Elastic rods or wires may easily be made to vibrate, and
when uniform in structure, in equal times ; the number of vibra-
tions increases with the diminished length of the rod, being in-
versely as the square of its length. Thus, if a rod twelve inches
long perform three vibrations in a second, it will, if shortened to
TlBXATIOm OF BODS. 199
one balf, perform twelve TibratioiiB, and if of bat three inchee in
length, forty-eight in the same time.
375. A -vibrating cord or wire, or an elastio rod fixed at one end^
will not neceflsarily vibrate in a plane, bnt any one of its points
may describe either a circular or an elliptic path. The circnlar
vibration of a oord has been aptly illnstrated by Prof. Tyndal by
attaching one end of the cord to a rapidly-rotatioe mandrel, at a
point slightly eccentric, the other end of the cord being attached
to a fixed support, with the intervention of a swivel By properly
timing the rotation, the cord will divide itself into several vibra-
ting spindles, separated by nodal points. *
To the mnltitnde it is probable that the facts of science are
more acceptable when decked oat with adventitious ornament,
and in deference to this taste. Prof, l^ndal has employed a cord
covered with tinsel, or a strioff of bright beads, and has illuminated
the fusiform subdivisions with coloured electric light.*
Also, it is a law of vibration, that a body may have two or mors
modes of vibration* impressed upon it simmtaneously ; thus a cord
or rod mav vibrate in its entire length, and ?rith one or «^^ 266.
more nodal subdivisions, at the same time, and the
motion of each particle is the ag^^te of its separate
motions : this is called the jmneijple of the super-
po§iHon of small motions.
This may be beautifully seen by the Oaleidophone, a
contrivance of Prof. Wheatstone, made by fixing a
silvered glass bead ik. Fig. 255, on the top of a steel
wire B. On making this wire vibrate, the curved ^th
of its extremity will be visible by the motion of the httle
spot of light reflected from the surface of the bead.
Manv curious and interesting combinations of on-
eqaal vibrations, in two planes at right angles to each I
otner, may be produced oy making use of rectan^lar /^ [^
wires or rods, the width of the sides of which is in L, J
some simple numerical ratio, as 1 : 2, 2 : 8, 4 : 5, &c. "^^^17
With a rod, the sides of which are as 1 : 2, a parabolic curve may
be produced, which will sometimes pass successively through the
f<^wing phases,
2^.266.
while others will present more complicated combinations of two
unequal circnlar or elliptic motions.
* Th« Bsm« tMta appMkn to prevail respeotiog tome of the grander ob-
jeeta of mitiire, rinoe it baa reeently become the faahion to aimuarljr iUnmi-
nale tbe FaPa of Bchaffhaqaen, aaa the Beiohenbaoh, the GiMbaeh, fto., ia
Switcerknd.
200
DTKAMIOB.
^. S67. Bjrraflecting a rsjof h'ght from the
Bhimng Bur&ce of a vibratiDg wire,
Dr. YooDg was enabled to obeenre
the curioQB corveB deacribed by its
particles; some of these are repre-
sented in Fig. 257, but their vanetj
is endless.
376. Vibrations are performed either transyerselj or longitadi-
nally with regard to the axis of
the vibrating body. The former
may be illustrated by fixing a
wire to a proper support, ab,
Fiff. 258, and drawing it at its
middle out of its straight po-
sition; the vibrations shoirn by
the dotted lines are tran$ver$e
to the axis of the wire.
Fix a weight, d, to one end of
a properly-suspended piece of
^.258.
^.258.
f
^
brass wire, c d, Fig. 259, coiled
into a loose helix. If the weight
be raised towards o, and then allowed to fall, it will advance to,
and recede from, d alternately, the wire performing a series of
2aii^iM7tna2 vibrations.
377. The longitndinal vibrations of a row of particles may, like
the transverse vibrations, be either stationary or progressive, the
stationary undulations resulting, as in the former case, from the
reflection of progressive undulations. A progressive longitudinal
wave is well illustrated in nature by the passa^ of a li^ht breeze
over a corn-field. Here, taking a row of ears in the direction of
the wind, each ear of com is successively deflected by the pressure
of the air, and then returns to its former position : and the pro-
gressive accumulation at one point, and recession at a succeeoing
point, correspond to the elevation and depression of a transverse
or normal wave.
These longitudinal vibrations may be conveniently illustrated
by an ingenious apparatus designed by Prof. Wheatstono for that
purpose ; this consists of a long cylinder, about four inches in
diameter, enclosed in a box, the axis passing ^roogh one *end of
the box, that the cylinder may be rotated at pleasure by a small
winch. A series or oblique rinn, about a quarter of an inch broad,
are drawn on the surface of the cylinder, and a slit of the same
width is made in the box, parallel to the axis of the cylinder,
through which a small portion of each ring may be seen. If the
obliquity of all the rings be equal, but the same phase of each ring
be successively placed at equal axial and equal angular distances
round the cylinaer, when this is rotated in the box, the appearance
of a progressive longitudinal undulation will be producea. If any
portion, as one-third, or one-fourtli of the length of another equal
crHndar bs taken, and k traurarae ring be pUoed at Mah end of
thu portian, and a leriee of obKqne rings between then gi«dnallj
iTWTfiatiTig in obliquity, and agvn giadoalLj diminuhing, ibt tazue
wiiea bein^ i«pe^«d oo either nde oT e*ch truiiverae ring in lb«
refeiM paadoD, the ntatioa ef this cylinder in the box will produo*
*' Bnnoe of a stationary loDgitudiiial Tibration of a row c
Theeffect U moit itriking if the box be blackened, and
the appeannoe of a stationary loDgitudiiial Tibration of a row of
partidea. The effect i* moit itrikinR if the T ' " '----' ■-'
wiat» ring! be drawn od a black cjSnder.
Similar «ffi)ota, althoogb perhaps not eqaally perfect, may, bow-
errar, bepndoced by meani within the reacbof anyof anr readan.
For this pnipoae, a eeriei of eqindiBtant undnladng Knee are to be
drawn on a iheet of paateboani, m in Fig. SCO, each t\
kad parallBl to itself; iwbind a
. of pasteboard the appe
grenirs wave trill be prodaced. If straight hnea I
narTTTw slit in auother sheet of u
. . .. — . — , „ ._ _,.^ratDS, tl
width of the lines and of the slit should be the same ; and tt
eBect will be most striking, if the liaes be leil white, aed the
ground aa well as the screen l» blackened.
378. One nmarkable aDirersal law gonms all lb«sii vibratory
morements, that no matter what theit magnitDdo, tbey are always
meironons, that is, the excursion of each particle on either side
202 DTMAXICB.
of its normal position is perfonned in ec^naX times. Thus, in the
case of the cord vibrating in its entire length. Fig. 253, all
pNortions reach the straight line (the position of rest) at the same
time.
It has been observed (290), that, generally, the amount of
elastic force is proportional to the displacement ; therefore, in the
case of vibrations, each particle will tend to retnm to its position
of rest with a force proportional to its distance from that point.
But it may very readily be proved by analysis, that if a particle
be urged towards a given point by a force varying as its aistance
from that point, it will reach the point in the same time from all
distances. Hence vibrations will continue isochronous until the
particles resume a state of rest ; their motions having been grada-
ally overcome by imperfect elasticity, and by extemiu resistance.*
On the subjects treated of in the five preoedmg chapters the
student may consult with advantage any of the foflowing works :
Peschel's Elements of Physics, translated by West; Moseley*s
Illustrations of Mechanics ; Gregory's Mechanics ; and Ferguson's
Mechanics, edited by Sir D. Brewster ; and the Monographs in
Brewster's Encyclopedia, Lardner's Cabinet Cyclopaedia, tne En-
cyclopndia Metropolitana, and the Library of Usetul Knowledge.
Amon^ the Continental authors, the works of Pouillet, Poisson,
Biot, Hauy, Qoetelet, and some others, will repay a careful studv.
The laws of Statics and Dvnamics are treated mathematically
in Wood's Mechanics, edited by Snowball ; Whewell'k Mechanics,
Dynamics, and Mechanics of Engineering; Eamshaw's Statics
and Dynamic^ ; Wilson's Dynamics ; and Moseley's Engineering
and Architecture. In the Prmcipia of Newtoo, and Euler's Letters
to the Princess of Anhalt-Dessau, many of the subjects are treated
geometrically.
* Ai thii ii s very ftincUmental inropositioii in Aoonttioi, the proof maj
not be anwelcome to our more adrMioed reedert. Let • be ibe aiitasee or
ear |>srticle from ito pontion of reat, a— « tbe •p«oe deeoiibed, aiid v the
Tefoci^ ecqaired, ftt tbe end of tbe time t ; and sinoe/oc • by bypotbeos,
letit=:|A*, tben ;&#&■— v.tf^v;
integrating this, and coReeting, we obtain
therefore —djt=v=i ^/it,, -/«»*— •*'
wbenoe di Ctrfaiobs-i )=-i ^L,,
integrating fbifl and eorreotiag, we obtain <aJL.eoe~^-l.
If in thii equation we pat • s o, wbioh ie tbe oaee when the pertiole reaches
ita position of rest, we obtain t =s ^\ ,
which, being independent of a, it the tame for all dittancee.
Since e*sfi (a*— «*), it ^peara that vse onlj when «as +a» or— a ; the par.
tide would therefore (reiiataaoet apart) oontinne to oscillate throogh eqoal
■paoee on either side of ite pocition of rest.
303
CHAPTER Vn.
HTDROeTATIGB ; THS PBOPEBTIES OF FLUIDS AT REST.
379. Fldhw, or liquids, are characteriied by the eztTeme mobi-
lity of their molecules, in consequence of which they are unable
to retain any distinct form like solids, always assomine that of
tbe vessels containing them. Fluids obey all the laws which have
been explained in the preceding chapters, with such modifications
as depend upon their molecular constitution. They obey most
strictly the law of gravitation (59), and are capable of assuming
motion, in the same manner as solids, in cases where the ready
mobility of their particles on each other does not interfere. A mass
of water, or other fluid, in falling from a given height, would pro-
duce effbcts as important as an equal mass of any solid, if no
opposing cause existed ; and the reason why no one would fear
tne falling of a pailful of water on his head from an elevation,
capable of giving to the pail itself a degree of momentum sufficient
to fracture his skull — is that, in falling, the water is opposed by
the air, and, from the ready manner in which its particles allow
of separation, it becomes divided into a kind of irregular shower,
producing no effects likely to be dreaded from their mechanical
violence. If the particles of water were tied together by increased
attraction of aggregation, as by freezing, then its mechanical
effects would be as serious as those of other solids.
380. Fluids have been divided into elastic and inelastic ; this
distinction has, however, no foundation in fact, for if a small
quantity of mercury or water be allowed to fall from some height
on to a hard substance, as a flat stone or sheet of glass, it will not
remain where it falls, as a lump of moist clay woold do, but the
particles rebound from the surface bv their own elasticity, and are
scattered in all directions: thus showing that the particles of
which the fluid consists possess a certain amount of elasticity.
Fluids have also been divided into compressible and incompres-
sible, but this distinction is by no means well defined, for it is
anite impossible to draw a distinct line of demarcation between
lose fluids which, as water, and alcohol, are but slightly com-
pressible, and those which, like air and all gases, are readily
compressible, and consequently, evince a large amount of elas*
ticity ; the properties of the one class are common to the other,
with but slight modifications. The physical characters of fluids
genendly will therefore first be considered, reserving for an ensuing
S(H Bn>B(wriTia.
chapter the conridsralfon of the propertiea pecnliar to (he eminentlj
elutic, or gMeoDB fluids.
SSI. Liqnidi, properlv to called, of vMch water miy be taken
M the tjpe, are Got slightl]' comprtsBibls ; thia cbaracUr, indeed,
wu Tor lome time doubted, as the celebrated eipariment, per-
formed b; the Florentine academiciaoi, of sucIorinB water in a
hollow ball of sold, and caonog the fluid to percolate
Hf, m. the porei of the metal b; the pleasure of a screw,
vai for a long time oonridered coucluBiTe on thia
Canton, the compreaeibilit; of i
preuure of our atmoepbere, equal to about fifteen
. pounds on each aquere iuch^ was estimated at
0-OOO044 of iU bulk; while Mr. PerkiriB las since
estimated the compreasion under the aame pressare
at 0000048 ; and Profesaor Oenited, b; means of an
extremely accurate set of eiperiments, has fixed on
rather more Iban 46 milliontbe. or ^^ nearlj, as
the dein«e of eompresdon experienced b7 a giren
balk or water, for each addibonal pressure of one
atmosphere. The appamtaa nsed by Profeasor
Oenited, and to which he gave the name of
pieaimeUr, consisted of a verr strong glass Tessel
A B c D, ha Tine firmly cemented on to its apper part
a abort iron cylinder e f, in which a piston o, capo-
' He of being moved bj the screw B, moTea air-tipht.
A bottle K, into the neck of which ia firml; fixed a capillary
tube I., fiimisbed with a acate grsdnated into fractions of as inch,
ii plat^ in the glaaa vessel a n c d. By a previoas experiment,
the contents of the tobe l, as compared with the bottle k, are
ascertained. This consisted in first weighing carefully the empty
bottle and tube, and Ellinf; both with mercury at a given tempe-
ralore [32° F.), and weighing again; the difference of these two
wei^hta would be that of toe contained mercniy at 82' F. A
portion of the mercnry contwned In ihe tube was now expelled by
heat, and the temperatnro again reduced to 32° F. ; the mercury
will now be found to baie fallen in the tube, leaving n scale-
divisions empty. The bottle is again weighed, and it is evident
that the difference of the two last weights will be the weight of
mercnry at 3!° F. contaiaed in n divisions of the tube. Bat the
capacities of the bottle and tube must he proporlional to the
weights of mercury at tlie tam» ttrnptrattire which tber contain ;
if, uierefore, w be the weight of mercury Sllini; the bottle and
tube, and ie the weightoftbat contained in n divisions of the tube,
the value of each auile-division in parts of the whole volnme will
be — . In aome of the tubes nsed, one inch in leogth held
CX>1IP1IB8IIBIUTT OF FLUIOe.
206
80 mOIiouths of tbe contents of the bottie. The whole apparatuB,
bottb and tube, being filled with water, or other fluid of wmch the
compressibility is to be determined, the screw h is tamed, the
piston o descends, and the pressure being communicated through
the fluid in A B c D, the contents of the bottle k are compressedi
the amount of compression being measured hj the descent of the
fluid in L. The compression of the fluid is shown by the descent
of a bubble of air in the tube, entangled in the upper part of l,
belbre placing it in the larger vessel ▲ b c d. By means of this
apparatus. Oersted determined the compressibility of the following
fiiuds for each additional pressure of an atmospnere in millionth
parts of the whole bulk to be for
Mercury, 3; Alcohol, 21 ; Water, 46; Ether, 61.
382. In these experiments of Oersted^s an important source of
error has been loet sight of, namely, the compression of the glass
vessel itself; for the above results are manifestly the sums of the
oompression of the bottle, and of its contents. To obviate this
error, the following modification of the piezometer has been de-
vised by M. Kegnadt. The stem of the bottle, instead of being
entirely endosed in the outer vessel, passes air-tight through its
cap, and a tube vnth a stop-cock, coming from a vessel oi com-
pressed air, is attached to its open end ; and thero is a second
tnbe, also furnished with a stop-cock, which connects the air-vessel
with the outer yessel of the piezometer. By these means the
pressure may be exerted either inside or outside the bottle, or on
both conjointly ; and by comparing the results thus obtained, the
error arisin^^ m>m the compression of the glass may be eliminated.
The following more correct table of the compressions of various
fluids has been deduced from the experiments of M. Grassi ; the
compression is expressed in millionth parts of the bulk, and is
that due to the pressure of one atmosphere.
FlnM.
Temp.
(Cent.)
Compr.
Flnid.
Temp.
(Cent.)
Compr.
PreMUR
ID Atmo-
■plierM.
Mercury
00
2-96
Ether .
00
Ill
3-408
Water
00
50-3
It
If
131
7-820
i«
1-6
51-6
1)
140
140
1-680
,1
4-1
49-9
fi
13-8
153
8*362
n
10-8
480
Alcohol .
7-3
82-8
2-302
n
134
47-7
}i
If
85-3
9-450
n
180
46-1
II
181
90-4
1-570
ft
260
45-6
11
II
991
8-970
If
34-5
45-3
Chlorofo
rm 8-5
62-6
II
II
430
44-2
11
12-0
64-8
1-309
>i
530
441
II
12-5
76-3
9-200
306
From thia Table it appeara that the compreuibilitj of £aidl il
not congtaDt ; it varieB conaidenibl; nith tempAratura, hut not in
the utme direction in all Quids, Tor wbila the compreBubilitj of
vateT is fonnd to diminish with increase of temperatare, it ia
aagmented by the same means in Ether, Alcohol, and Chloruform.
It DiBj also be observed in the latter flaids that, >t the same
tempeiature the amount of compi«ssion bean an mereaiing ratio
to the angmented compreBaing force.
383. Johnion'i Deep^ta Pratvrt-gavg*. — Thiainttrnnenthaa
Fit. MS. iiofio oonstructed for the pnrpose of measuring the
depth to nhich it has been immersed in the sea
by the amount of compreaaion sustained by the
' water in a cloaad cylinder, a. Fig. S63. This is
furnished at the upper part with a tap, B, having
an opening, c, for the admiselon of water, and
another, d, for the eicape of air. The piston-rod,
a, passes trough the ituffing-bax, r, and as it is
forced into the cylinder, A, by the preunre of the
water, it carries the index, i, np the scale, H, by
meaoi of a projection at its lower end ; when lh«
instrument is relieved (Wim pressure, the piston
rod recedes, and leaves (he index, i, at tlie highest
point to which it has been carried, and thus the
amount of compresidDn snataiaed, and hence the
depth of immersion, are determined. The piaton-
rod and scale are protected by an external tube,
with a door in the side of it, as ahown in the
figure. It must be borae in mind that in order to
obtain a correct reanlt the temperaturea both at
the anrface, and at the point of groateat comprea-
aion, mnat be ascertained, and the requisite correction applied.
3d4. Liqulda, on account of the eitrome mobility of their par-
ticles, are capable i^ communicating pressure eierdoud on them
equally in every direction, a properly conati-
ffr- 2M. tnting Che most important characteristic of
fj thia class of bodiua. Let abdc. Fig- 361,
be a vessel oonlajaing a liquid destitute of
weight, and therefore theoretically unacted
, opon by the attraction of the earth ; and let
■ the shaded portion p be a solid piston, also
r deatitulD of gravity, moving air-tight in xo,
and exactly cove ring the auiface of the liquid.
Now, as F IS without weight, it doea not press
upon the fluid, and the sides of the veaael
Ho may he pierced without ita escaping. But ir
we place on f a weight of 100 pounds. It will
attempt to descend, and would reach the bottom of the vessel were
it not opposed by the liquid. Accordingly, the upper lajer of
fluid X beooDM praaed bj tlie pitton, and woald fall, if not nip-
ported bj tbe aattittceDt stnilam y, which tlios in tarn bacotnat
pressed ; this mU oa the Ujer i, and this dd ihe subjacent hvjeis,
trail ami tting the pressure eieiied b; the weight irith which the
piston is loaded to the bottom of the Teasel.
Also, from the mobilitj of the particles, those of any giTsn lajeT
in contact with the sides of the Teasel would be farced out laterally,
Boless resisted bj an eqoal pressure exerted b; the sides of the
TBSsel, Ihey must, therefore, exert the same pressars against the
•idea of the Teasel.
And as the whole base, b p, supports the pressure of 100 ponuda,
it follows that one half the bue rapports bnt SO, and 001 of the
bass bat (hm poond, &c. From these considerations we ma.; safely
infer that,
A. Preaaure ia transmitted bj flnids in all directions:
B. The transmitted pressure is equal in oTer; portion of the fluid:
' c. It is proportional to the area of the surface preaeed.
These general laws joaj be proved eTperimentAlly by a closed
TESsel filled with fluid, in the upper surface of which are two
^Krtmea, to which pistona are fiitad, haTine unequal areas, at
10: 1, fbrexample. Then if one pound weight be placed on the
smaller pialon, ten pouads will be required to keep the larger one
jnitsplsoe. If the larger piston be placed in the side of the TeaaeL
Uie preasnra agaiuat it, ansiDg from the gravity of the fluid itael^
must firvt be oonntetacted, when the aame reaull will be obtained.
Also, if an Drifioe be made iu any part of ihe veaael, the fluid will
eticape in a jet, when any pressure is spplied to either piston.
385. Liqnids can never attain a perfect state of rest, and be in
G0ii](4ete equilibrium, onJesa the particles in the upper and eipoeed
layer form a surface perpeadicular to the direction of the forces
acting upon it; and eTery molecule of Ihe mau of Suid eipe-
riences eqnal and coutrarf presiures. To render the flrst condition
inUlligible, let a e h/, Yig, 265, be a Tassel containing water, or
olhorflnid; to attain aperfectequilibrinm, the
snriaee of the fluid must be level and in a "'■ ■
plane perpendicular to the llnea g, repreaenting
the dnections of the earth's attraction on the
particles bed. If, instead of ronniiig a level
aniface, the fluid be supposed to be bounded by
s cnrre aAe(fe,asiiuiIlhoTiEontal layer, aathe ,
line bd, will be pressed by the weight of tbe
molecnlea aboTe it; this preasura will be trans-
mitted lalerally (398), and the molecules of
flnid at b will be acted npon by this lateral
jn'Cssnre, and pnahed ontwards, because there ^
u nothing to oppose tbia action ; immediately
other paiticUs, acted npon in a Kmilar mauner, are pushed Oflt in
208
HTDROBTATIOS.
their tuni; and this efllBCt continues nntil all that portion of
fluid, standing above the horizontal line hd^iB depressed to one
level surface, and then the our?e bed yanishes, and a horizontal
surface, extending from atoe, perpendicular to the lines of pres-
sure ^, is prodo(^. The fluid will then he in equilibrium, pro-
vided the second condition obtains, that every molecule in the
interior of the mass of fluid experiences eqoal and contrary pres-
sures. That this is the case is evident, for everjr particle of fluid
receiving the pressure of those above it tends, m consequence of
the equality of pressure (374), to transmit the same pressure
lateraUv ; and if the pressure on two sides of a particle be unequal,
it will be acted upon by the stronger force, and continue to move
until it has attained a situation where all the pressures acting
upon it are eoual. The only exception to the law of the level
surface of fluias at rest arises from the capillary attraction, or
repulsion, exerted by the sides of the containing vessel (88).
It may readily be shown bj precisely similar reasoning, that the
common surface of two Uquids of unequal densities is horizontd,
when the liquids are at rest.
386. The construction of the Spirit-level depends on the fact
of liquids assuming a horizontal surface. This instrument con-
sists of an hermetically-sealed ghiss tube, nearly filled witb alcohol,
and enclosed in a case, leaving one side of tne tube exposed to
view. The tube beine very slightly convex in the middle, the in-
strument is so adjusted, that when it rests on a perfectly horizontal
surface, the bubble of air shall occupy the middle point of the
tube. Any inclination in a surface on which a spirit-level is placed,
is indicated by a departure of the bubble from the middle point.
887. When two or more vessels, of any gnven dimensions, com-
municate togetiier, the same conditions m equilibrium exist, as
when a fluid is contained in a single
FSg,2M, vessel. Let a, b, Fig. 266, be two
diflerently si^ vessels connected by
^ the tube c; on pouring water into
one of them up to the line 1 2, it will
"^ be found to present a level surface
in both ; ana the fluid in each will
be at the same elevation ; for if the
j-i-p ^**e^ ^^ ^t instead of being at i,
p "tm — :ii±~|""r yf^YQ at m wf , it is obvious that the
layer of fluid pp would be submitted
to unequal pressure, being in b
pressed by the long column Jo, and in a pressed only by the shorter
column mjp, and consequently equilibrium could not exist (375).
Therefore the narticles of fluid acted upon by the greater pressure
will move, ana attain a state of rest only when the level of the
fluid is the same in both vessels. If the diameter of the vessel n,
be small enough to mantfest a sensible amount of capillaiy
n-
^
Thia law i* equallT -nlid when the connected tmuU ptesent the
ereatot Taiiet; id Boape or size. If the tabes i, d, c, ii, R, F, be
filed ioto a common reBcrroir, L u. Fig. 267, and water be poared
iotoD, itwillattuneiActlTthe
aame elaTatioD in each of the fif. H7.
tobea, notwithstanding the dif- t-nr s s r
feieiKM !n the flgote and nze.
The only ciicmrrtance intro-
dncJDg the slightoat exception
to this law is capillarit; (38),
bywhich, if any of the tnbea or .
TeaaeU in the abore GgnrM be
^vrj narrow, the water, or other
fimd, will have a tcndent^ to riM to a higher elevation than in
the wider ones ; and the eloTatioD above the ctnumon level will
be exactJT what ii doe to capillaritr.
3S8. The hydroitatie Itstl acta on this principle ; it condete of
two piecea of the tame kIsm tnbe conDecled bj a fleiiole tube,
and when nearij filled with water, may be nted to ascertain two
pointi in the ume horizontal plane, in litnationB not risible from
oach other, aa in two diSerent parte of a mine. The two poinla
at which the water reiti in the glaaa tubes will evidently be
hotixontal, whatarer conna the flexible tnbe may take between
If the gUa tnbes be oonnected by a rigid metallic tube,
mounted at ita middle point on a tripod stand, the apparatna ii
S89. The Mtne priadple oF "wat<
nplain the oEt-obeerved phenomem
which is merely a small hole bored, fre-
miter, ti of claj or hnrd Blone, and c c, a permeable bed w of und
or graiel between them. If now the porous stratum be tapped
in a valley, as in the 6gpn, frota which the ground mes conri-
derabl^, waler will continue to flow so loDg aa the source of
water is higher than the mouth of the well.
890. The above law (387) appliea onl; when tbe connaunicfttine
^ ^^ veaiiela are filled wilh the »am« fluid; for if fluids of
' difierenl densities incapable of mixing, as water and
i mercury, be used, the elevations acquired by each
will be found to be in the inverse ratio of tbeir spe-
cilic gravities (112). Let mercury be poarad into
ihe tube acb until the bend □ is fillod, then pour
water into n, and it will be found that to raise the
mercury in a to the height of one inch, a column of
water, rather more than 13} inches bigh, will be re-
qalredin b: in coasequence of the relative gravity of
mercury, aa compared with water, being as 13'6 : 1.
391. A beautifiil example ofthe truth of thin law of equilibrium
of fiaids is presented in the figure of tho surface of oceans and seas
in a calm state, by which the cause of their superficial curvature
becomes immeiiiatcly apparent. It follows that, in common with
everything belon^ng to our globe, the seas obey tbe farce of
gravitation ; and ere also nibservient te centrifcgal force (330),
the oceans end seas therefore necosaarilj assume the apheroidul
furm, in common with the solid elements of the earth's crust, but
not subjected to the superficial inequalities of the latter. On this
account, where a standard place of obsen^atiou is required for very
accurate barometric, or other meleorolc^cal observadons, so as to
enable obscrvrrs in different paita of tbe world to compare the
results of their observations, the level of the sea, or a given dis-
tance above it, is always chosen. Among minor causes affixing
the regular curved surface of the great mass of waters on our
S;lobc, may be mentioned those which oriso from certain physical
eatures of the earth itself; tbe mountainous elevations on its
surface attracting, by lateral gravitation (60), the water of sees
and oceans towards them. If the mountains of tbe CordJIIenu
were about 100 times higher than they ore, the seas would, by
tbeir attraction, be elevated into liquid mountains on both aides
of the coasts of America, and the ports of France aud Japan be
left dry. The peculiar directions of winds and currents are soureea
of disturbnncc to an imporlant extent, caueini; elevations ia parti-
culnr and isolated masses of water : thu^i the level of the Bed 8ea
at hi^h water is more than thirtv-two feet higher than that of the
Mediterranean. The level of the Pacific at Calloo is mare ele-
vated than the accan at Cartbogena by twenty-three feet; whilst
the ocean at Dunkirk, and the Mediterranean at Barceloua, are at
the same eleratiou,*
• PoaOlel, Fbjiique, p. lit.
PKB8S0RE OV THE BABE OF A VESSEL.
211
-W
J^'
Q
392. The pressiire of a flaid on the hottom of the containing
▼eaael, is altogether independent of its shape, and is equal to t?ie
weuffU of a cdumn of fluids of which the base is the same as that
of the contauMig vessel, and the height equal to that of the oon-
iainedfluid.
The best mode of proving this ^' 270. *
statement is bj means of the ap-
paratus contrived by M. Haldat,
Fig. 270, consisting of a bent
tube, ABC, having at a a collar
cemented on, into which vessels
of different shapes, d, b, f, ma^
be screwed. Toe tnbe a b c is
filled with mercury up to the
level of the dotted line a c, and
the tnbe ap fixed into c. The
cylindrical vessel d is then
screwed into A, and water poured
in as far as h \ the base of the
oolomn of water vnll of course be
equal in area to that of the sur&ce of the mercury in the tube a.
The mercury will then rise to a certain height in a, as p ; in
consequence of the pressure of the water in d on the surface of
the mercury in a. Then unscrew d, and fix on a the conical
vessel, E, and pour in water until it has attained the same vertical
height, as in D ; on examining the mercury in a, it will be found
at the same point p as when the cylinder d was fixed on a. Be-
move E and replace it by f, and on pouring in water to the same
height, the mercuij in b will attain the same elevation as before :
proving satisfactorily, that the pressure exerted by masses of fluid
on a given base is quite independent of their quantity ; for the
pressure was the same when either of the differently sized vessels,
D, K, F, were used, each containing very different quantities of
water ; in each, however, the actual base formed by the surface
of the mercury, and the height of the column of water were the
same, and the pressure, as above stated, varies solely with the
vertical height, and area of the base, of the column of fluid. In
the case of the funnel-Bhaped vessel, e, the inclined sides support
part of the weight of the fluid: in that of f, the downward pres-
sure on the base is counteracted by an upward pressure against
the portions of internal surface, that incline inwards.
It is easy to calculate the amount of fluid pressure on the bases
of containmg vessels, by taking b for the area of the base of the
column, II for its height, and d for the density of the fluid. The
pressare upon the base b will be equal to b x h x d, for b x h will
be equad to the volume of the fluid ; and to have the weight, this
product must be multiplied by the density, d.
393. From this law (392), we are enabled, with a given bulk of
p 2
flniti b . ...
> icbmJ. For, witb » qDantitj of fiaid f^
certain Bmonnlof preware c«n be exerted on a Riren are*, when
the vertical beigbt of (he fluid nh; ten timei thai preamra can
be produced b; DarrowinK the capacit; of the TSsael, >o that tb«
vertical 'height of the nuid maj be 10&. and conTeraelr ibe
pressure may be leBSened to j^i bj eo inclining Ihe sides of the
Teasel, tb«l the Yeriical height of the flnid maj be only ^ Bj
BTaiiing oonelves of tbia law, a cask may be readilj' h
irith , ., ^
into the bunghole. On ponring wMer into the tnbe, preitsnre n
exerted, equal to the internal area of the vessel, multiplied by tlie
height of tne column of water in the pipe, aud an amount of force
sufficient to burst the cask with Tioleuce will be genersltd. The
well-knowQ philosophic toy, called the hydrostatic bellows, or
hydroetntio paradni (nbicb is no paradox], illustrates the same
fact. This consists of two boards, connected loosely by strong
leather; into the upper board is fixed a long tube, on pouring
water into which, tbe bnarde are forced asunder, eien when
previously pressed together by a conndeTable weight. In tbia
manner, when tbe space between the boards is nearly filled vrith
water, and a man stands on Ihe upper board, an ounce of water
poured inlu tbe pipe will exert sutncient force to elevate him,
notwithstanding (he weight which the fluid prenare ia required
.. a constant rah'o between tlie extent oT
BDrficei pressed upon by a continuoos mass of fluid, and tbe amount
of the presBurea they sustain, explains the enormous pressure that
Bramab's Hydraulic press is capable of exerting. This machine
consists of two strong hollow cylinders a a, c d a, Tig. !T1, com-
municating with each other by means
of a pipe B D ; h, q, are two solid cy-
" 'en, workine iu water-tight collars
. and c. The cylinder v, the dik-
er of which is large compared with
that of 4, supports a platform f, on
which the substance to be pressed ia
placed, q is capable of being moved
up and down by means of a lever m t^
having its fulcrum at H. D is a valve
opening upwards, and n a valve openiur
into the space a a ; e is a cistern BIIm
with water ; and i is a cross piece fimlr
secured to theuprigbtso.n. Toeiplain
the action of this machine, suppose 9
to bo in its lowest positim, and tite
space between the solid and ludlo*
niTEBIUL FLUID PRiaSUBE. 218
cylinders to be filled with water ; then, on elevating q, the pressure
of the ataiospheie acting on the surface of the water in e, forces it
through D into the space previously occupied by q ; on depressing
q, the valve d closes, and a portion of tne water in o d is forced
through the valve n, which prevents the return of water into c d,
and causes m to ascend ; and these actions are repeated with each
soccessive stroke of the piston q, until the substance between i and
F is sufficiently compreraed.
The pressure may at any time be relaxed by unscrewing a ^lug
at IK, which allows the water to escape from the cylinder ▲ b into
the chamber b.
Suppose the diameter of the small cylinder, or piston, to be half
an incn, and that of the larger cylinder five inches, then the ratio
of their areas would be as 1 : 100 ; and suppose the piston to be
worked by a lever (110) by which an advantage of 5 : 1 is gained,
then for every pressure of one pound on the lever, a pressure of
500 pounds would be exerted by the press.
395. Two ingenious applications of the principle of the
hydraulic press to special purposes have been made by Messrs.
Tangye, ot Birmingham. One of these is the hydravUc Ufting
jaeky A, Fig. 272, in which the pump
18 enclosed in the head, a the solid ^' *"'
piston being continuous with the stem
which rests on the foot. The required
resistance may be overcome by placing
either the upper surface of the head,
Aj or the toe, b, under it, as may be
moet convenient.
The second is the hydraulie punch-
ing machine, d f, Fig. 272. In this
the cylinder and pump are enclosed
in the head of the machine, d, and
worked by the handle, b. The punch,
c, is continuous with the solid piston, and after having been pro-
E oiled through the plateplaced beneath it, is withdrawn from the
oie by the lever, F. The lower part of the machine is a solid
iron casting, to sustain the pressure of the punch. Boiler-plates of
any ordinary thickness may be readily punched by this machine.
396. Since the pressure against the internal surface of a vessel
depends on the extent of the surface, and not on the capacity of the
vessel, it follows that, as the sphere has the smallest surface compared
with its capacity, a spherical vessel will be the strongest for resist-
ing internal pressure. Consequently vessels made for the purpose of
withstanding great pressures are usually either spheres, or cylinders
with hemispherical ends, unless internal stays are employed.
397. In accordance with the general law of fluids exerting
pressure equally in all directions, it follows that each layer of
fluid presses as powerfully upon the superposed stratum, as it does
SI4
tff. 173. upon the snbj&oent ooe. Thni it is srideiit
thnt kll the particleB compoainf; any par-
dcokr Btratnm of fluiti, ■* n^. Fig. 378,
mnst be presHod npon hj all above then), in
tbe Bamo mimner as if they supported a
solid piston equal to tbo Suid mass n vp m.
If then we regard a portion onlj of the
layer mp, as a 6, we can readilj onderetand
that tills is at once pressed jrom above
dowDwards by the colnmn dabc, and from
below opwoTda bj an exactly equal force,
in such a manner that, if a solid cylinder
were immened in the fluid with it< base
resting on at, the upward pressure would tend to raise .it out of
the fluid. These theoretical considerations ma; be readily reri-
fied by means of an apparatus consisting of a
lit. ^*- stout ^losa tube g, ¥ie. S74, the bottom of
which is ground perfectlv flat, having a plate
of brass, B, resting against its baset and re-
^ toined in situ by tbe striuK v. On immersing
the whole in a Tessel filled with water to no,
the plate will be pressed against the mouth d
the tube b; the upward pressure of the fluid.
L If water be then poured into jr until it nearly
reaches the external level nn, tbe plate will
obey tbe attraction of gTavitatioD, and wiU fall
to the bottom of the vessel, as tbe iipuord
pressure of the water below tbe plate b, be-
comes neutralized by the doumaard pressure
of the water in the tel>e g.
On account of this upward pressure of fluids,
if a hole be made in the bottom of a ship, the
^ water mshes in; to efibotnally oppose which,
B force must be applied, equal to tbe weigfat of a column of wsler,
of which the base is of tbe same area as that
of the aperture in tbe vessel, and the length
eqoat to the depth of the bole from the snrlace
oftbewater. Hence in vessels of largedranght,
the under surfaces should possess considerable
strength lo enable them to oppose the upward
treisure exerted by the water in which they
, 398. As a consequence of tbe law of equal
presBcre, evei^ portion of the sides of a con-
taining vessel is exposed to pressure, corrS'
■ponding to tbe weight of the fluid pressing
•gainst it. In tbe vessel of water a en.
Tig. !T5, if a particle of fluid situated at b be
prened %7 th« ccdnmn of iral«r xb, it will, for immdi atraadj
stated, be at Ibe tame time pivnail upwards (397) bj an eqnal
force, and thia pnssure will U canimDDicated laterally to the
t£
of the same intensitj as that whicli acts
contained in the correnponding horiunital
399. The lateral presaore is pmportioiuil
to the depth of the fluid ; for in the lewiel
B B, K^. 276, the fluid column A c transmits
its pressure through the horizODtal lajer
c D to D ; and the column n r jHesaing open
the Isjer ra, has ita pretanre transmitted
by F o to a ; then the pieMnre at a must be
greater than that at a, in the same pro-
portion as s P is longer than a o : and there-
fore generally the pressure of a fiuid upon a
eiTen small portion, or element, of surface,
IB proportional to ita depth below the sor- i
fiwe ta th^nid.
400. When the presanre upon the baas of a cubical
water is knows, the lateral preMore can ^^ ^tt.
be readily caicalaled, for (be pressure
upon any one side of a cubical veBsel
GUed with fluid, U one half of (he
preasnre on the base. Let a e be the
cubical vesael, bisected by the oblique
plane cdoh, and let k, k, be corre-
sponding elements of the side and sur-
face ; then the pressure on it will be the
weight of the vertical prism K if, and
the same being troe for every other
element, the whole pressure on the side
DE will be the weight of the priam
A B c i> a H, which is half the cul/e ; and each Bide
'pressure. But the base angtai
a the
,«al
to the whole weight of the flnid, since the sides of the rcBsel ai
vertical, bence the total pressure against the surface of a full cubi-
cal vessel is three limes the weigCl of the cnnlnined fluid,
401. CoUrt of Freuiere.— If a gir^n surface be oi posed to the
presaare of a flaid, that point of the surface, about which the
preaaores upon ita seieial partg are so balanced on all sides, that
tbey may be aostained by a single presanre in the opposite direc-
tion, is called the centre of pieiaure. The pressure of a fluid
against any point of a surface must always act in the direction of
a normal to that point, since otherwise motion of ibe particles of
flnid mnat ensue, which is contrary to the hypothesis of the fluid
!16 aCDtOMATtC*.
being at r««t: but it will bs Bufficiaot to contider plaoe idi&om
only, aguDst which the directioai of preuura are ■& panltel.
— — LetiB, Fig.a78,be»pl«ne"nri«>«
immersed id a Said, and let d ■ be the
inlenection of the sariace of the fioid
viththe plane a a produced to meet
that surface. Take any point m io x ft,
draw mc perpendliiilar to na, «ax
Terlical, and ax horiioQlal; and let 9
U tbe angle tn c;i:. Tbrn the preaanra
in B tmall element of the aurfaca, tm,
.a the weigbt of a colnmn of fluid of
which die base IB n and the beightmai;
bat m K = c m . sin 6, and if u be ths
weight of an unit of volnme of the fluid, then a.n.m^ or
u . m . c n . sin 9 will be the presanre on m. The moment of this
preHura round the aiiii d ■ (77) is
cm.uj.n.am.iinO, or u.sin S.m.cm*;
and conseqnentlj the inm of all similar moments is
10 . (dn e . 2 (m . c m«).
But this ezpTesaion is precisely similar to that fnan which the
centre of oscillation (347) orpercuaaioa (3&I) haa been determined ;
and br pursuing tbe same steps, a trimiW resollwill be obtained;
(he centre of pressnre of the surface ii is therefore found to co-
incide with the centre of oscillation of a plate of the same form,
and of uniform thickness, round the axis d a.
If o be the centre of presanre, and o, the oenire of gravity, of
the surfsce a a, it has been ahown (S49) that tbe product
L'oxao ia a conatsnt quantity; when therefore a a ia Tery amall,
CO must be Tery large, and oo Tanishes, when co is infinite,
which is the case wlien ab is horiaontal: bence it appeara ihat
the centre of pressure of a horizontal plane surface immeised in a
tiniJ at any depth coincidea with tbe centre of mvity; and that
if tbe iiDmersed plane be oblique, or vertical, the centre of pres-
sure approaches the centre of ^vity aa the depth increaaes.
If the surface be a tcuOuigle, one side of which coincidea with
Ft« «7B Bo tm '*'* surface of tbe fluid, aa ac.
Fig. 279, the centre of preaanre
~ D is found by binectiag ab, ci>,
in a, F, joining ar, and taking
K o = I K p, meoanring from ■.
If the stirrsce be an isoaeelcB
■ triangle abc, Fig. 280, of which
I the apex a coincides with tho
I surface of the fluid, and the b«ae
ac ia horiaoolal, biaect bo iD D,
D and take aooJ a d, then o ia the centre of preoanre.
BQUIUBSIUM OF FLOATIKO BODIES. 217
If the base bg coincide with the surface of the fluid, then
DOs^DA.
^ The position of the centre of pressure in these and other par-
ticular cases, such as those mentioned in reference to the centre
of oscillation (347), may be determined experimentally by means
of a Tessel containing water, a yalve in one side of which consists
of a rigid plane of any proposed form, connected with the aperture
in the vessel by some neziDle water-tight material, as India>rubber
cloth. The point at which alone the pressure of the water on the
Slane can be counterbalanced by a single pressure can then be
etermined by trial.
402. The resultant of the pressure of a fluid on the several
points of the surface of a solid, either wholly or partly immersed
in it, may be determined by means of the following perfectly legi-
timate hypothesis; that any portion of a fluid at rest may be
supposed to become solid, without havine its equilibrium disturbed.
Suppose then any portion v of a fluid at rest to become solid,
therefore since its weight, and the pressure of the surrounding
fluid, are the only forces acting on t, the resultant of the pressure
of the fluid on tne surface of y is equal to the weight of y, and
must neoessavly act upwards in a vertical through the centre of
gravity of y (90). But Ihe fluid will exert the same pressure on
the surface of any other solid, that occupies the same space which
y occupied in the fluid : hence —
The ruuUarU ofapresture ofafiuid on the surface of a solid
immersed in it is equal to the weight of the fluid displaced, and
acts upwards in a vertical line through the centre ^gravity of
the fluid displaced.
403. Instead of sup]K>8ing v to become solid (402) we might
have supposed the fluid surrounding y to become solid, without
altering tne pressure at any point in y. In this case, the pressure
at any point m the surface of y will be equal and opposite to the
pressure at the same point in the former case ; consequently, the
lesnltant of the pressure in the latter case will be equal and
opposite to the resultant of the pressure in the former : hence —
The resultant of the pressure of a fluid <igainst the inside of a
vessel containing it is equal to the weight of the fluid, and acts
downwards in a vertical through its centre ofgravitu.
The resuUant here mentioned, must be carefully distinguished
from the total ^essure, because, in relation to the resultant, prea-
sures in opposite directions, as those upon the upper and lower
boards of the hydrostatic bellows (393), will tend to neutralize
each other.
ZQUILIBBnm OF FLOATIHO BODIES.
404. When a solid floats in equilibrium, the weight of the solid
is equal to the weight of the fluid displaced, and the line joining
the centres of gravity of the solid and of the fluid disptaced xs
vertieal. For lb« ireig^lit of tbe solid, and the prcMnre of the
Huid on the surfooa of tbe portion of the aolidioiroeraed, >n the
oolf forces that net on the solid ; therefore, since the nlid is at
~ ' '' wight, and tbe reenltant of lbs preunra of tbe flaid or
vertical Ibroagh its centre of gravity; and the resultant of the
pressure of the fluid is eq'ial to the weight of the fluid displao^,
and acts upwards in a vertical threogli the centre of graritj' of
the Snid dinplBced : therefore, tehoi a $olid, &c.
4US. If tbe eqoilibrinm of a floating solid be ib'gbtl; diiturbed
b; making; it reiolve tbron^b a very nnall an^lein a vertical plane,
without altering the mianlitj of the fluid diapUccd, the rennltant
of the preBBure of tbe fluid on the solid in ils new position will stitl
be eqnal to tbe wel);bt of tbe solid, and will therefore have no
tendency to elevale or depress tbe centre of gravity of the solid :
bot since the resultant acts in a vertical through the centre of
gravity of the flnid displaced, it will tend to male the solid rotato
round a horisontsl axis through its centre of gravity, unless theas
two centres of gravity happen to be in tbe same vertical line.
Whenever the pressure of Uic fluid acting upwardk Ibrongh the
centre of gravity of tbe Qnid diaplsced tends to inertaie the anc'Ie
through which the solid has moved, the eqnilibrinm of the solid
will be tmtiabU ; whenever, on tbe contrary, the pressure of the
fluid tends to iliminiik that angle, (be equilibrium will be itaUe.
406. The ifrtowBfif.— Let a n 6, Fig. S81, be a floating solid
capable of being divided into two symmelHcal halves by a plane
jLob, which coincides with the plan«
Tff' !SI. of tbe psper; and let Q be its centre
of gravity, snd a that of tbe fluid dia-
plsced by A D B, the immersed portion
of the solid, when floating in equili-
brium in an upright porition. The
plane a a, which cmncides with the
surface of the Anid, is called (he plane
ofjhntatiim. Since the solid is sym-
metricat with regard lo ihe plana
A D &, tbe line a h, joining the points
□ and u, must be in that plnne. Sup-
pose now that tbe solid be made to
revolve through a small angle, S, in tbe plane adi, so that the
quantity of fluid displaced may be the same as before; let ot br"
be the new plane of floatation, a b and a b intersecting each other
Id c. Draw h r, a vertical through r, the centre of gravity of the
fluid displaced by the solid in its new position, and p r, nq, verti-
cals ihroogh p, q, tbe centres of gravity of tbe wedges, i r y" a,
BYj'b; also throogb H draw ur R parallel toab, meetiDg ce, k
vertical throqgh c,1d b. The point m is called the (wfoceHfre.
THB METACCHTBI. 219
I
If a body be divided into any number of parta, the moment of
Uie whole body with respect to a given plane is eqoal to the snm
of the momenta of each part with respect to the same plane (86).
Hence, since the density of the body is nniform, and therefore the
mass is proportional to the volume (275),
(volume of ai>&) EF+(wedge Aca)cr =3 moment of ao&d
= (volume of A D b) E h— (wedge Bob) on;
the negative sign being taken, because en is in the contrary di<
lection to c r, and b h.
But vol. of ▲ D B aa Tol. of 0 D 5 =: v, BUpposo ; then,
(wedge Aca)cr+(wedge Bcb)cn= v.eh— v.bf
=s V.HF = V.Hlf .0. . [a].
Now if m be a small part or element of the surface t a y\ and c m
its distance from the line y tK, then the thickness of the wedge at
the point m is c m . 0, and consequently the volume of the wedge
▲ cais
S (m . c m . 9) = 9 . 2 (f» . cm) = 0 . t a y' . c r,
similarly, vol. Qf wedge Bcb^d.Yhif .on.
But since vol. of a d b = voL of aob, subtracting from these
the common portion, a db,
vol. of wedge a c a = vol. of wedge b c d,
therefore YaY'.cr= t6y'.c»»,
and consequently c is the centre of gravity of the plane a y 6 t'.
But since vol. of wedge Aca = O.y ai* .cr^ multiplying each
aide by c r, we obtain,
(wedge Aoa)cr = 9 .Yar* .cr^;
similarly, (wedge Bo6)cn = ©.y6Y'.cn*;
.-.(wedge A c a) c r + (wedge B c 6) c » = 0 (y a y'. c r* + y J y'.c »*) [6].
Now (wedge a c a) c r is thcr statical moment of the wedge
round y y', and is the sum of the moments of all its elements ; but
the element of the wedge has already been shown to be 0 . i» . c m,
and the moment of this element round y y' is therefore
0 .m.c to',
and the sum of these moments is
0.^{tn.om*);
but this is the expression already found (346) for the moment of
inertia of the plane y a y' round y y' ; therefore y a y' . c r* is the
moment of inertia of the plane y a y' round y y' ; hence,
YaY'.cr* + Y6Y'.on*
is the moment of inertia of the whole plane round y y', and
YaY'.cr'+TjY.cn' "= A.i*,
wlure A 18 the tnt, of the plue of fliutation, tnd k ita r*din* of
gyration (349) round T v'; therefon, bj equating \a] and \b\ and
•nlMtitnting a., if for itd Talue in [&], we obtain
l.i*.T.Hll.
If Abe the beightof a c7liDdrTcalTeHel,oF which the base ia i,
jnit capable of oootsining the.fiuid diiplaced, then t — a. A, and
A.P.A.A.HH.ori'-A.HU;
that Is : — the raditu of gyra^on of tht plane offioata^iXK about
itf (mt o^ofeillaJton, u ameaufroportwnai b^iceenlhe htight qf
a e^liniineal ti«M«I eap^de of oontaiitmg lAe ^luiij ditplaced, of
tehvA A» bate U thtplaM offioaUaion, and IM dUttmet betiMM
tA« MKfn oi^ gravity of the fluid diiplaeed and the raetaetutn,
Thia eipreeiion hears a remarkable analog; to that relating to tbo
centre of oscillation in 341, namelj, t.*=:ca.ao.
407. A pressure acting in the direction r m, Fig. 282, will tend
Pig, 181. to diminish or increue the angle
I OH F, according as u i* abore or
below □ ; therelore the eqailihrinin
of the floating solid vill he elable, aa
A.ormutable.BSB, according as Mia
above or belov o ; and the amount
of stability will also eridentlj de-
pend on the depth of a below ii.
The two pressures at h and a being
equal, and acting in parallel and
opposite direct ion a, constitute a couple (SI).
Therefore the stability of a sailing veuel, or its power of re-
sisting the lateral pressure of the wiod on the Hits, depends on
the depth of the centre of frraiity of the veinel below the met&-
centre ; henco we perceive the necessity of accnmulating weight
in the lowest part of the veasel, iu order to depress the cenbe of
gravity : this is accomplished by means of ballast, when the
veasel is not otherwise loaded.
Rf . K3- '^^ conditioni of »qailibrinm of a floating
light wood — for example, American pine — in
the shape of a tcBnsverse section at a ship,
09 AB, Fig. 283, with an upright stem c, in
the centre of the deck.Burface, a, on which
slides a weight n, furnished with a clamping-
screw. The keel, b, should be lo loodi^,
that when the weight is at the bottom of
the Btem, c, tho block when floating will
right itself after being displaced laterally,
bat that when the wei^t is at the top of the
~'~~1, it will npset. The changes in the
QUANTITT OF FLUID DI8PLACr.D. 221
stAte of eqmlibriam, and the passage from stability to instabilitj
may be examined hj gradoallj shifting the position of the sliding
weight.
408. There is no Icind of Tessel afloat in which stability of
equilibrium is of so much importance as in the life-boat, Fig. 284,
in which stability is so fully realized, that if it be placed keel
upwards in the water, it will, like the child's tumbling toy, in-
stanUy right itself. This is effected by raising the bow and stem
of the boat considerably, and enclosing at each end a water-tight
air-chamber, and at the same time loading the keel heavily : by
this arrangement the interval between m and o, (Fig. 282, ▲}, and
consequently the stability, is rendered as great as possible.
409. When a solid is immersed in a finid, it displaces a quantity
of the latter equal to its own bulk, a legitimate consequence of
the impenetrability of matter (2). If this quantity of fluid be
lighter than the solid, the latter will sink, but if heavier, it will
swim : this has been already alluded to (61) as the result of
gravitation. But if the flaid displaced be the same weight as the
immersed solid, the latter yrill neither rise nor sink in the fluid, in
whatever position it be placed ; a circumstance arising from the
force of graritation acting equally upon the solid and the fluid
displaced, the quantities of the matter in each being equal. Fishes
appear to be in this state of equilibrium when immersed in their
own element ; and for the purpose of enabling them to preserve
this state at different depths they are prorided with an air-
bladder, by compressing or expanding which, they are enabled to
cause iheir bodies to acquire the same density as that of the water
in which they live. At a very great depth, the air in this air-
bladder becomes considerably condensed, and on suddenly rising
Id the surface it expands ; and it occasionally happens that this
takes place with sucn force, that the muscular efforts of the animal
are unaUe to control it, and the or^an is ruptured, causing au
extravasation of air into the surrounding tissues.
The well-known hydrostatic toy in which a hollow glass figure,
partly filled with water, floats or sinks in a vessel of water, by
preasing a piece of caoutchouc with which the vessel is covered,
u a popular illustration of these facts. Let ▲ b, Fig. 285, be a
glan vessel filled with, water up to cd, havinjs; a little figure of
uin g^ass, as a balloon, b, placed in it, in which a small opening
•xista at the lower part of b, so as to allow water to enter or escape
UXDB0STAT1C8.
1 it; preriouBlj allowiDg enough water to
ir the baUoon lo raadsr it tienlj of the lame
aierago density u the water in ad. Over the
moDlb 1 is tied a piece of ibeet cooutehouc. If e
0 floate to □ D, and the caver a be preased inwards
1 into the jar, the air above c d wilf be compregeed.
I the pressure wilt be convejed throngli the water
to the air contained in i; this will coosequentlv
be compressed into a smaller bulk, nnd enough
water will enter e to render it heavier than the
water, and it falls to b. On removing the band
and tailing off the pressure, the air in B expand*,
expels the water which had previously entered it.
Fig. iga.
is fundamental principle of Hydrostatics was linrt ob-
served b^ Archimedes, who, as bisteiy informs us, was accidentally
led to the conclusion that a bodj, when immersed in a fluid,
loses a portion of its vroight equal to that of the displaced flgid.
The truth of this may be shown experimental I r by suspending
from one of the aroiB of ■ balance a hollow cylinder, o, having
a cylindrical mass of any substance, i, capable of exactly fitting
into it, banging from it by means of a thread. Place weights in
the scale pan b until the solid cylinder a and the hollow one d are
exactly coonterbslanced ; then pour water into the vessel c until
A is completely immeTsed,
and immediately the pan b
will preponderate, the solid
cylinder appearing to have
lost a considerable por-
tion of its weight; then
poor water into the vessel d
until it is quite fiUI, and as
I soon as this is done, the
} batani« will once more be
in equilibtio. Now, as the
cylinder D is of BUcb a
size that tbe solid mass a
vriU eucth' at into its in-
terior, it iollows tiiot the
^ water with which D is tilled
is precisely equal in bulk to
the solid A ; proving most Batisfactorily that the appanint losn of
weight saffcred by *, on being immersed in water, is precisely
equal to tho weigbt of a mass of the fluid equal in balk to itaelf.
The apparent loss of weight io the maas A, observed on immersing
it in water, arises from the upward pressure (397) of the flnid
partly supporting the immersed solid, and opposing, to a certain
eitenl, the attraction of gmvitatiou.
PHIICIPLB OF ASCHIMEDBS. 223
411. It may be shown as the ooBvene of this experiment, that
the fluid appears to g<iin as much weight as the immersed solid
^pears to lose. This may be shown by attaching the bucket, D,
^g. 286, to one scale of a balance, and counterpoising a vessel of
water, placed in the other scale. Let now the solid a be im-
mersed in the fluid, and so supported as not to rest against the
vessel, when it will be found tnat the vessel will preponderate ;
but on filling the bucket with water, the equilibrium of tne balance
will be restored.
412. The principle of Archimedes (410) affords a ready mode
of determining the relative density or specific gravity (11) of any
substance ; for when a body is immersed m water and weighed,
it suffers, as above stated, an apparent loss of weight equal to that
of its own bulk of water ; then, by knowing this weight, as well
as the absolute weight of the body when weighed in air only, we
have all the elements for calculating the density of any substance :
for the density of any substance is the quantity of matter that is
contained in a unit of volume. Distilled water is generally taken
as a standard to which the specific weights of all bodies not
gaseous are referred, and its specific gravity is assumed as 1, or
unity; thus, if a boay is said to be of specific gravity 1'156, all
that is meant is, that a quantity of water, weighing 1000 grains,
is exactly eqnal in bulk to a mass of the substance weighing 1156
grains. A cubic inch of water, at the temperature of 62** F., weiehs
252*458 grains : hence to obtain the weight of a cubic inch or root
of any substance, it is only necessary to multiply its specific
gravity by the weight of an equal bulk of water.
413. The best mode of ascertaining the specific gravity of a
solid heavier than water, is to suspend it by a hair, or piece of
fine platinum wire, from a hook fixed in the bottom of one of the
pans of a balance, and by placing weights in the opposite scale,
to ascertain its exact weight, then immersing the solid complete^
in water it will appear to lose weight (410), and the exact weight
lost by the body when thus immersed must be carefiilly ascer-
tained. 8abtract the weight of the substance in water from its
weight in air, and divide the latter by the difference, the quotient
will be the specific gravity required. The rationale of this pro-
cess is sufficiently obvious : the exact weight of the body is first
learnt by weighing it in air ; by ascertaining its weight when
immersed in water, and subtracting this from its weight in air,
we learn the weight of a mass of water equal in bulk to the body
under examination, and by dividing the actual weight of the body
by that of an equal bulk of water, we ascertain the relation they
bear to ea<^ other.
Ex. A piece of copper weighed in air 2047 grains, and in water
1817 grains; then 2047—1817 = 230, and 2047-^230 = 8-9, hence
water being I'O, the copper was 8*9 times heavier than an equal
bulk of water.
224 HTOBOeTATICB.
414. If the substance be lighter than water, tie it to a piece of
any heavy solid, of vhich the weight both in air and in water is
known, sufficiently large to sink it in water. Weigh the com>
pound both in air and water, and ascertain the loss of weight;
then, knowing the weight lost by weighing the heavy body by
itself in water, ascertain the difference of these losses, and by thii
number divide the weight of the light body, the result will be its
specific gravity. The rationale of this process is very plain, for
the last loss s the weight of a quantity of water, equal in bulk to
the heavy and light bodies together; and Uie first loss r= the
weight of water, equal in bulk to the heavy body, and conse-
quently their difference is equal to the weight of a mass of water
of the same bulk as the light body.
Ex. A substance weighed in air 600 grains, tied to a piece of
copper, it weighed in air 2647 grains, and in water 2020 g^ns,
suffering^ a loss of weight of 834 grains. The copper itself losing
230 grains when weighed in water, the body must have lost
834— 230 =» 604 grains ; then 600-^604B '993, the specific gravity
of the substance.
In taking the specific gravities of small solids, a frequent source
of fallacy exists in the aidhesion of minute bubbles of air to the
surface ; these sometimes adhere with so much tenacity, that no
agitation of the solid in the water will dislodge them. When
very great accuracjr is required, it is desirable to boil the solid in
the vessel in which it is to be subsequently weighed, and to weigh
it without removal from the vessel.
415. If the solid be soluble in water, it must be weighed whilst
immersed in some fluid incapable of dissolving it, as alcohol, oil of
turpentine, &c., and its specific g^vity as compared with the
fluid ascertained. All that is required to determine its density
with regard to water, is to multiply the specific gravity thus found
by that of the fluid employed.
£x. A substance soluble in water was weighed in oil; its
specific gravity, as compared to oil, was found to be 3*7. The
specific gravity of the oil was 0'9 and 3*7 x 0'9b3'33, the specific
gravity of the substance as compared with water.
416. The specific gravity of a fluid may be discovered in several
ways ; the readiest mode is to compare the weights of equal bulks
of distilled water and of the fluia the density of which we are
seeking. For this purpose take a phial of convenient sise and
carefully counterpoise it. Ascertain first the weight of water re-
auired to fill it, and then the weight of the same phial full of the
nid under examination ; and, on subtracting from the latter the
weight of the bottle, the weight of the fluid will be ascertained.
Divide the weight of the fluid by that of the water, and the
quotient will be the specific gravity. In order to obtain the
greatest accuracy, the bottle should be stopped with a short piece
of capillary tube, the bore of which may oe very readily filled
exactly with the fluid in question.
THE HTDROMXTBR. 235
Ex. A counterpoised bottle held 500 grains of water, and 412
gimins of alcohol; then 412h-500» 0*824^ the specific grayitj
required.
417. Another and veiy convenient mode of finding the specific
gravity of a fluid is founded directly on the fact of immersed
Bolidfl displacing a bulk of fluid equal to their own (410). Forliiis
purpose take a glass ball of which the loss when weighed in water
IB known, then weigh it while immersed in any other fluid, and,
subtracting this from its weight in air, ascertain this fresh loss in
weight. Then its loss when weighed in the fluid, divided b^ its
loas when weighed in water, will be the specific gravity reqmred.
£z. A gUiss ball lost 30 grains when weighed in water, and 24
when weighed in alcohol ; and 24-^30=3 0*800, the specific gravity
of the fluid required.
418. The hydrometer. The specific gravity of fluids is fre-
quently very conveniently ascertained by meais of this instrameut,
ttie action of which depends upon the fact that a floating bodv
displaces a bulk, equal to itself in weight, of the fluid in which it
floats (404), and consequently that a solid of a given weight sinks
deeoer in a lighter than in a heavier fluid. Instruments of this
Idna are made of various materials, usually metal or -^ ^
g^asa, according to the uses for which they are in- ^'
tended. Their action is confined within a very
Hmited range, unless they are of inconvenient
length, and their indications are by no means ma-
thematicaUy correct, still, for veiy many important ,
practical purposes they are extremely useiiu. For r~^
determining tne specific gravities of acids, and other
chemical fluids, a glass hydrometer is used ; it has
this advantage, that' it cannot be falsified by any
change of form from external iigury. The construc-
tion and mode of graduation is as follows : — ^Procure
a thin glass tube blown into the shape of the figure
A B, Fig. 287, and from four inches to a foot in length ;
place in the narrow part of the tube ac a thin slip
of paper, and pour in mercury until, when immersed
in distilled water, the whole instrument will sink to
within half an inch of its top or bottom, accordingly
as the instrument is intended for fiuids heavier or lighter than
water. Then thrust, by means of a wire, a fragment of cork and
one of sealing-wax into the smaller tube, d : by holding it near
the flame of a candle, melt the wax arouna the cork, 'and then
idlow the whole to cool. In this manner the mercury will be kept
in the ball b, without any danger of its falling out on inverting the
instrument. Replace the tube in distilled water, and very care-
folly mark with a file the point where the stem a is intersected by
the surface of the fiuid : let this be a, then immerse it in a solu-
tion of salt) of which tne specific gravity is known ; suppose this
Q
MLon toe lent poini musi m oaeerrea, sna laeir
from 1, which vtll give the ipecifio ^vitr reqni
Syket't hydrom^er ie ■ modificatian of thii i
aaea in commerce for ascertuniug, b; meana of
33S
U> he 1*030, and mark with a Ble ths point where the Item Ii inter-
sected by the nufaca of the aolnlion; let thii be b. With a pair
of compnsBei lake the distance ah, on a slip of paper of the same
rize asthntprevionsl; p1u:adin a, anddiride this into thirty eqnal
partB, and from the wme acale divide the vhole length of the
paper nntil it has oxtj equal parts mailed upon it ; and nnmber
those in fivei : diatinguishing every tenth diviKion with a darker or
longer line than the others. Then introdnca this paper into the
stem A, in place of the first piece, and push it down nntil the mark
a eorresponds to zero, or 0 no (he paper acale ; when this is done,
the latter may be retained in Its piaco by a little Tamlih or gam ;
and the top being closed by the blowpipe, the instmment ia com'
Sleted. To ascertain the specific grantj of any fluid, of greater
engitrthan water, by this hydrometer, immerse it in the flnid, and
when it floats at rest, note the p<Hnt of the scale lo which the level
of the flnid coireapQpds. Thus, if the stem sink lo 15, the specific
gravity of the fluid is 1015, ainca each scalejivimon cOTreiponds
to 0001. If the fluid be of less density than water, the degrees
below the sent point must be obeerved, and their value subtracted
' y required.
' 's instrument nach
^, , }f their specific gra-
vity, the Blrcngth of alcoholic liquids. This instrument is usually
made of copper, and consists of a solid hall conaccted by a small
roand stem with the hollow bnib, which is surmounted by a fiat
stem on which the graduations are marked. In order to bnng the
plane of floatation within the limits of the graduated stem, in flnida
of veiy difTenjDt specific gravities, this instrument is provided with
several email weights lo be placed on the Bt«m just above the solid
ball, each of which corroBponds with a certain number of onitt of
Bpeciflc gravity, water being; taken as 1000. It must be borne in
mind that any accidental indentation of the hollow bulb will falsify
— - the indicatione of this instrument, hj altering the
volutae on which the graduation is based.
419. NiehoUon't Hydrometer,— TitU is an instru-
ment by which the specific gravity of either a solid^ or
a fluid may be determined. It consists of a hollow
cylinder, ■ r. Fig. 388, to which a dish, c, is attached
by a slender wire, c k, placed in the axis of ■ p ; and a
heavy dish, n, is attached to the lower end, t, of the
cylinder. Let it be reqnin-d,
I . To compare the tpecific gravittei of a iciid and
ajltad.
Let a be the weight which, when placed in c, canses
the instrument to sink in the fluid, liU its surface meets
B 0 in a givon point, a. Place the solid in c, and let i be the
weight that must be added to make the instrument link to n ;
then place the solid in d, and let t be the weight placed in o that
tlierefbra
12J
will nnk th« instniment to the aame poiat, h. Than, nsglsctiag
the ve^bt of air displaced \>j the solid,
veight of the solid- i—i ;
uid iFeight of the not id - weight of the fluid diaplaced = a;ipamt
weight of the solid in the fluid,a;i— i,
tbereloTe might of the fluid diHplaced''(—i—(t—T)^Y—i;
... , ED. gr. of the solid z— i
Bp. gr. of the huid V— X
8. 2^ compare tke tpeeifie gravM»t of tino Jliad*, a and B.
Let ir be the weight of th8Dj^">'aBter: i the weight that mait
be placed in c, to Buik the instraDient to h in the fluid a ; and t
tlie coirespoDiliiig weight when in the fluid a; then,
veight of fluid i. displaced "W + z,
weight of fluid Bdi«pUced=w+T;
bat the volnine oT fluid displaced it the same in botii cases ;
sp. gr. offlgid A w + x
sp. gr. of fluid b w + r '
420. Hare't SydromtUr. — This instmineDt aflbrda a readj
means of comparing the specific gravities of two fluids, a and B ;
and conseqnently, when that nf either '\i known, the
other maj be delermlned. b c, r i. Fig. 289, are two ^- *•*■
rertieal glass tnbea conunonicating at their upper
extremities, a, p, with ■ cavity, o, supplied with a
stopcock, through which the air may be withdiawn
from it The lower extremities of the tubes, c, E,
are immened in the fluids a and b, contaiued in two
ir a portion of the ur be now withdrawn from o,
the fluidB will be raised in the tubes b; the external
pcennTeaf the atmosi>bere; and suppose the fluids
A and B nse to the points p, q, respectivety, the sur-
faces of the fluids in the Tessels being at c and a
letpectiTel;. If the atmospheric preBsure = n, and
the pressuie of the air in i> = m, then
n-ii = (8p.gr.ofA).cp,
also n-M=(»p.gr. ofB).B.);
.. , tp.gr.of* nq
Iherebn Hp.gr ofB^cp'
In order to avoid errors dne to oapillaiT attraction (ST), a second
obeerralion mntt be made at two other points, p*, ij', and the dis-
tances, PF',<(q', CBrafnllrmeMored; then, ascaptllarity will aEfect
the pant* f, r', alike, as well as the points q, tf, the colamiu p t',
olelj b/ the vune diflarenM of preuares ;
gp. gr. of i. qq'
■p. gr. of a F F
421. TTit SteraonuUr* — The object of this inBtnment ii to
measure ibe volmaeB of small solids that cannot be immened in
Alt KM), ^'t^''- ^°^ ^bis purpose the lower eitremitiei of two
e<lnal gUra tabes, a n, c f, Fig. 390, are inaert«d into
ui iron box, b, Bnpplied with a slop-cock. The tnbe, or,
is graduated, and lennioates at its upper citremit; in
a cj'liadricai vessel, f b, the surface of vbich at e it
grannd plane, and capable of being closed Bir-tight bja
plate of glasa smeared with grease. Within F a is a cup,
r, containing the bod; of which tbe volume is required.
In using this instrument, remove the piate i, and
pour in mercurj at D, until its aarface meets po in a
^ren point, f ; then replace the plate e, and let a por-
'ion of the mercni7 run ont by opening the Blop.«)ck
it b; after which let the surfaces of the mercury in fq
ind D a meet F c in the points H, 0, respectively. Let
u be the volume of the space occupied by the air, befiire
the solid was placed io ibe cup, r ; v, the volnme of the
■olid ; K, the area of a seclion of the tube pc\ h, the
altitndeof the mercury in the barometer: a, thedeiuity
of mercury. When the surface of the mercniy wsa at
„-i.. .;.: ;„j.i. „ ._j :^, p,^
H occnpiea the
and its pressors — ^ . <r (A —
u may be determined by a limilsr process, the cop, f
empty. K il found by weighing the mercury occupying
ponlou of the tube re. A cubic inch of mercury at 16° C.
3429t grains nearly : therefore, if the length of the col
mercm-y in f c = a inches, and it« weight — u graias, and i
pressed in parts of a sqoare inch,
beinj
I given
■reighs
= 84!94.K.(i,andi- -
• Thli iutnuneBt m
SPBCI7IC aSATITT OF GA8B8.
If w= the weight of the bodj, its specific gravity = — .*
422. It must he home in mind that, in all determinations of the
Bpecific gravities of bodies, water of uniform density at any given
temperatmre must he employed ; and this condition is fhldlled hy
using distilled water. But as that fluid expands with eveiy incre-
ment of temperature above 40^ F.^ it follows that very erroneous
results will be obtained, unless the water used in determining the
densities of bodies be at the time of observation at some con-
venient standard temperature, as that of 60° F, And if this be
not practicable, a correction must be made by calculation, so as to
reduce the results obtained to the assumed standard. For this
purpose, the following table, exhibiting the specific gravity of
water for every temperature from 37° to 80° J^, will be found of
great service.
Tem.
Bp.Gr.
Tem.
48°
Bp. Or.
Tem.
69°
Sp. Or.
Tem.
70°
8p. Or.
37°
100093
1-00076
1-00008
0-99894
38
100094
49
1-00072
60
1-00000
71
0-99882
39
100094
60
1-00068
61
0-99991
72
0-99869
40
1-00094
61
1-00063
62
0-99981
73
0-99866
41
1-00093
62
1-00067
63
099971
74
0-99843
42
100092
63
1-00061
64
0-99961
76
0-99830
43
1*00090
64
1-00046
66
0-99960
76
0-99816
44
100088
66
1-00038
66
0-99939
77
0-99802
46
1-00086
66
1-00031
67
0-99928
78
0-99788
46
100083
, 67
100024
68
0-99917
79
0-99774
47 100080
68
1-00016
69
0 99906
80
0-99769
To ascertain the true Bpecific gravity of any body which has
been weighed in water of any temperature above or below 60° F.^
we have only to multiply its specific gravity as found by ex-
periment by the specific gravity of water at the temperatiure at
which it was employed.
Ex. A substance was weighed in water at the temperature 42°,
and its specific gravity found to be 6*20. Then its specific gra-
vity, reduced to 3ie temperature of 60° i^., will be
6-20x1-00092 = 6-20478.
423. The specific gravity of a gas is ascertained in a similar
manner to that of a li(}uid, onl^ the standard of com]>ari8on is
changed, dry atmosphenc air being here assumed as unity, or,^ to
avoid decimals, 1000. Let a copper or glass flask, furnished with
* Tbe descriptions of this and the preceding initrament are plsced hare
in connexion with the sobject of speoiflo gravities ; bat they wm be better
onderatood after reading the chapter on Fnenmatacs.
230 HTDBOBTATICS.
a stop-cock, be weighed when filled with air, and then again after
being exhausted by means of an air-pump as perfectly as possible;
the difference of these weights will give the weight of air con-
tained by the flask. Then fill the flask with the gas under ex-
amination, and carefully weigh it, this weight, tninui that of the
flask, will give the weight of the gas. The weight of the ^as
divided by that of the same bulk of air will give the specific
gravity of the former as compared with the latter.
Ex. A ^lasB flask, carefully counterpoised, held 5*7 grains of
atmospheric air and 6*4 grains of defiant gas ; the specific gra-
vity of the latter was therefore 5*4-t-6'7« 0*982.
In examining the specific gravity of gases, they should be care-
fully freed from moisture by being passed over recently ignited
chloride of calcium ; and the results obtained corrected for tem-
perature in the manner described in all chemical works.
It is also necessary, in order to obtain very accurate results, to
exhaust the flask two or three times successively, and refill it with
the gas of which the specific gravitjr is required, in order that the
residual atmospheric air may be so diluted, as not sensibly to afiect
the result.
424. The following questions will illustrate some practical ap-
plications of the knowledge of the specific gravities of the bodies.
A. What is the weight of a cubic inch of copper?
The specific gravity of copper being 8*9, or, more exactly, 8*879,
we have to multiply this by the weight of a cubic inch of water,
which, at 62° F., is 252*458 grains : therefore, by logarithms, —
log. 252*458 » 2*40219
log. 8*879= "94836
log. 2241*5 » 3-35055. Ans. 2241*5 grains.
B. A glass flask being filled with mercury at 0** C., the mercury
appeared to weigh 3156*613 grammes, and the weight of the air
contained in the flask was 0*281 grammes; therefore the true
weight of the mercuir contained in the flask was 3156*894
grammes. When filled with pure water at 0° C, the water ap-
peared to weigh 231*888 grammes; and the weight of air con-
tained in the flask was 0*281 grammes ; therefore the true weight
of the water was 232*167 grammes. The same volume of water
at 4° C, when its density is a maximum, would have weighed
232*193 grammes: hence,
sp. gr. mercury at 0® C. ^ 3156*894 ^ 13.50590
sp. gr. water at 4"'C." 232*193 "
Two other observations gave for this ratio the values, 13*59578
and 13*59602 ; the mean of all three is 13*596, very nearly.*
* Begnsolt, Aanales de dumie, t. 89, p. 296.
TABLB OF SPECinO QRAY1T1E8 ; WATBB s 1, AT 4" G. 231
425.
MeUds.
Lithiom ....
Potassium ....
Sodium
Msgnesium . . .
Alnminiam . . .
Anenic ....
Tellnrinm ....
Antunony . . . .
Zinc
Cast iron . . . .
Tin
Steel
Manganese . . .
Cobt3t
Copper, cast . . .
' wire ...
Nickel
Bismuth . . . .
Cadmium . . . .
Silver
Lead
Palladium ....
Thalfimn ....
Tungsten ....
Gold
Platinum, forged
laminated
Mercurj
Organie Bodies.
Wood of ponlar . . .
■ cedar . .
■ lime . . . .'
•"^■^■^^"^ Hall • • • •
" UlHHSD • • •
' oak ....
Ugnum-TitflB ....
Cork
White wax ....
iTory
0-593
0-865
0-972
1-760
2-615
5*881
6-258
6-860
6-862
7 207
7-285
7-816
8-000
8-513
8-788
8-879
8-800
9-882
8-900
10-474
11-445
11-650
11-880
17-600
19-358
21-837
22-069
13*596
0-383
0-561
0-604
0-845
0-852
0-925
1-330
0-240
0-960
1-826
Inorgamc Non-MetalUe Bodies,
Ice 0-865
Amber 1078
Sulphar 2086
Glass, crown
flint .
Rock crystal .
Marble of Paros
Diamonds . .
Oriental rubies
2*488
. . 3-829
. . 2-653
. . 2838
3-501— 3-581
. . 4-283
Liquids,
Ether
Alcohol ....
Bectified spirits . .
Ammonia ....
Sea-water ....
Milk
Oil of olives . . .
turpentine . .
cinnamon . .
cloves . . .
— bitter almonds
Acetic acid . . .
Nitric acid ....
Sulphuric acid . .
0-715
0-792
0-837
0-960
1026
1-030
0-915
0-869
1-010
1036
1-043
1-063
1-451
1-841
0€ues. AiBs 1 ; Bab. at 30 in.
Hydrogen
carburetted
Ammonia ....
Nitrogen ....
Oxygen .....
H^dro-8ul]^hnric acid
Nitrous oxide . . .
Carbonic acid . .
oxide . .
Cyanogen ....
Sulphurous acid
Chlorine ....
0-069
0-555
0-590
0-972
1-106
1191
1-527
1-529
0957
1-806
2-234
2-470
Weighis of given Bulks of Water and Air^ for oaleuUUing the
absolute Weights from the Specific Oravities of Bodies.
Bar. 30 in., therm. 62** F. loos.
Cubic inch of dist water in grains . . . 252-458 . 2 '402 19
foot in ounces avoird. 997-13697 2-99875
in pounds ditto 62-32106 1-79463
Weight of 100 cubic inches of air, in grains 80*49 . . 148416
CHAPTER VIII..
In the preceding chkptar the conditiaoa of eqmlibrium uf the to-
called non-elRBlic fluids have been considered; ia the present,
some of the laws that govern the motions of tliese fluids vrill be
investigated.
426. Fluids, escBping from orifices in vessels containing them,
obsj, like solids, the law of grevitaticTi, and their motinii is acce-
lerated in a corresponding manner. The eipro'sion of this fact ie
known aa the theorem of Torricelli, snd ma; be thus
0 the diitanee o/iU rurface of the livid above the centre of
the ori/iee. Floids obey this Ian without any relation
Fig.Xil. to their density, their Telocity solely depending upon
the depth of the orlSce, from which ihey escape, below
!the level of the fluid. Thus, if a Tessel bo filled with
water to the height of the dotted line, d, Fig. 291, and
.1 three apertures be made in the aide of the ressel at
i, B, c, the water will escape from each with very dif-
ferent velocities. At A, it willposBCBB the same velocity
caa if the patiicles of water had fallen in racuo from ii
to A, whilst at B and c the eacapiiiK current will nos-
suBB the same velocity as if the Bnid composing it had
fallen from n to b, and from n to c. From this fact we
learn that if two equal vessela be tilled with fluid, and
allowed to diacharge their conteuta by equal orifices at e<|ual
dejtthB, one of them being kept mite fu& bif the addition offieth
finid, the quantlly of water discharged in the some time from the
latter vesael, as compared wilh the qoantity escaping from that
which vat allowed to empty itself, will be as 2 : 1.
This result is analopius to that previonaly obtained (299),
with regard lo the molion of a body acted on by a uniform acce-
lerating force, namely, that tho space described from rest, in any
l^ven time, is one half that which would have been described wi;h
the last acquired velocity continued uniform. It followB, also, from
(306), that V = Jig.h, h being the depth of the orifice ; or, in
other wor]s, the velix:ity of a flnid issuing from an orifice varies
as the squflra root of the depth of the orifice. Alao, since the
ooantity of fluid paaung througfa an orifice in a given time must
YKKJl OOKT&A.CTA. 233
be proportional to its Telocity, it follows that the qnantit j of fluid
dischamd in a men time will be proportional to the square root
of the depth of tne orifice : thus, for example, twice as much fluid
will escane from an aperturo at the depth of 8 inches, or feet, as
that discharged bj an equal orifice at the depth of 2 inches, or
feet.
427. £aeh particle of a spouting fluid may be cousidered as a
projectile, aua will therefore describe a parabola (307), and as it
appears (309) that a bodj falling freely from the directrix of the
parabola to toe point of projection will acquire the yelocity of pro-
jection, it follows that a line coinciding with the surface will be
the common directrix of all the parabolas described by spouting
fluids. It may also be readily shown, that if a fluid spouts from
apertures in the yertical side of a full yessel resting on a horiasontal
plane, the horizontal range is greatest when the aperture is at one
half the height of the yessel, and the ran^ is then equal to the
height. AGo, the same range on the horizontal plane will be ob-
tained from orifices at equal distances aboye and below the middle
point. These results are precisely analogous to those preyiously
obtained (313) respecting projectiles. If a circle be described on
the yertical height of the nuid as a diameter, the horizontal range
of the fluid spouting from any aperture will be the horizontal chord
of the circle, drawn through the centre of the aperture.
428. When a fluid escapes from a circular onfioe having a yery
thin edge, in the bottom of a yessel of suflBcient depth and capa-
city— as, for example, a foot in width and depth, the following
phenomena may be observed : —
A. The particles of fluid descend vertically to yrithin about three
inches of the bottom, and then move, in more or less curved paths,
towards the orifice. This may be best seen by mixing with the
fluid some particles of matter of visible magnitude, and, as nearly
as mAybe convenient, of the same specific gravity as the fluid.
B. The surface of the fluid gradually falls, remaining horizontal
until within a certain distance of the bottom, when it forms a hollow
cone, immediately above the centre of the oiiflce, the surface of
which is convex mwards.*
C. The cunnent of fluid havine escaped from the vessel, contracts
in diameter to a distance from the orince equal to half its diameter :
the diameter of the contracted portion of the vein being to that of
the portion nearest the orifice as 5 : 8.
p. Every fluid vein, moving vertically downwards from a circular
orifice, is composed of two well-defined portions, which meet at the
narrowest part, or vena eontraetOf as it is called. The portion
nearest the orifice is perfectly transparent, like a rod of glass or
crystal : its section is circular, and it gradually decreases in dia-
* This Bnrfaoa is formed bj the rerolation abont its exis of a bjperbolio
oorre of the fourth order, the eqaation of which in its simplest form is
334 HrDKODTaiHica.
meter, until it jniDs the second portion of the carreni, which >■
Dearly onsque, and apparently much agitated, consisling of s mul-
titude of drapH, each produced bj an annular dilatation of a portion
of fluid at tbe orifice of the veear-], and undergoing, during the
time of ita falling, it series of periodic vibrations^ bj vhich each
drop alternately elongate* and contracts. A senea of pulutinitt
thni ocean at the onGce of the vesael, their nnmber lieiag in the
direct ratio of the rapidity of the cunent, and in the inTerae ratio
of the diameter of the orifice; they are frequently ioffidantiy rapid
to produce a distinct maaical sound.
G. In consequence of the contraction of the fluid vein (C)
liqaida escape with greater rapidity from a conical tube than
IVom a cylindrical one of equal length, provided the truncated
a^pex of tne rormer corresponds, in situation and MCiion, to the
poJDt of gnaUtt eontraetum of the fluid currant.
The following are the nnmerical results of some eiperimenta : —
A *e«el with a nmple bole discharged . . 6! quart* U 100*1
„ with a pipe two diameters of hole in
length 8! „
A vessel with tiie aame [npe inserted half
way in the orifice, only 6! qouto in 100* :
When the bottom nf the vessel was the para-
bolic curve described by the particlea . . 92 „
With a beU-moQth added to this .... a moxwiiim.
429. It appears from the experiments of H. Lacontnre that not
only the focm of the fluid issuiug from a fnnnel-ahaped vea>e1, bnt
also the quanti^ delivered is mndified by receiving the fluid on a
horizon tsl plane placed at various distances from ue orifice. If a
■mall quantity of lycopodium be suspended in the 6uid, the
isaoing stream more readit; breaks np into globular beads, the
number of which diminishes, and the size increases, as the oppoa-
ing plane is brought nearer to the orifice, as a, 6, c, d^ Ttg. 293.
In a, b, and e, the quantity delivered is sensibl;r the same as vben
the stream is unimpeded. In d^ when only a single bead remsini,
the Sow it considerably diminished, and in e the discharge is at a
minimum. In the pention /, the fiow is mneh greater than wheo
baskbr's mill.
235
JB9g,fg3.
n
unimpeded bj the opposed plane, and it reaches a maximam at
the distance represented in g.*
430. In a vessel fall of water, the down-
ward pressure of any column of fluid, as
A B, Fig. 293, acting on the horizontal
layer, c d, communicates an equal pressure
on the opposite sides of the vessel (398) : if,
then, an aperture be made at c, the pres-
sure there becomes noil, and fluid escapes,
whilst the pressure remains active at n.
As the pressure at c against the side is
removed, and that aeainst d continues in
action, the vessel, if suitably suspended,
will move as if repelled in a direction op-
posed to that of the current escaping from c.
Barker's Mill. — The movement, arising from this reaction
against the sides of the vessel, is readily illustrated by means of
the apparatus, ▲ b c. Fig. 294, consisting of a large glass tube, ▲,
closea at both ends with corks ; two tubes, b, c, bent twice at
ri^ht angle^ are fixed in the lower cork, their ends at e and r
being bent in opposite directions. Fill a with water, place the
f»rk, o, in its place, and p. ^.
suspend the whole, b^ the *^'
thread, h, from the ceiling.
The apparatus will remain
at rest, for no fluid can es-
cape, as the pressure of the
air against tne open ends,
B F, exceeds the. gravitation
of the fluid (427). Then
remove the cork o ; the at-
mospheric pressure will act
on the water in a, which
will now descend by its own
gravity, and passingthrough
the tubes^ b, c, and escap-
ing at B, F, will produce a
rapid rotation of the appa-
ratus, in a direction contrary
to that of the cunrent of
escaping fluid. This motive power is known by the name of
Barker's mill ; it is, however, of very little practical utility, as a
mechanical agent. ^
431. When a fluid passes through a tube, or channel, of which
the section is greater at one part than another, the velocity of
the liquid is necessarily greater in the narrow than in the wide
parts, as the same quantity must pass through every section in
• Les Hondes, Xay 10, 1868.
236 RTDSODTVAVICS.
iV* SK. the same time. Thas, if in the tobe A b, Fig. 2d6,
water be allowed to run throngh in a Btream, so as
to keep it constantly full, ite Telocity at o or d will
be much greater than when traversing tiie wide
parts, E, F, in proportion as the section of s or f
IS greater than that of c or d. The momentam of
the fluid will be equal in every transverse section of
the tube ; for as it is e^ual to the auantitv of matter
multiplied by the velocity (276), although the quan-
tity of fluid contained in c d, is less than in b f, yet
its velocity is proportionably grater. For the same
reason, when wat^r flows through a funnel, its velo-
city is much greater when passing through the tube
than when traversing the wider part of the instru-
ment ; and hence also the current of riven is more
rapid under the arches of bridges than at any other part.
432. Natural fountains are sometimes formed by the water
escaping from some concealed reservoir through a channel, or
fissure, in the strata containing the supply of fluid. On the
water escaping from a point below the reservoir, it possesses a
velocity regulated according to the theorem of Tomcelli (426),
and therefore sufficient to project it upwards in the form of a jii
tPeau,
Artificial fountains are constructed on a similar
FSff, 296. principle ; thus, if the tube a o, Fig. 296, be filled
^^ with water, it will escape from the aperture at o,
\f in a jet risine to an elevation somewhat less than
that of the cdumn of water in a; according to the
experiments of Marriotte, attaining an elevation of
5 feet, if the column of water in the reservoir be
5 feet 1 inch high. The elevation of the jet d'eau
would be equal to the height of fluid in the reser-
voir, if all triction from angular bends or projec-
tions, &c., as well as the resistance of the atmo-
^ sphere, were removed. The greatest elevation is
\\ obtained when the fluid escapes through an aper-
ture pierced in a thin plate of metal, avoiding all
conical tenninations, or ajviageB, as they have been
called.
433. The remarkable phenomena exhibited by the Geysers or
thermal springs of Iceland, have been satisfactorily explained by
Prof. iSpdidl. These consist of long vertical tubes or shafts,
which become graduallv coated with a siliceous deposit from the
water, and are supplied by some reservoir at the bottom ; the
depth of the tube was in one instance ascertained to be 60 feet.
The water at some considerable depth, perhaps 40 or 50 feet, is
at a temperature above the boiling point, at the ordinary pressure
of the atmosphere, but is maintained in the liquid state by the
FBICTIOH OF FLUIDS. 237
preBsnie of the Boperincumbent column of water. When thii
over-boOing water is, by a sudden accession o( pressure from
below, raised above the point at which it is constrained by pres-
sure to remain liquid, a portion of it immediatelr assumes the
gaseous form of steam, and driyes up, in a magnificent jet, the
column of water above it : and similar effects recur at uncertain
periods. These phenomena may be artificiallj illustrated by a
metallic tube seven or eight feet long, filled ¥rith water, placed
Tertically, and surrounded by small charcoal fires contained in
wire baskets, at the bottom, and about one-third of the length
from the bottom, the lower fire being the larger of the two. If
the orifice of the tube be surrounded by a broad and nearly flat
fonnel-ahaped vessel, the water, after being ejected, will return
into the tube, and the same result will be periodically repeated.
434. Friction is found to take place between solids and liquids,
and eyen between the particles of fluids themseWes ; but is not
susceptible of exact measurement, as is the case with solids
(52 — 56). A stream of water is always more rapid in tbe centra
than at the sides, as, being deeper there, the current flows on die
sorfitoe of lower strata of fluid: whilst, in the shallow portions of
the river, the water is exposed to the friction of the rough and
miequal bottom. In the centre, also, the stream is somewhat
more elevated than at the sides ; as, in its rapid course, it draws
the water from the sides of the river after it, by the friction of its
particles. M. de Buat has given the best practical rule for ascer-
taining the yelocity of rivers, when the sectional area, and the
fall in a given distance are known. Supposing the whole quantity
of water to occupy a rectangular channel, the width of which is
equal to that of the stream, from bank to bank, and the area of
the transverse section equal to that of the whole transverse seo*
tion of the stream, then the depth of water in this channel is
called the hydrauUe mean depth. The velocity will be nearly
proportional to the geometric mean between this depth, and the
fall in a nven distance. If the course of the stream be very
tortuous, tne yelocity will be considerably below the value thus
obtained. The practical method of ascertaining the bulk of water
dischaiged by any given stream, is to ascertain the approximate
sectional area by soundings, and to observe the average time occu-
pied by bodies of nearly the same specific g^ttvity as water,
immersed in different parts of the stream, in passing a measmed
space of 50 or 100 feet. The product of the sectional area in
square feet by the velocity in feet per minute, will give the num-
ber of cubic feet of water discharged per minute.
Li the transmission of water through pipes, the friction against
the inner surface of the pipes is so consKierable, that it is found
necessary to allow one-third or one-fourth more diameter to the
pipes, than would theoretically be requisite to transmit the pro-
posed quantity <^ fiuid, provided it could traverse them without
238 HTDHODTKAMIOB.
friction. And the same law is observed in the distribution of
floids in the animal economy ; the sum of the sectional areas of
the branches into which any large blood-vessel is subdivided, is
always considerably greater than the area of the trunk from
which they have beea derived.
A sufficiently accurate estimate of the velocity will be obtained
by supposing the height of the head of water from its Rurface to
the dischaiging orifice to be diminished in the same proportion
as the diameter of the pipe would be increased by adding to it
^th of its length, and then taking four-fit'ths of this height.
Thus if the diameter of the pipe were 1 inch, and its length 100
inches, we must suppose the efiective height to be reduced to one*
third by friction, ana the velocity must l^ calculated for a height
of four-fifths of this, that is, a little more than a quarter of the
actual height. If the pipe had been two inches, the head would
have been supposed to be reduced only one-half by friction,
and the latter pipe would discharge at least five times as much
water as the former, although it has only four times the sectional
area.*
In the ajutage, or escape-pipe, of a foun-
^p. 8»7. ^jjj^ ^ similar fact is observea ; for if it be
bent abruptly, and not with a regular and
gradual curve, the passage of fluid is much
obstructed. Thus,^ fluids escaping under
equal pressures, will rise much higher if
passing through the tube A, Fig. 297, than
throu^ B.
This may be most readily shown by in-
serting two such pipes into two orifices in the
same vessel, situated in the same horizontal
plane ; and the larger the orifice of the jets,
the more uneouafwill be the altitudes to
which they are observed to rise : the prejudicial resistances in-
creasing rapidly with the increase of velocity of the issuing
fluid.
There is a remarkable confirmation of this principle in the
tortuous course ordinarily pursued by enlarged veins and arteries,
the coats of which do not oecome thickened in proportion to their
increased calibre; they are thus protected from the effects of
sudden augmentations of pressure, which their comparatively
attenuated coats would probably be otherwise unable to sustain.
435. The motion of nuids in conical tubes is illustrated by sn
experiment of Bemouilli ; he found that water, in passing rapidly
from the narrow to the wide end of a conical tube, a b, Fi^. 298,
would empty the vessel, o, filled with water and communicating
with AB, by a small lateral tube. Dr. Barry found that a similar
effect was produced by a descending current ; for when water was
* Idbrsiy of UseAil Knowledge— Art. HjdrMiliee.
U
This pbyaicalfact maTprob*b1j be best «ipliiiiieil tbag: — Each
monsg particle bu acquired > certain amoont of energy, hj
which it exerta apreMore in the directioD of the aiia of the labe,
thiongh which it u paadng. If flowing lowardi the imaRtr end
of a conical tnbe, A, Fig. 300, this
pn»iit«, a h, maj be nooiied into rtg. Mo.
a c perpendiijalar, and e b panllel to
the naa of tbe tnbe; of these ae
ii wholly efiectiTo againit the aide
of [he tnbe, and if an apartnre, or
lateral branch exiita, the fluid will
eacspe thence. If, on the contraiy,
■he finid Sow tonardB tbe larger
end, as in B, then c b, the portion of
ihs preanire ab reMlved perpendi-
cularly to the aide of the tube, acta
enlirely Jtom its snr&ce, and if a
Interal Invncb enter at that point,
immersed in a small Tcaael of fluid,
u c, Figa. Sgs, 299, atmoapheric preaaure on the floid in c, bei[
tnmBniitted throngb tbe brancb-tabe, and unoppoaed at ita point
JQUclion, trill cause tbe fluid to enter the conical tube at that
point. And it is eiident that the greater tbe value of e 6, that
a, the greater the relocit; in tbe conical tube, the greater will b«
tbe lateral infloi; this result ia conflrnied by eiperitDent, Ibraa
the Telocity of the fluid in a increaaea, that in o maj be railed
tbivagh a higher Tertical colman.
In the circulating ayatem of animals, the arraDgementa of tbe
bk>od-*easeli frequently exemplify these principles. Id the arterial
syrt«m the blood, aa it is too well known, Bowa rapidly from an
apertnre, or &om a divided branch, in accordsuce with wbat has
been Mated respecting A, Fig. 300 ; and the branches are Ere-
qoently given off obliquely, to facilitate tba onward current in
',5
240
HTDBODTHAMICS.
them. In the venous system, on the contrary, in which the blood-
corrent flows as in b, the branches usually enter the larger vessels
more or less perpendicularly, so that a current of blo^ passing
along one vessel, may assist m emptying a lateral branch ; or two
currents entering a larger trunk at tne same point, may thus
exhaust the contents of a small vessel entering between them.
In the human bodv, the termination of the lefl spermatic vein in
the renal vein, and that of the thoracic duct in the angle formed
by the internal jugular and subclavian veins, afford remark-
able examples of such hydraulic arrangements in animal
structures.
436. The applications of the phvsical properties of fluids to the
purposes of domestic economy, and the wants of civilized life, are
extremely diversified, and afford some important objects of study
to the mechanic and engineer. An outline of the construction of
a verv few of these valuable contributions of science to art will not
be misplaced in this chapter, as this will afford an opportunity of
expl&inin^ to the student their modes of action on the principles
al]^ady laid down.
Among the various instruments used to elevate fluids above
their former level, those termed pumps are the most important.
Their theoretical construction is extremeljr simple : they mav be
divided into two chief sections ; the first including the sucxing
and lifting, the other the forcing pumps.
The suckinff or suction pump, as it is incorrectly termed, con-
sists essentia]^ of a hollow cylinder, a b. Fig. 301, having a valve,
E, opening umoardB, fixed in its lower extremity. A piston, c,
furnished witii a valve, also opening upwardSf moves in the in>
terior of the cylinder. If the lower end of the pump terminate
in a pipe immersed in water, and the piston be depressed to e, the
air between c and b will escape bv the valve in c, and on elevating
the piston, the capacity of the space below c
being increased, tne pressure of the air con-
tained in it will be diminished (499), and in
consequence of the pressure of the air on the
water at the bottom of the tube being thus
diminished, the pressure of the atmosphere on
the surrounding fluid causes it to rise in the
tube, until equSibrium is restored. On again
depressing and elevating the piston, a further
diminution of internal pressure takes place,
and a further portion of water is elevatea, and
we may suppose this repeated until the water
reaches the valve, s ; at the next elevation of
the piston, a portion of the fluid rises through
B ; and on once more depressing c, this water
raises the valve in the piston, and passes
through it, so that on again elevating c, a
K^.801.
2f
1
3>
colamn of water ii raised with it, which eTentiully ncapM through
the sida tabe, or epont, o. Oa thai contJoaiDs kltenutely to
ruae hhJ deprera the piston, witer ma; be raiaea from the reser*
m the height of that pniot aboie the leTel of the water in
well or reaerrair wei« leu than that of a cijlumn eqaa] in weight
to the atmoaphers, which ii aboat 3S feet ; but aa the piston of an
ordinary pump ii eapabte of producing only a vetj imperfect
numnm, it followB that the height to whioh water may b« ruaed
hj theas meana will be condderablT leM than 32 feat The action
M the lifting-punip ia ao limilar, tnat a distinct account ofit i»
DimeoeaBarj ; aa nanallj coiutriicted, it diSbr* chieflv, from the
Emp jnit described, in the pistno entarine the cylinder fiom
low, instead of from above. The following experimsnt will
ahow (if indeed ■ demoaatralion can in the present d«7 be deemed
necessary) that the elsTation of water by the commoD pump ii doe
to stmospberic pressure alone, and not it) any hypothetical
"^Dciple of snction," as was once supposed. FImo a (all re-
oeiTcr over a vessel of water on un air-pnmp pUte, and let the
Btem of a noatl modei-painp paw air-tight through the cover of
tbe reoeiver. While the receiver is foil of air the pump wilt work
madily, bat with difficulty when a partial vacunm is formed in
tha noeiver; and if tbe edianation oan be carried so far, that the
preacore of the air become* lesa than the weight of a column
nachiogfromthevalTeoftheptitap to the surface of tbe water in
die veaMl, the pump will not act at alL
437. Whenever it is required to raise water to a greater height
than the lifting-pump oan efiecl, it becomes
peceaaary to malie use of another construedon, Sl3.90t,
called toe forcing-pump, which differs from
the last in the position of its valves : the
piston, B, Fig. 303, moves ur-tight in t}ie
cylinder, fo, as In the sucking-pump, but it '
baa no valve. A valve opening upwards is
filed in the lower part of Uie cylmder ; and at
a, a lateral tabe, o a, is Sied, having a valve,
D, opening outwards. On n being depresMd,
the air is forced through the valve d ; ood if
the pomp has its lower end plunged in water,
OD raising a, the fluid will, when the air has
bean expelled as before, rush in through c,
in consequence of the dimioiahed pressnra on
ita aorfaoe. And on depressing the piston,
this portion of water will be forced through the
valve p, ont of tbe nde tube, aa, as, in coneeqasuce of tbe valve
c opening upwards, it cannot escape downwards at that point.
B lUBODTa AHKM.
The height of o abore tha r«Mrroir is limited, m in tbe pnoed-
FIm SOS '"8 case, but there is do limit la the tteiglit
to which the tube, a i, mKj wceiw), iinriaed
CDDiidenble, the
labour of working Che pump is much IncniwBd
'"""'" Bisilv of oTercominK all at onoo
'the • • ■ ■ - -
I br the ne
I the ineiti
r .
the inertia of the wholn column ii
each deaceot of the iiialan. This in
euce m*7 be obviated by the uae ot an air-
Teuel, DF, Fiff. 303, in which h is the lower
extremity of the ascending tube. When (he
surface of (he water in nt- HaeB above H, the
preaaure of the air which is condensed iD
r K, the upper part of d f, Ibroes the water
ap H F in a continuous atream.
43S. That most'Tslnable acqniaition to modern medicine, the
well-known stomach-pump, ia sq instriiment ot this deacriptioo;
the tube introdooed into the atomach being alteroatel; cen-
Dected with the lower end, or the side lube k, according as it is
required to iqject fluid into, or lo emptj the conlenls oC, the
ttuinach.
439. 2%« Oaiifornia Aimn.— This most useful and oomprehcD-
UTe form of pump is due to the genius of oar transatlantic breth-
ren. It is a double-acliue horiiontal pomp, and is tvpresented
Hg 304 '^ ' 'E' ^^ altacbnd to the upper end of a
' verlicalboard : AB, isabehtleTer,bj which
the sliding-bar, c. is carried backward and
forward : the handle a ma; be inserted into
a socket on either side m the piece, a, as
may be most coovrnient; or two handli'S
~ua; be employed, if great force is required,
IS when used for a Gre-engine. The piston-
od connects a solid piston with the bar, c.
In the chamber, k, abuve tbe barrel, are fmu
valvea, two opening inwards connected
with the inlet-pipe, H, and the other two
opening outwards connected with an air-
chamber, D a, which is clamped down od
(he chamber e by two swing-bolts and
nuts, E, F. D is tbe outlet [upe, and » a
plug, which is screwed down when Iha
pump IS required lo be used aa a forcing
pump (437), and removed when a gentler flow ia required, aa in
that cane the surface of water in the air-vessel is relieved from
any pietsuln greater than the ordinary pressure of the atnio-
aphsie. As tM water la received at one aide of the piston and
delivered at the other, during its tnolion in boih directioos, the
flow is nearly continuous, even without tho air«hamber: tbe
parti an (oKd tad little likely to get oat <rf ord«r, uid the
valTU ftr« readily got &t, if requiiite.
440. The fir^^ngioe a t, compound forciDg-pomp, coEtfilting of
two forcinK-pninpa placed on oppoaite aidei of on air-Tea«e), with
trbich bo£ commimicata. The falcrnm of the leter by which
bolh pamna are worked, is placed midway between them ; codm-
qnantlj, Uiey act alternately in charging the air-TeneL la order
to obtun B very forcible Jet, it ia neceaiary to preToot the eacape
of aiiy portion of the content! of the air-iessel, nntil the confined
air it conaidenbly eompreuod.
In the present day the portable iteam-engine, and the 6i^
engine ate moet oaefi^lj combined. In the beat eteam fire-enginei
the itaam a got np in about ei^t minntet from the moment of
setting light to the fire, eipecially when aided by the cnnent of
mr prtidnced by the rapid locomolion of the engine itself; and the
K>pQlBi*e power is »ery much greater than that of the ordinary
nd-ensiDe*.
441. In the pnmps already described, water U railed either by
atmoopberio or machanical preianre; soma inetmnienta will now
be described in which the momentom of one portion of fluid in
motion is eSeclive in raising another portion.
The ffadraulic Jtam. — A s, Fig. 305, is a pipe, descending
obliquely from ■ reservoir of water, *, to the lower part of an air-
Tenel,a,into the aide of which ii inaertsd the ascending pipa,fB.
c is a imall air-vessel ; B, a Urge and light Talve opening down-
wards ; D, a heavy bolt-Talre openiog npwaids; and B, a small
valve opening sideways into o.
Snppoae the Talves' k, b, closed by the pressure of the water in
XB,a closed by its own weight, s and o filled with air, and r h
filled with water up to the level of the water in a. r.>et the valve
■ be depressed and opened; then the water in abwiII move in
the direction ±n, and flow oat at n, until the ctirrent becomes
•officiently rapid to raise the valve a, and thus to close the orifice.
Hm water in a b having its motion thus suddenly checked, will
exert a very great pressure on the inner sur&ce of the chamber, a,
and having raised the valve n, will msh into the air-vessel, a, and
■p the pips Fa, compressing at the same time the air ia a andc
244 HTDBODTMAlllOt.
Aa KKin u tlie momentum of the water in l B is expended, and it
becomes qiueiw«iit, Dcloaes, and the preieore of the air inccaiuei
the water in i B to recoil Blightlj, nntil the air in c occnpiea »
larger apace than it did under the pretsare of the atmotphei^ ; at
ttjs instant, the internal preianre at B being lew than that of the
atmoephere, b deicendii, and opens, and the action of the machine
19 renewed. In this manner the water ascBods in r a at each sdc-
oeasive impulse, until it reachea the point to which it is desired to
elevate it. A purtion of the air in a and o is taken np hj the
water, wbich abaorbB a conaiderable qnanCity of air under a high
pressure ; to snpplj the waste arising from this cause, the machine
18 prorided with the valve e, which opens and permits the sir to
enter, during the recoil of the water ia a b. The hjdraulto or water
ram maf be advantageously emploj^ed whenever the qaantitj of
water reqnired to be r^aed la inconsiderable, and the expenditure
of fluid in working the machine is of no consequence. The
qnantitv raised at each atroke will obvionsl; be leas ai the height
to which it is required to be raised incieaaet. The ram ma; be
advantageousl; employed in raising water from a perpetual spring
to a ciitem at a considerable height above it.
442. The Centrifugal Ptasp is another machine in which the
molJTe power results from the momentum of a portion of fluid in
motion. This will be understood b; supposing Fig. 2di to be in-
verted, and made to rotate rapidly, the end o (now lowest) being
immersed in water. During rotation, the fluid in the anus n, C will
bj its centrifugal force tend to fl; outwards towards a and r, and
to escape from these orifices, the pressnre of fluid in the tnbe l.
will therc&re become less than that of the atmoephere, which vtill
cause a fresh portion to enter the tube a at o. The heisht to
which water may be thus raised is, as in tha oaw of the lifuoK
pump, limited to less than 30 feet. Ther« it, however, a disalT
vantageons expenditure of power in working thia maohine, and it
is now rarelj if ever employed in practice.
443. AppoU'i Centrifugal ftanp. — An ingenione and very
anccessfdl applicalion of ceaCrifngal force as a means of raidng
Fia 306 water, was made by the late Mr. AppahT.
* The essential part of this machioe is
1 represented in Fig. 806. A circular disc
I of metal, c, is fixed transvenelT on an axis,
I AB, and two equal discs, d, b, having large
I apertures, as h i, in the middle, are al-
I tached to c, by two series of onrved parti-
I tions, f, o, &c This chambered wheel is
I placed nearthe bottom of an upright ehaft,
^ ive recUngutar tube, between two
il fnista, the edges of which approaoh
' ven near to the maniD, h a, and to a simi.
lar margin on the other sicfe ; the reservoir from which the water is
lo be raised to the t(^ of the shaft has access Ui these cones, and
lam r
nicaffnu
THS 8TPR0V. 245
llherefore to the central epAce of tlie wheel BOTronndrng the axis,
▲ B. When a rapid rotation is communicated to this wheel, the
water included between the nartitione passes outwards by its
centrifugal force, and up the snaft, haying no other egress ; and
ita place is continuously supplied through the cones from the re-
servoir. Many readers wiU probably remember a considerable
afaeet of water that was thus raised so as to form a cascade at the
International Exhibitions of 1851 and 1862, by a wheel little more
than one foot in diameter.
The most advantageous application of Appold*s pump is in
raising large quantities of water to small altitudes ; for example,
it has been Tery successfully employed in draining fens, &c.
444. Tke Ciain and Bucket Pump is a simple application of
power in raising water in a series of buckets, which are attached
to an endless chain passing over two drams or pulleys, one of
which is placed beneath the sur£ftoe of water in the reservoir ; the
buckets, m passing over the upper dram, are tilted over, and
•mpty their contents into some convenient receptacle.
445. Capillarity and adhesion have been employed in raising
water by means of the rope jmmp, wbich consists of an endless
hempen band passing over two pulleys, one of which, as in the
preceding, is placed oelow the surface oif the water to be raised,
and power is applied to rotate the upper pulley. The fluid absorbed
by, and adhenng to, the band, is partly pressed out of it in passing
over the upper pulley, and partly driven off by centrifugal force,
when the motion is sufficiently rapid, and is received in an appro-
Sriate vessel. The Quantity of water raised by this apparatus
epends entirely on tne velocity of the band ; in one insUtnce it
was found that when the pulley made 1000 revolutions per minute,
83 j^ons of water were raised 135 feet in the same space of time.
This and the nreceding are now seldom employed.
446. The ayphon. — ^This well-known hy^ulic instrument, con-
Bista, in its simplest form, of a bent tube, a b c. Fig. 307, having one
of its branches longer than the otber. On
immersing its shorter leg in a vessel of ^' ^*
water, applying the mouth to c, and ex-
hausting the air, the pressure of the atmo-
sphere on the surface of the fluid, d b, i%ill
force it to ascend in the tube : as soon as
this has become filled with water, remove
the end o from the moutb, and the water
will continue to flow through the syphon,
as long as the end a is immersed. The
theorv of its action is simple : let « 6, Fig.
308, be the short, and $ d the long leg of
the svphon filled with water. If the lag sd
terminate at n, the pressure at h and n
vrould be equal, and no fluid would escape : but f d being longer
than hihj the distance nd, there must of necessity be greater
246
RTDS0DT1IAMIC8,
1^.809.
Fig. SOS. pressure exerted at d than at 6, and hence the
water escapes at d. If h is immersed in a vessel of
water, the pressure of the atmosphere will cause the
latter to rise in the tube, and thus 1>J this instru-
ment the vessel is readily emptied. The length of
the legs of a sjphon is calculated from the top^ #
to a line corresponding to the level of the fluia in
which the short leg is immersed. If the long leg
of the syphon be immersed in the water instead of
the short one, and it be filled with the fluid by
exhausting it with the mouth, the upwardpressure
of the air against the water in the shorter leg will be sufficient to
drive it back into the vessel : consequently, no syphon will act,
unless the leg outside the vessel be sufficiently long to reach below
the level of the fluid.
In order to prevent the entrance of any portion of the fluid into
the mouth, the longer leg is usually provided with a small side
tube, which opens into it near to d, Fig. 308, and, running up by
the side of it for some distance, is then bent outwards for tne con-
venience of applying the mouth. In order to exhaust the m>hon
for the purpose of filling it, the end d must be stopped by the nnger,
if not furnished with a stop-cock.
447. A tube, a b c. Fig. 309, with its
extremities curved upwards, is a useful
modification of the syphon ; its action
is readily understood. jBeing filled with
water, and one of its legs immersed
in the vessel d, the column of fluid
above ▲ will press upon the water in
the extremity of the tube, and no cor-
responding pressure being applied to
the fluid in c, it overflows and escapes
from the orifice, forming a little jet
d'eau. This instrument is termed the
Wirtemher^ tyfhcn.
The common scientific toy, called Tantalus*
cup, consists of a glass vessel. Fig. 310, in
which the bent tube, a b c, is concealed. The
long leg A passes out through the stem of the
cup ; on pouring water into tnis glass, it will be
retained as in any other vessel, until the hori-
zontal branch, b, becomes filled, and then the
water will escape through this syphon, until it
falls below the orifice of the leg, c. The mouth
of a little image is often fixed at b, to represent
the fabled Tantalus ; and as soon as the fluid
rises to his lips, it escapes through the syphon.
448. Another philosophic toy, illustrating
Fig, 810.
WATER-WHKELB.
247
flome of the principles already laid down, ^*9' 311.
is known under the name of Hiero's foun-
tain, and consists of three Tesrels, c, d, e,
connected bv the tubes a, b ; the tube n,
connecting the upper part of c with the
upper part of d, whilst A passes air-ti^ht
tnToagn d, connecting the reservoir e with
the bottom of the vessel c : a jet tube f
passes through the reservoir e, and extends
to the lower part of n. To use this appa<
ratus, the vessel n, and the reservoir e,
are nearly filled with water. The water
in E descends through the tube a into c,
forcing the air contained in the latter up
B into D, above the surface of the water, ^
on whicn it exerts a pressure eouivalent to the height of the
column, A : thus the water in d is forced to rise through the tube r,
in the form of a jet ePeau. The mode in which this apparatus
acts is, consequently, analogous to that of the compressed-«ir
fountain (511, c), dineiing only in the manner in which the com-
pression of the included air is effected.
449. The obsolete hydraulic instruments, called the Persitm
wheelf and the Screw of Arehimedee, were so constructed, that
some portion of a curved or spiral canal occupied by water, al-
though gradually elevated, remained lower than the acyacent
portions on either side ; but the action of these machines is so dis-
advantageous, that a detailed description of them is unnecessary.
450. Currents of water are frequently used as sources of power
in moving machinery, by means of the well-known contrivances
called wfiter^wheela ; these are of three different kinds, called re-
spectively underahot-f breast-t and overahot-wheeU.
A. The underahot wheel is furnished around its circumference
with radial float-boards, and is immersed in a running stream to
the depth of the float-boards. This kind of wheel is used where
little or no difference of water-level can readily be obtained, or
where the water is intended to act on it in either direction, as in a
tidal stream. Wheels of this description are usually broad ; the
breadth being sometimes e<^ual to, or even exceeding the diameter.
When an undershot wheel is not required to work in both direc-
tions, it appears from the experiments of De Parcieux and Bossnt
that a decided advantage is gained by inclining the float-boards
towarda the advancing stream, at an angle of 20° to the radius of
the wheel produced. The water then becomes partially heaped up
on the float-boards, and acts b^ its gravity as well as its momen-
tam : also they leave the retiring stream with less resistance.
It appears, as the result of experiment, that the effective power
of the wheel is greatest, when the velocity of the float-boards is
aboat one-half tuat of the stream.
248 BYDBOOTHAMIGS,
B. The hreatt4ffheel differs from the former both in its con-
stmction and mode of application. In this the float-boards are
placed obliqnelj aronnd a continuous cylindrical surface, and are
enclosed at each eiid bj a flange extending as far as their outer
edges, so that the descending portion of the circumference of ^ Uid
wheel consists of a series of buckets or wedge-shaped cavities
capable of retaining a certain portion of fluid. A hreoMtwork of
masonry is bnilt np to near the circumference of the wheel, from
its lowest point, to about one-sixth of its circumference : and the
water rushing down the breastwork, and fillin|^ the buckets, acts
on the wheel both by its momentum, and by its gravity. This
form of wheel is best suited to localities where a moderate supply
of water, with a fall of six or eight feet, may be obtained.
C. The overshot wheel differs from the preceding in the form
of the buckets, and in the much smaller ratio that^ the width of
the wheel usually bears to the diameter. It is available only in
situations where the fall is not less than the diameter of the wheel,
but may be driven by a much smaller quantity of water than
either of the preceding forms. The water is received from a trough
at the upper part of the wheel, and acts almost entirely by its
^avity. The circular rim that forms the base of the* buckets
IS called the solSy and the lateral flanges, the shrouding. Each
float-board consists of two, or sometimes three, distinct portions ;
the inner portion is radial and is half the depth of the bucket ; the
iBt« 4,4 middle portion is considerably in-
^' '"• clined to the radius^ as in Fig. 312 ;
and the external still more so. The
water is most advantageously em-
ployed when only a little more than
the radial portion of each bucket is
filled : in tnat case it does not com-
mence escaping until the bucket has
reached the position a, about 35*
from the vertical. Smeaton has in-
ferred from experiment that in wheels
of medium size, the velocity of the
drcumferenoe should not exceed three
feet per second, but that in large
works it may be somewhat greater.
It appears also that the power of an overshot wheel is more than
double that of an undersnot, of equal magnitude.
451. I%e Turbine. — In some parts of the Continent, the em-
ployment of horizontal water-wheels with vertical axes is much
more frequent than that of the vertical wheels generally empk^ed
in this country : to these the name of turbine is applied. Their
general construction, subiect to various modifications, is that of
a series of oblique radial float-boards, on which the descending
current of water is made to impinge in the most advantageous
THB BCBBW-PBOPBLLBB. 249
direcdon, and acting both hj its grayity, and its vis Tiva {340)i in
driTing the wheel.
452. J%e PaddU^lieeL — ^Having briefly noticed the principal
fonns of mechanism by means of which a cnrrent of water may
be rendered available as a source of mechanical power, it remains
to notice those in which the inertia of water is applied as a means
of locomotion. Of these, the earlier in point of date is the paddl&-
whedf the action of which is the converse of that of the nnder-
shot water-wheel (450, A), while the constmction is nearly iden-
tical. The power applied in rotating the ^ddle-wheel niaybe
represented by a coaple (81), the arm of which is vertical. The
lower pressure being counteracted by the resistance of the water,
the upper one is wholly effective in producing progressive motion.
In the paddle-wheels oif ordinary construction, the fixed float-boards
encounter a prejudicial resistance both in entering and leaving the
^ter : for, in order that the float should enter or leave the water
with the least possible resistance, it is manifest that it should be
in the direction of a tangent to the point of the curve that the
circumference of the wheel is at that instant describing. This is
a cycloidal curve, not exactly what is generated by a ^int in the
circumference of a circle roUing on a straight line as in 830, but
▼hat would be ^nerated by some point in a radius produced.
As the direction of this curve is inclined from the radius towards
the vertical, both at the points of entrance and exit, the resistance
from this cause can be diminished only by moveable floats, the
position of which is more frequently governed by an eccentric, but
atiU better by a peculiar arrangement of link-work ; for the details
of which the reader must be referred to the practical treatises on
this subject. It may, however, be remarked, that the increased
cost of production, and greater liability to derangement, are in
practice found to be scarcely compensated by the motive power
saved ; and, in consequence, fixed floats are generally employed.
^ 453. The ScrevypropdUr. — Another means of marine locomo-
tion now very frequently employed is the BcrewpropeUer, The
form of this instrument is that of -the sorew of Axohimedes, which
is generated by a straight line intersecting perpendicularly at its
middle point an indefinite stndgbt fine, along which the centre
moves, while at the same time uie line rotates uniformly. The
form of this may be more familiar, an being that of a spiral (or,
as it 18 commonly called, geometrical) staircase. The direction
in which the screw acts is at right angles to tiiat of the paddle-
wheel : it is placed in a vertical rectangular cavity purposely 1^
for it, in the stem of the vessel, and being entirely submerged, is
much less liable to injury from ooliision or impact, and to the in-
equality of action to wmch fte paddle-wheel is HaUe, when the
vessel rolls heavily.
The appropriate pikih of the sorew, or angle at which the edge
of the blade is inchned to the axis, depends upon the <velooity of
250
propuUion: in Bcrem of the beat construction, the bliulei are
moveable, and the pitch capable of acyustment. The aerev-pro-
peller is lo a certain extjitit tbe converu of the tarbine (451).
464. The WinAniU-tait. — The preaaore of the air in mntian is
nsed as a source of mechanioal power by meaaa of a familiar con-
triTince, the windmill -Mil, tbo action of tho air on which is pre-
ciiolj' analogous to that of a cnrrent of water oo the tnrbine (451).
The form of the sails nearlj coin cities with that of I he Archimedes'
screw ; the; do not, howeTer. extend to tbe anis, as the central
portion wonld be almoet ineffectiie. The direction of the aiia of
the Tanee (wbich in alwajs a little inclined from a hoiizoatal line),
BhoiUd be brought to coincide with that of the wind. For thia
purpose, tbe head of the windmill is made to rerolTe; and the
revolution is DSDallr governed bj a small secondnr; vane, placed
at right anfclea to the larger one ; and as this ia always acted on.
hy the wind, except when the plane in which it revolves coincides
vrith the direction of tho wind's motion, it ie alwayi effective in
turning the mill head tonnd to (he wind.
465. The Sleam-tngine, — In regarding the geneml and peculiar
propertiea of both tbe elwtic, and comparatively inelastic, liuids,
we cannot help being struck by the numerous ways in vhich they
are so admirably fitted to snpply tbe wants of man, and by which
they are made availahle in adding to hia various comforts, and
mioiatering to his wants. Of this there in no more conapicnoua or
more important example than the lUan-engint, hy which, in many
inatancei, man ia converted Irom a mere aource of labouring force,
to the far higher and more intellectual office of controller and
director of almost resiitleas power. The leading features of th«
construction of the sleani-engine, in ita three pnncipal varieties,
will now be explained, but without even an attempt lo enter into
many of the important details of their mechanism, that are more
Buitable to a practical tr
n tbe steam-engine itself.
The simplest fortn is tbe at-
motpherie eteam-engine ; * D,
Fig. 313, is a bollow cylinder,
ooiomnnicadng with a steam-
boiler by meens of a pipe, c ;
n ia a valvo opening down-
wards, and closed by a spring ;
B I), a pipe leading from n
cisieraaf cold water, e; Hisa
piston oonneded with onee^.
Iremitf of a lever, T. OF ; &vm
the other extremity of Iha
lever is suspended f h, tbe rod
by which the motivo-power of
tbe engine is communicated ;
reight eqnal to h^
351
mixm connected with F l opens tbe steun-cock c, whenerer K
dsKend* to b, and cIoub it, when m aarends to a. The cock d ie
opened in ft nmilsr manner when M cornea to A. and ia cloaed
again aeon after M begini to dsBcend.
Soppoae H to be at B, and the preianre of ateam in the boiler to
be a little greater tban the pressure of the atmoaphera; then, when
c ia opened, the steam raabeB into u n, and the preeanns on tbe
upper and lower eurfeceB of h being nearly e^ual, the weisht a
will caiue u Id aacend. When h risea to a. c la cloaed and d ia
i^ned, whea a jet of cold water iaines into the cylinder and
cmdenaes tbe BlemD, leaving a Tacnam below ■ ; and aince the
pnasDiv of the atmosphere on h ia equal to twice the weight of
K, H will deacend with a moving force eqnol to the weight of h.
When H arritea at b, c ia opened again, and u aacenda aa before.
The water remaining inns escapes thronph the valve a, which is
forced open by the pressare of tbe atesm when first admitted.
466. WiM'i Sltam-engiiu.—A.B, Fig. 314, is a hollow cylinder
cloaed at both enda : L a F ia a „
iever, one end of which is con- _ '^^ "*'•
nected with the piston n, by a
rod, A K, passing throngh a
•tuffiog-boi, or a team-tight col-
lar, at A ; the other end of the
laver is connected by a link f a
with the crank of a By-wheel,
> H ; D ia a vessel, called the
condenser, into which a tittle
ctid water tnay be injected ;
B a is a tnbe which connects
X B with tbe boiler, and with
the condenser, d. At ■ and s
ara placed valvea, ao connected
with 1. p, that when h cornea
to A, a com monicatiou is open-
ed between a h, tbe chamber
abore the piston, and the
bmler, which ia closed when u has deaceifded abont one-third of
AB ; alao between ub, the chamber below the piston, and the
condenier. When u comea to b, aimilar commnnications are
opened betweoD MB and the boiler, and between am and tbe
condenser.
Sappoae K to ascend from b to A, the space below m being filled
with steam from the boiler; as aooD as m arrivea at A, the commu-
nication ia opened between h b and d, throngh which the steam
paaaea, and b«comet condensed, leaving a vacuum in h b : at the
same time a conmDnication being opened between ah and the
boiler, steam msbea into a m, and m ig forced downward* by tbe
_1J
252 HTDBODTHAMICB.
fnU prMsnre of the steun during one-third of its descont, and after
the commanication between a. if and the boiler is cut off, bj the
diminished pressure of the steam in the cylinder. In the same
manner, when m arri?es at b, a yacunm is produoed in am by the
condensation of the steam, and m is pressed upwards hy the steam
admitted into the lower chamber. The condensation of the steam
in D is promoted by a jet of cold water, which is remo?ed as fast
as it ooUects by a pump, p; by which, also, any air that may hare
been mixed with tne steam is remoTed.
In practice the steam is admitted to, and escapes from the
cylinder, not by means of separate valyes, as represented, for the
sake of cleamess, in the diagram at r and s, but by means of a
slide-Talve ; for a detailed description of this, and for the relative
position and dimensions of its ports or apertures, as well as for an
account of the various and important accessory contrivances, such
as the parallel motion by which the upper extremity of the piston-
rod is made to describe a curve very nearlv coinciding with a
straight line, and the various means of regulating the supplv of
steam and water, &c., our readers must be referred to the standard
treatises on the Steam-engine.
467. The HSghprBUwre JSUaa^-enpnB.-^Th.^ construction of the
cvlinder, piston, and valves, in this engine, is the same as in
Watt's engine ; but the steam in the boiler has a pressure many
times greater than the pressure of the atmosphere, and, instead of
being condensed after each stroke of the piston, it is permitted to
escape into the open air.
Biqypoee m (¥ig. 314) to amend from b to a, the space m b being
filled with steam from the boiler ; as soon as m arrives at a, a
oammonication is opened between m b and the air, at the same
time that steam from the bailer flows into a m, aod m is forced
down towards b by the excess of the pressure of the steam above
the atmoei^ric pnsaure ; and the return stroke of the piston is
effected in a similar mannec.
458. Stationairy en^^ses of large sise are usually constructed
on the lano^pTeamure^ or oondensing principle, both on account of
the increased danger of high-pressure steam, and also because it
is found that fuel can thus be more economically employed.
In the construction (X portable and heomotiw engmes the high*
pressure principle is adopted, by which the weight and bulk of the
condensing apparatus is saved. In these the weight of the beam
is also avoided, and the pistons (of which there must be two) act
directly on a right-angl^ crank (218, II.), in order to maintain
uniformity of action. In order tnat a large quantity of steam
may be generated in a small space, in locomotive boilers the fur-
nace-heat is transmitted through a large number of parallel tubes,
surrounded by the water, by means of which a great extent of
heating surface is obtained.
In Marine engines, since the stability of a floating vessel is in*.
THE BTEAX-HAKMKH. 253
creaflod by depreasine tlie centre of gravity (407), the beam, and
the heavier parts of ihe framework of the engine are nsnally placed
hek>w. In screw-engines the action is generally direct, the pistons
acting on cranks connected with the screw shaft. For this pnr-
poee it is necessary that the j>iston8 and cylinders be placed
transveraelj : and as the space is very much limited, the diameter
of the cyhuders frequently exceeds their length. Also as the
heads of the cylinders most necessarily be brought near to the
screw-shaft, the piston-rod instead of being solid is a hollow cylin-
der, with the bottom of which the crank rod is connected, as it
would work disadvantageously when inclined at a large angle with
the piston-rod. Engines of this* description are usually called
tnink-engines.
459. HydrcttdiCf or Water-engine*. — ^In some oases, in which
the demand for power is occasional onlj^, and for short or uncertain
pmods, it would be inexpedient to maintain a constant supply of
steam; and steam-pressure on the piston may then be advan-
tageously replaced by water-pressure, if a due supply of water can
be procured from a sufficient altitude to afford the required pres-
sure. The admission of water into, and its exit from, the cyhnder
is effected by means precisely analogous to those employed in the
steam-engine. The jBydramic cranSy for shipping or unshipping
heavy goods, and the 'Hydraulic Uftj for raismg weighti to the
upper part of a hif^h buikling, are examples of machines worked
in this manner. The power of these machines is to be estimated
on the same principles as that of the Hydraulic press (394).
460. The Steam-hammer, — In the manufacture of heavy articles
in wrought iron, such as anchors, large steam-ensine cranks, and
armour-plates for ships, an immense advantage has been gained
by the introduction oi Nasmyth*s Steam-hammer. This machine
is, in fact, a direct-acting steam-engine, in which the cylinder is
inverted, and the piston-rod connected with a ponderous mass of
iron, haTinga steel face, which impinges on an anvil placed be-
neath it. The steam-pressure, being sufficient to lift the hammer,
will, when admitted above the piston, cause it to descend with at
least double the accelerating force of g^vity ; and hence the
powerful effect it. is capable of i^roducin^. So completely is the
tremendous power of this machine within the control of the en-
gineer, ^at uie writer has seen nuts cracked on the anvil, without
bndsing the kemeb, and a few moments afterwards, a mass of
timber of nearly a foot in sectional area on which the nuts were
placed, reduced to spHnters by two or three blows. In order to
aooompHsh the former feat, the hammer is coaxed into a gradually
augmented oscillation on the elastic cushion of steam beneath the
piston, by small successive admissions and emissions of steam ;
the latter, by putting in action the full force of the machine.
It has recentiv been proposed to invert in some degree the
arraogements of Nasmyth's Steam-hammer, and while the end of
tliepiilon-rodii itUched to the solid frkmeiroik of the nttchiiw,
to make the ojlindere pert of the nuui coiutituliDg the hmnmBr;
bat the writer ia unable to ea; whether the pnctical advantage,
or otherwise, of this mode of constniedoD hw lieen determiQed by
•xpeKence.
461. Siemem' gi/romtiric gimenior. — It is a well-knowo bet,
and one capable ofeai; demonstration, thiit if a cjliodrical veaael,
cootiiiiiing a fluid, be made to rotate oa its axis (being Terticatj
the anrface of the fluid will assume the form of a paraboloid. A
yerj ingeaioos application of this has been made by Mr. Siemeos
in iho conatractian of a goternor. This
"#■ »«■ ooEBiats of a cup, c, Fig. 315, of a i*t».
bolic fonn, open at both top Knd
bottom, the latter dipping ilightl/ be-
low the SDrface of a liquid coDtaioM io
an eiteinal casing, B. The cup it
supported bj four radial featbera, whicb
connect it to a hoes fixed upon a ceo-
Iral spindle, the ipiudle pasting down
throogh a tobniar support, a, fixed to
the bottom of the casing containing the
liaiiid. The spindle jnit mentioDed,
which is funiialied at Its lower end
with a pinion, is placed in the saise
ver^cal line as a lertical shaft, u,
situated beoealh it, and driTea from
the main shaft of the engine b; suit-
able gearing. At the top of this shatt i«
fixed an annular wheel (176) >, having
its teeth in the ume horiioatsl plane
as tbe pinion at the lower end of the
cnp spindle. Between the pinion an
the cnp spindle and tbe internal leetb
of th» wheel on the lower ahaft,
and gearing into both, are placed two piniiins, as shown at x ;
these pinions levoliing freely npon stade fiied to an arm, which
can, in its torn, revolie npon the central spindle. Bj means of a
lerer and connecting link, this arm is oonnected with the vertical
Mm of a bell oisnk fixed upon tbe axis of ihs throttle valve, the
other or honsoolal am of this bell crank carrviug a weJf^t. The
whole apparatus is supported hj twu legs, e, a, suitably attacbed
to the firameworfc of the engine.
The aotion of this apparatus is as follows : Wben the engiae is
in motion, the rotation of the lower vertical shaft would tend to
make the two intermedlale pinions nvolve round it ; this laotion
is, howevsr, resisted by (he vei^ht fixed on tbe throttle valve
lever, and the intennediate pinions are consequently made to
drive the pinion fixed on the cup spindle and thus cause the
8ZXMBSS' OTBOMBTBIC OOTBRXOK. 255
rotation of the cap. When the cnp w made to reTolye, the liquid
rifles within it (its surface assuming a fonn approximating to that
of the cup, as shown hy the dotted lines), and eventuflily over-
flows the uj^per edge. The portion thus overflowing is caught,
and its motion amsted, hy a set of radial vanes, u, fixed to the
external casing, and from these it falls upon another set of vanes,
L, cast upon the exterior of the revolving cup. The arrangement
of these vanes is shown in the figore. The action jost mentioned,
together with the power absorbed in setting in motion the fresh
Ut^uid drawn in at the bottom of the cup, proiduces a resistance to
the rotation of the latter, which is constant at a g^ven speed, an
increase in the speed augmenting this resistance by increasing the
overflow. It follows, therefore, that if a constant driving power
be provided, the cup will continue to revolve at an uniform velo-
city. A practically constant driving power is obtained bv the
arrangement of differential gearing which we have alreaay de-
scribed ; the " pull" which the intermediate pinions are capable
of exerting upon that fixed upon the cup spindle being limited by
the resistance afforded by the weight fixed on the lever of the
throttle valve. So long as the speed of the engine is such, that
the teeth of the internal wheel move at the same rate as that at
which the teeth of the pinion on the cup spindle are driven by
the pressure produced by the weight on the throttle valve lever,
the mtermediate pinions merely revolve upon their studs ; if, how-
ever, the speed of the en^ne be au^ented, the fact of the rate
of rotation of the cup spindle remaining uniform will cause the
intermediate pinions, and the arm carrying them, to revolve round
the central spindle in the same direction as the internal wheel is
driven by the engine ; and this direction is so arranged that the
motion thus given to the arms carrying the intermediate pinions
raises the weight on the throttle valve lever, and closes the throttle
valve, thus diminishing the speed of the engine. If that speed
falls below the proper rate, a series of movements, the reverse of
those just descnbed, takes place.
It will have been undentood from what has been already said,
that the cup vrill not maintain an uniform velocity, unless this
velocity is such as to cause a continuous overflow over the brim of
continuous oveziiow may be found by the following equation :
» =
H'-'l^^^r')
6-2832 B
In this equation n = the number of the rovolutions of the cup
per second ; h =s the height of the brim of the cup above the
256 HTDBODTVAMZGB.
level of tlie liquid at rest in the external casing ; r « the redins
of the lower opening of the cup ; r = the radius of the brim ; and
^ s: the force of gpravitj. All the dimensions should be expressed
in the same units of measurement. The level of the liquid can
be seen by the glass ^uge affixed to the side of the external
casing, as shown in Fi^. 315 ; the cock at the bo.ttomof the gauge
affords the means of withdrawing some of the liquid, if requisite,
or, on the other hand, an additional quantity can be put into the
casine through a hole at the top, which is closed by a screwed plug.
The dip of the cup, and the speed at which it will be dnven,
can thus be adjusted to any required amount Radial feathers
are formed around the lower part of the casing, in order to prevent
the main body of the liquid from obtaining a rotaiy movement ; and
loss from evaporation is obviated by making the casing air tight.
Any liquid, such as water or paraffin oil, may be employed.
The gyrometric governor has stood the test of practical applica-
tion very successfully. One of them has been at work for some
months at Mr. Siemens' telegraph works at Woolwich, and has
answered exceedinglv woU. The rapidity with which it acts is
very remarkable, and it has been found by experiments that, in
the case of an engine fitted with this governor, two-thirds of the
load could be suddenlv removed without causing any perceptible
^ change in the rate of revolution. The figure, which is drawn to
*.a scale of one-twentieth, represents a eovemor having a cup 8 in.
in diameter at the top, and 8 in. high alxyve the level of the liquid.
The resistance which a governor of this kind opposes to a change
of speed is very great, and will enable it to be employed for re-
eulating the speed of engines by the link motion, or for working
Uie gate of a waterwheel, or in other cases where considerable
power is required in the eovemor.
462. Flmds are capable of assuming nndulatory movements
analogous to the vibrations of solids (869— -378), differing, however,
in some respects, in consequence of the different physical arrange-
ment of their atoms. If a pebble be allowed to drop into a cum
piece of water, a series of npples will be| generated, diffused con-
centricallv from the point of impact, and becoming more and
more shallow as they recede from that point. On a small scale,
thesd are best observed bv dropping a glass ball on the surface of
mercury contained in a shallow vessel.
463. At the point where die pebble touches the water, a de-
pression will be produced : this will, from the ready transmission
of an applied force in all directions (384), produce a circular eleva-
tion of tne water round it. The particles of water thus elevated
above their previous level wiD, in their turn, fall, producing an
elevation of the next circular series of particles. Thus the initial
motion vrill be gradually propagated from the point of impact, in
a series of gradually extenaing circular ripples, until opposing
causes allow the eqmHbrinm to be restored, c^ Fig. 816, wiU
« (371),
257
Fig. 319,
ripple Bur-
rooDding it. ■, the adjoiDing circle of
depreuioD, &c. Tbe wLite circle! repre-
•eDt the elevations, SDd the ahwled oQeg
the deprenioDi of Iheae circalar w»«b.
The particles of oater thus diapUced j
merelf moTO id iniall vertical circles, *nd
ue Dot reall; arfted from the ceotre to the
bulk of the pond or brook, althoDeh it is
difBcnlt to bellere at first light that the
irater does not moTe laterally. This will,
howeveT, he admitted, on rafemnK tothe vibratioDiof a
or after watchiog the motiona oipiecee of straw, &c.,
face of water; tbe; will move np and dawn with each ripple, hut
acarcel; leave the place where Drat olMcrved. These wave-like
nKrvementfl are not only propagated laterally, but in all other dire^
tiona, aa might indeed be eipecled irom the laws already aQ-
noQDced, and aiteod downwards to a vertii^ depth equal to 350
^mea the elevation of each undulatioa.
An entire ondidation ^ „
ccuiat., M in the c«H ^' "''
of the vibratioQ of solids,
of a phase of depreaiion
mud one ot elevatiaa, and
the analogy ma; be ren-
dered more obvious bj cod-
caiving a series of circular
ondiilalioDs,diTided at ac b,
F!g. 317, so M toprewDt >
vertical HctioD. Tiie phases
irf elevation and depression
will preaenl the series of
cDrvea shown bj the line
A'tftf,
In non-elastio fiaids, vitcctity, or cobenve force, is the prin-
cipal agent in bringing the undu-
lations to rest; whilst, as it haa
been alreadj stated, m the case of
•olida and elastic fluids imperfbct
alasticinr produces the same result.
464. UndulationB, wben imiuDg' '
ing against a salid, are reflected
back in accordance with the ordi-
nary laws of reflected motion: a I
senea of nndulaticD^ propagated
ftom a centre, c. Fig. 318, and
reacbii^ a plane* obttocle, aa, will
Rg. SIS.
258
be reflected Irom it in the Bsme Ibnn and muiDer u if tbe; had
been propagated from * point, », placed at the name distance aio
from the opposite side of the Sied plane. In this way, nndola-
tioni geDsrated ID the centre of a circular voaitel may reach the
boandaricB of the Qnid, and,
74. SIS. on impingiDgagainit the walla
of the vessel be reflected
back to the centre, and ao on.
465. In conaequcnce of the
nndy reflexion of nndula-
tions. thej may be prnpa-
I gated in any direction by
meaoe of properly-arranKM
concave anrfaces. In UiiB
way nndnlatioDB gpnenited In
one of the foci, i, of on eUipae,
Fig. 319, mny have tbair
conjoint effects propagated to
the other, b, as here Bhown.
466. If two undnlationa meet, their reanlting movement trill
Tary accordine to the circum stances nnder which they come
in contact, "xaoa, if two ondnladona meet in the lame pkiue,
the resulting wave will be eqnal
yij-SM. ,(, the sum of the two separate
ones; bnt if in epoMile phatet, to
their diflbrence. Hence it is qnits
' posuble for two wajei of equal in-
tensity, tratelling in opposite diieo-
tions, to meet, and completely de-
itroy each other's motioo. This is
lermed the interference of loava.
The two equal aeries of undnlationa,
prapagatea from the points a and n.
Fig. 3S0, will, at the points when
they meet ID oppoute phMes, intei^
^■»1- fere and loae tbeir motion,
whilst, at the points of inter-
section of the cresta. the agita-
tion of the water will be iolen-
467. When a series of nnda-
btions impin^s upon an ob-
stacle io which an apeitoiv
eiiati, those which reach the
opening will pss^ through it,
the rest being reflected (464).
Those which pass through, nn-
deigo a peculiar change in their
T7XDULAT10HB OF ELASTIC FLUIDS. 259
carve, in consequence of atrildng against the edges of the opening.
Thus a series of undulations, propagated from c, Fig. 321, and
reaching the opening, ▲ b, in a fixed obstacle, will be propagated
throush it, so as to nil the space abed. The curye of tne con-
centric waves will be altered at f, f ; g, o, &c., from the influence
of the edges of the opening a b, becoming deflected in the direction
F K and o L, in the direction of the arcs of circles drawn from a
and B respectiTclj as centres. An analogous phenomenon, known
as inflectum, is obeeired to take place when a raj of light is
partiaQy intercepted by a solid witn a sharply-defined edge. See
468. It may readily be observed by experiment in a cistern or
other large yessel with vertical sides, that if a wave impinge very
obliquely on a side of the vessel, a portion only of the motion is
reflected ; and a peculiar heaping up of the particles of fluid ap-
pears to move along the side of the vessel. To this phenomenon
Mr. Scott Bussell has applied the term lateral aceumulatum. It
is by this kind of action that a sonorous wave runs along the
curved surface of a building, as in the well-known example of the
whispering gallery of St. Paul's Cathedral, so as. to afiect the ear
at a remote point ; and not by a series of successiye reflexions at
very obtuse angles, as has sometimes been supposed.
469. When elastic fluids or gases, as atmospheric air, are sub-
mitted to mechanical force, they are capable of assuming certain
alternating movements, analogous to the vibrations of solids (376)
and the undulations of water (462), and other non-elastic fluids.
These motions. of gases difier, however, in some particulars from
those assumed by water, in consequence of their physical con-
dition, their component particles being held together with a very
weak attractive force (9). Suppose a certain amount of force, of
momentary duration, be applied to a portion of air at a, Fig. 322 ,*
under its influence, the adjacent particles
recede equally in all directions, so as to fill ^^* 322.
a larger space, as b. Now in thas expand^
ing from A to B, it follows that the air pre-
viously contained in the space a b, most be
driven off; but its inertia (278) opposes an
obstacle to this taking place, it accordingly
becomes momentarily condensed, the atoms
approximating under the influence of the
expanding force at a. The particles at ▲
then collapse, but their elasticity again causes
them to en>and, and these futemations
continue until the efiects of the applied force are lost, and the dis-
turbed portion at a regains its state of rest. The concentric por-
tion of air, B, compressed under the influence of a, in its turn
dilates and acts on a shell of air external to it ; this, in its turn,
on another, and so on ; thus the initial force acting on a exerts its
8 2
260 HrhRODTKUfics.
influence on concentric portions of air, its eiTects gradaally dimi-
nishing with each, until they become too feeble to produce any
influence on more distant portions, as in the case of the ripples
of water (462).
470. In the case of these o$eiUation», undukUianSf or pulses of
air, it is obvious that we must regard them as extending equally
in all directions in the free air, and limited only by the shape of
the containing vessel when the air is confined in small spaces.
Therefore the effects of the united oscillations or pulses extend
equally in the course of radii from a centre to every point of the
surface of a sphere.
471. The remarks made on the reflexion, transmission, and in-
terference of undulations of non-elastic fluids, equally apply to the
elastic fluids, or gases : it being borne in mind, nowever, that the
vibrations of elastic fluids are commonly lonaiiudinalf that is, the
motion of the individual particles is in the direction of the motion
of the wave. Two waves of air concurring in the same phase will
exert an influence on surrounding particles of air eqoM to their
sum, and if in opposite phases, to their difference. This subject
will, however, be again reverted to when treating of sonorous un«
dulations, and of tne oscillations of ether, in explanation of the
dj-namical theory of light and heat.
Theory op Tides.*
472. The present chapter would be incomplete without some
notice of a very important class of natural phenomena; — the
Tides of the Ocean. In the calmest weather the vast body of the
waters that wash our coasts, advances on the shores, inundating
all the flat sands, and then as gradually retires to its former level ;
and twice every day is this vast ucean wave observed alternately
to advance and retire, independently of all casual disturbing
causes.
In searching for the cause of this remarkable phenomenon, phi-
losophers readily conceived that since the Sun ana Moon each cross
the meridian twice in the twenty-four hours, these bodies might
by their attraction influence the waters of the ocean. Accordingly
various theories have been adopted for the calculation of the tides
on this hypothesis of solar and lunar attraction, of which the most
noted have been those of Bernoulli i and Laplace. Universal gravi-
tation being admitted, there can be but one universal and correct
theory based upon it for calculating the oscillations of the ocean ;
but, in consequence of the difficulties of the analysis, which have
hitherto been insurmountable, other hypotheses must be resorted
to, in addition to that of gravitation, in order to obtain an approxi-
mate solution of the problem. The irregularity of the depth of
the ocean, the manner in which it is spread over the earto, the
* Fntft Meohaniesl Philosophy, p. 663 •< ttqq.
THEORY OP TIDES. 261
position and decUvitj of the shoTes, and their connexions with
adjoining coasts, cannot possibly be subjected to rigorous calcula-
tion, although these and similar causes greatly modify the more-
ments of the great tidal waves. All we can accomplish is to
suuilyse the geDend phenomena which must result from the
attraction of tne sun and moon, and to deduce from observations
such data as are indispensable in completing for each port the
theory of the ebb and now of the tides : these data are arbitrary
quantities, dependent on local circumstances.
473. The theory of Bemouilli, which has been termed by the late
Dr. Whewell the EquiUbrium theory^ sssumes that the attraction of
the moon causes the ocean to assume at every instant the form it
would have, if the earth and moon were stationair. It is foand
hj calculating the tides on this hypothesis, supposing the pole of
the prolate spheroid, which is nearly the form of equilibrium, to
lag behiod the moon, that results are obtained which accord very
well with observation, in some of the more ordinary phenomena of
the tides.
Laplace, however, has taken a different course ; he has calcu-
lated the attractive forces of the Sun and Moon upon the ocean,
and the results are found to contain some constant, and some
periodic, terms. He ^ assumes that in consequence of the friction
and resistances to which the particles are subjected, the waters
would soon have assumed a form of equilibrium under the forces
which are represented by the constant terms ; and then, taking it
as a general dvnamical principle, that the state of a system of
bodies in which the primitive conditions of motion have disap-
neared under the influence of resistances, is periodic, when the
mrces themselves are ^riodic, he obtains an expression for the
height of the tide, which is the same as that obtained by the
Equilibrium theory of Bernouilii. But there are so many assump-
tions in Laplace*s theory, that we may, as far as we know, d prior i^
as readily adopt the Equilibrium theory ; the accuracy of the
theory must in either case be tested by a comparison of the theo-
retical results with observation. This laborious task has been in
a great measure accomplished by the researches, and under the
auspices of Dr. Whewell and others, and maps of co-tidal lines
have been laid down with considerable accuracy.
474. The highest, or Bpring-Hdea^ as they are called, are ob-
served to oocur at a certain interval after the new and full moon,
at which periods the attractions of the Sun and Moon conspire to
elongate tne fluid spheroid. The lowest, or neap-tideSf occur at
the same interval* after the moon has attained the first and third
quarters, at which periods the attractions of the Sun and Moon
are the most opposed to each other, and therefore jointly produce
the least elevation of the tide-wave.
475. The interval after new or full moon at which the spring-
tide occurs is called the Establishment of a Port. In the port of
2 62 H ITDEODTN AMICS.
London, calcolation and observation yery nearly coincide in de-
termining the interval to be two and a half days.
476. If two tide-waves reach any given place by different
routes, there will always be more or less interference between
them, according to the interval between the similar phases of the
two waves (466) ; if they meet in opposite pbases, and are of eonal
depth, they will exactly neutralize each other, and there wouloi be
no tide ; but if, as must almost necessarily be the case, the depth
of one tide-wave be a little greater than that of the other, there
will be only one small ebb and flow in the twenty-four hours. This
singular fact has been observed at Batsham, a port of Tonquin,
lat. 20** 5(y N. The waves seem to come by two channels, one of
which runs irom the China seas between the continent and the
island of Luconia, the other from the Indian sea, between the
continent and the island of Borneo.*
If there be a small interval between the similar phases, there
will be two high and two low tides, separated by a corresponding
small interval of time. This phenomenon has been observed in
the Frith of Forth, in Scotland.
References, — For further information on the contenta of the
last two chapters the student is referred to the monographs in
the several Oyclopsedias already mentioned, and to the works of
Gh^g^ry, Youn^, Pla\'fair, Pouillet, Biot, &o. The whole subject
of fluid mechanics will be found to be treated mathematically with
great conoiseness and eleffance in Professor Miller's Hydrostatics
and Hydrodynamics, to wnich the Authors are indebted for several
paragraphs, as well as illustrations. The advanced reader may
also consult with advantage Moseley's Hydrostatics, and Pratt's
Mechanical Philosophy.
* KewtOD, Prinoipia» torn. iii. prop. 24.
mm^^
263
CHAPTER IX.
PVEUlLiTICS; OB TBS FBOPEBTIE8 OF ELASTIC FLUIIM.
477. The great mass of gaseona matter, sarrounding oar earth,
and extending to a conHiderable distance from it, is termed the
atmotphere, or attmaepherio air. This, like the denser fluids,
obeTs laws similar to those treated of in the preceding chapters,
witn snch modificadons as its eminently elastic character pro-
dnces. Like the less elastic liquids, ^ases obey the attraction of
CTATitation, and the conditions of equilibrium and equal pressure
(374), explained in Chapter VII.
Atmospheric air freea from moisture cousists, in 100 parts^ of
Kitrogen, by volume 79 by toeighi 76*9 ;
Oxygen, „ 21 „ 231.
A variable, but small proportion of carbonic acid, and likewise
of aqueous vapour, is always present in the atmosphere. As an
average. It may be assumed that 1000 parts of air consist of
Nitrogen, 788; Oxygen, 197 ; Aqueous vapour, 14; Carbonic acid, 1.
478. In consequence of the atmosphere being retained at the
earth's surface by gravitation, and at eveij point sustaining the
pressure of the superincumbent stratum, it is much denser near
the level of the sea than at some distance above it : thus, at an
elevation of 3 miles it is 4 the density of the air at the sea-level ; at
6 miles it is J ; at 9 miles, | ; and at 15 miles, ^ of that density.
The greatest part of the atmosphere is thus evidently always
within 15 miles of the surface of the globe, although, from certam
astronomical phenomena, it is supposed to extend to a distance of
40 or 45 miles ; and here is, in all probability, its utmost limit.
Dr. WoUaston* has shown that, at this elevation, the attraction
of the earth upon any one particle is equal to the resistance arising
from the molecular repulsive power of the medium. Another proof
of the finite extent otthe atmosphere is found in the fact of the
sun, and the planets, being destitute of any similar media sur-
rounding them ; for if it were supposed to pervade infinite space,
such large masses of matter as the planets must surely nave
caused a considerable poition to gravitate towards them. Other
philoBophersf have supposed that the extreme cold of the upper
• FhiL TrvM. 1828, p. 90. t Phil. Trvis. 1826.
264 PNEUMATICS.
regioDs is sufficient to prevent tbe unlimited expansion of the
atmosphere. Dalton,* reasoning on one of Newton's propositions,f
adopted the opinion of WoUaston.
479. Tbe extreme elasticity of gaseous fluids arises from the
intensity of the molecular repulsioni which, instead of being nearly
equally oalanced, or exceeded, by the intensity of molecular at-
traction, as in solids and liquids (9), tends continually to separate
the atoms still further from each other, and to press against the
Kidos of a vessel containing them with sufficient force to rapture
it, if sufficiently weak, were this effect not checked by external
pressure. Unlike the far less elastic liquids, gases never present
a level surface free from pressure, for they tend continually to
expand themselves into space until repulsion is balanced by
gravitation.
480. The weight of 100 cubic inches of atmospheric air, at
60** F., the height of the barometer being 30 inches, has been
coQiputed at 30*9 grains, by Kirwan ; at 31*1, by Davy; at 30*5,
by Sir G. 8huckburgh ; and at 30'2, by Mr. Brande.
481. The atmosphere exerts upon all bodies immersed therein
a very considerable pressure,^ which would be sufficient to crush
animal structures, ir in obedience to the laws of equal and con-
trary pressure, tbis effect were not prevented. Let a piece of
bladder be firmly tied over the brim A of a strong glass vessel a b,
Fig. 323 ; it remains perfectly flat, and gives no evidence of any
2~ g23 pressure upon it, the pressures on ltd
'^' opposite sides being equal. Then
place the vessel on the plate of an
air-pump (502), and exhaust the air
from beneath the bladder; the
upward pressure which prevented
the weight of the atmosphere irom
exertine its effect being removed,
the bladder curves inwards under its
influence, and at last dves way with
a loud report. If the bladder should
Srove strong enough to sustain the pressure of the atmo8j)here.
roppine a small marble, bullet, or round stone upon it will
generally induce its rupture.
If a plate of glass were placed on a, instead of the bladder, it
would, if sufficiently thin, be broken by the pressure of the atmo-
sphere. This pressure is, in round numbers, equal to fifteen
pounds upon each square inch of surface.
482. Atmospheric pressure is exerted upon everything on the
surface of our globe ; nothing is naturally exempt from its in-
fluence, any more than from gravitation, to which force this
pressure is indebted for its origin (478).
• Phil. Trsiiff. 1828. t Principia, Book Ji. prop. S, p. 202.
2SS
If • vmmI be filled wilb floii], snd invGrted, the orifice, if Ui^
being covered with aajtbing that
will keep the surToce of the fluid Fig.Sii.
eotira, the preuure of the air will
prefenl the escape of the flnid, pro-
vided it be greater than the presanre
of the fluid oa the cOTerjag of the
TeaaeL Thia maj be illustrated by
filling B gl&B3 Combler, A, with water,
placing a piece of writing paper, BB,
OTer ita mouth, ftnd careTully invert-
ing it, as in fig, 334. It wiU be
found that the fluid will nut escape,
for the upward presaure of the Btmosphere will exceed the irravit^
of the water, and accordingly the veHBel will remain fall. Thi> Ui
weci»ely onali^iu to the upward pressure of water, aa explained
m397.
«3. If » wide glaaa lube, a b, Kg. B25, be partly filled with
water, and inverted in the veasel c, filled also with water, the
floid will not fall in the tnbe, but remain anspended jrig. ju.
at a higher level than th^l of the exlernal portion,
In appearoDce contrary to the law of gravitation,
of which it is, however, the simple effect. For ths
atmosphere, presaing upon the surface de of the
water in C, acta upon that ia a B, and keeps it
elevated in the tube ; for the opposing pressare of
the BurTounding atmosphere on the fluid wUhia the
tube is cut ofi' by the end i. being closed. But if
we perforate the upper extremity of the tnbe, the
presmre of tho air la eqnalty exerted on the water
m A and c, and accordiriglv in each it acquires the
same level. If the tube a a be filled with water,
Mid he of any length under about thirty-three feet,
the preaaare of the ntmoBphere upon the surface of
the fluid io which its opeu end la imineised will be sufficient to
keep it full of water.
If, instead of filling and inverting the tube, the upper end be
connected with a good exhausting pump or syringe, and the air
in its interior removed, the pressure of the atmosphere upon the
water in tbe cistern, in which ila lower end is immnned, will force
that liquid into ita interior, up to a certain elevation, averaging
about thirty-three feet.*
At tbis elevation the weight of Ihs column of water is balanced
by tbe preaaare of the atmoephere ; and, of course, any cbange in
the pressure of the latter will be attended by a corresponding
change in tbe elevation of the water in the tube, farming a bOTo-
iHtier, or measurer of aerial pressure. An inetiiiment constructed
* Bojla'tWoili*; Dr. Bhan'B. edition, lT£S,TaLii. p. Ua.
266
PHEUMATXC8«
.D
Ftg. 326. in this manner was erected in the hall of the apart-
ments of the Rojal Society, at Somerset House;
but water barometers, in consequence of their
leneth, and of the constant changes in the tension
of the aqueous vapour in the upper part of the tube,
due to changes of temperature, are useless as
instruments of observation, and accordingly the
mercurial barometer is universally employ^.
484. The mercurial barometer is constructed
on the same principles as the water barometer,
but the tube being filled with a fluid 13*58 times
heavier than water, is required to be but -^ih. as
long as that of the water barometer. A column
of mercurv thirty inches in height, counter-
balances tne average pressure of a column of
atmospheric air of the same diameter.
To construct a mercurial barometer, let a glass
tube, AB, Fig. 326, about thirty-two inches in
length, be carefnUy filled with pure mercury ;
then, closing the end e with the nnger, immerso
it in a vessel of mercnry, c. On removing the
finger, the mercnry in a b will fall to a certain
distance, leaving a column in the tube, of a height corresponding
with the atmospiieric pressure at the time.
It may here be remarked that, in practice, small quantities of
air and moisture are found to adhere to the interior of the tube
with so much tenacity, that in order to completely expel them, it
is found necessary to boil the mercury in the tube itself; this
plan is always adopted in well-made barometers, as the pressure
of any air or vapour, however small the quantity ma^ be, in the
chamber above the mercury, would falsify the indications of the
instrument.
The space above d emptied of mercury has been, until quite
recently, the nearest approach to a perfect vacuum which could
be procured by art ; for on depressing the end b deeper in the
mercury, the whole tube becomes completely filled; the fiuid
metal again falling on elevating the tuoe. The space above d
necessarily contains a small quantity of mercurial vapour, and is
termed the Torricellian vacuum, from the experiment naving first
been made in 1043 by Torricelli, a pupil of Galileo. The height
of the mercnry in the tube is always measured from the suriace
of that in the cistern c ; and this elevation is the measure of
atmospheric pressure at the time. The elevation usually assumed
as the standard in this country is thirty inches, and to this all
measurements and weights of gaseous bodies are referred.
485. Several modifications of the barometer are in use : of these
the most common, and at the same time the least trustworthy, is
the wheel or syphon barometer. In this, a portion of the lower
end of the tube, five or di inches Iodk. i» bent upwards, and in
this portion of the tnbe the morcurj fallj, u it rises in the npper
portion, and met vtrtS, the actual height oftheoolamnbeiastnat
of the upper garface above a horiiODtal plane passing throagh the
lower auiface. A pleoe of glass attached to a string passing aver
* pnllej, floats on the mercury in the lower bend, arid as the
ibing is kept in a state of tensioa by a small weight or count«r-
poise, a morement of the pulley correspondi with that of the
•orface of mercury, and is indicated by a hand or index attached
to the pulley.
^Vhen the areas of the upper and lower inifoces of the mercnry
are equal, the movement of either will be exactly one half thn
Tariaboa of the height of the coliunn ; also the apparent height
will be independsnt of capillary depression {*l), aiuoe both Bur-
bces will be similarly affected from that cause.
4S6. When the barometer consists of a tube immersed in b
dstem, as in Fig. 326, it is evident that the surface of mercnrv in
tbn cistern will be ruaed by a small quantity, as the npper sarface
falls, and nice vertd; therefore an obserration of the variations of
tbe npper suriiwe only, although sufficient for the ordinarv pur-
poses of a weather gl/ui, is not sufficient when accurate results are
requited. Several methods have been adopted in order to avoid this
■onrce of error, of which three onlv require to b« mentioned.
I. A pointed cone, a. Fig. 32T, is attached by its base ^. „
to tbe upper snrface of the cistern, and the bottom of '
the cistern consists of some tieiible material, as leather,
which is capable of being raised by a screw, c : before
making an observatiou, the sarfaco of the mercury in the
dstem must be nused or lowered by means of the screw,
v, so as JDBt to touch the point of the cone, which ct
incides with the zero point ol tbe scale. The exact col
tact is readily effected, by making the point and its image
seen by reSectioii from the surface of tbe mercury to
coincide. For this purpose the cistern i — ' """ "'
gJaes.
II. The scale, d, is sometimes attached to the point
B by a strip of brass, and both are raised or lowered by
means of a screw (not seen in Fig. 327), in order tt
obtain, as before, an exact contact of tbe point and sur
lace. This arrangement possesses an important advan
tage over the former, namely, that tbe temperature cor
lection (487) is very nearly effected by the metallit
connexion of the scale with the sero point. Initm-
menta thiu carefully constructed are commonly called
tlandard baromettn.
m. In some portable, mountain, or marine barometers, M they
•re sometimes called, the cislem consists of a piece of similar
tuba, connected with the barometer ttibe by a shmt bend of stoat
S6S
tube, hiving a ranch sm&Uer bore, as in Fig. 328, th« object «f
which ia to check violent oscilUtion of the meKaiT in the tobe,
n, g{g_ hy which the upper closed end of the tube, a., mty b«
reodil; broken. The scale (of bnus) is moveable on
the tube, and the zero point, a horizonla] edge, in
brought to coincide honzontalij irilh the euHiice of
the mercury in the ciHlem c, which baa a email
apeiinre near the top, to allow acceu to the Bur-
rounding air. A horiEonta] edge attached to the
vernier u then made to coincide with the surface of
the mercurr in A, and the abBolute height of the
column ii thuB reod'oCT. la order to render the in-
aa compact as poaaible, and also to bring
the centre of gravity of the whole tube to coincide
with the axis of the item produced, the lower part
of the tube is bent twice at the aame angle, as aeen
'n the fignre between a and n.
In order to prevent an^ small sir-bubblcB, that may
happen to get past the bend, from risine to the top of the tube, an
oir-lrap is frequently employed : for this purpoBe the lower end at
the tube, t, is drawn out to a fine point, and the conical end is
hermetically sealed into a larger tnbe, B, lo the upper part of
which the supposed air-bubble will rise, and the vacuum above
the mercury will not be vitiated. As an additional security, the
fine point of A is aometimes bent np in the form of a hook.
In order to inaure a vertical position, the mountain barometer
is saapendeii from a tripod stand ; the case itself ia Bometimes
divided longitudinally into three equal portions, to form the tripod.
This instrument is always inverted for safety in transport
In making accurate barometric obBervations, great caremnetbe
taken in placing the tnbe exactly vertical, becaase, siiice the pres-
tore depends on the height of the column alone (484), the appa-
rent height, or length of the column, will be in eiceis of the Kal
height, whenever the tube is placed obliquely.
487. When it is required to make very accurate observations on
die pressnre of the atmosphere as indicated hy the length of the
column of mercDiy in the barometer, care moat be taken to mnko
certain corrections for the temperature of the air aa infloencmg
the expansion of the mercury. On tlus account, all aocarately
reported barometric observations are reduced to a fixed tem-
perature, which is generally that of freezing water, 3!° F,, b; suh-
from the apparent height of the cunmn a small quantity,
'' panrion of the column v ' ' ' ' ' ' "'
uivalent
'ery nearly mtn, part of its hulk for each degree of Fahrenheit's
scale, and the diminnlion of its density will be in the same propor-
tion. Consequently, the height of the colnmn, which increases in
the same proportion aa the density of ihe mercury diminishes.
THE OOnCAL BABOMSTEB. 269
miut be reduced in that proportion. If Ms the obserTed height
of the mercniy at the temperature S2*^ + t° F., then the reduced
height will be
A (1 - 00001 X t) ;
If, then, we subtract the ten-thonsandth part of the observed
height of the column of mercury for each degree of temperature
above 32** F. at the time of observation, we shall obtain verjr
nearlj the equivalent height of the column at the freezing point.
488. Capillary repulsion, by depressing the surface of the mer-
car^, is another source of error, and must be allowed for in the
estunation of the height of the barometer. This increases with
the decrease in the diameter of the tube : its amount is shown in
the table already given (41).
489. The Conical Barometer*— It has been shown (345) that
the pressure on the base of a vessel depends only on the depth of
flaic^ and the area of the base, and not on the form of the vessel ;
a g^ven quantity of mercury will, therefore, if iotroduced into a
conical tube, constitute a column of varying altitude, according to
the portion of the tube it occupies, the column evidently becoming
elongated as it approaches the smaller end, and vice verad. If,'
then, a slightly conical tube of small size be sealed at its smaller
end, and filled with mercury to the depth of 31 inches, and then
inverted, it is clear that, the pressure of the atmosphere being
unequal to sustain a column of 31 inches of mercury, the column
will descend in the tube until the height becomes such as will be
sustained bj the existing pressure of the air ; and the space
through which the column will descend, corresponding with a g^ven
change of height, or in other words the openness of the scale^ will
depend on the acuteness of the anffle of the cone. It may be, for
example, that the column may descend five inches in order to
become one inch shorter : in that case an actual rise or &11 of one-
tenth will be represented by half an inch on the scale. As, how-
ever, it is practically impossible to obtain glass tubes of uniformly
increasing bore, and four or five feet in length, the only available
mode of constructing a barometer of this kind, as it was stated hj
Sir J. Leslie^t is attained by joining together two tubes having
uniform but unequal bores, the sealed end bein^ that of the longer
and narrower tupe. If, for example, the sectional areas of the
tubes were as 4 : 5, the mercury must descend five inches in the
smaller tube to fall four inches in the larger, and it will then be
shortened one inch, and the augmentation of the scale will be the
same as in the former supposeacase. This form of instrument is
recommended by its simplicity, and its ample range ; but the
wider bore being necessarily small, probably less than ^ of an inch,
in order that the mercury may not^ fiow out on any slight concus-
sion, the column moves very sluggishly. A much larger bore may,
* Invented bv Amontons in 1605.
t Encyclopttdia Britaonio^ 7tli ad. 1842.
270 FHEUMAT1CB.
however, be employed, if a loose piston be introduced beneatb the
colamn, as proposed by Mr. Whiting. This consists of a steel disc,
loosely fitting the tnbe, to the centre of which a bnbble of glass
is attached, which, floating up in the mercury, keeps the disc in
close contact with its lower surface.*
490. The height of the column of mercury undergoes several
regular variations iji the course of the day; they are termed
horary vcsriathns, 11 appears from the observations made at the
equator by Humboldt, that the maximum elevation takes place
at nine o*clock in the morning ; past this hour it becomes less,
until four, or half-past four in the afternoon, when it attains its
minimum ; it again ascends until eleven at nieht, when it reaches
its second maximum ; and once more descend to four o'clock in
the morning, after which it reascends until nine. Thus, every
day, the mercurial column is at its lowest elevation at four in the
morning and afternoon, and at its greatest at nine in the morning
and eleven in the evening. The amplitude of these variations is
but small, being calculated b^ Humboldt at only 0*07874 inch.
In £urope, these horary vanations are marked by changes of
atmospheric pressure, depending upon accidental causes, which,
at the equator, are nearly without action on the barometer. As
far as these horary variations have been observed in our northern
latitudes, the maximum in winter appears to be at nine in the
morning, the minimum at three in the afternoon, and the second
maximum at nine in the evening. In the summer the maximum
elevations are at eight in the morning, and eleven at night ; the
minimum being at four in the afternoon. In spring and autumn,
the times of these variations are intermediate between those of
summer and winter.
The difference between the greatest and least of these daily
pressures, or the diurnal osdUationf as it is usually called, is
equivalent to the pressure of a column of mercury, the height,
hf of which is expressed by the following formula : — ^f
A = 0*1193 (cos lat.) ^- 00149.
491. To obtain luxmrately the mean diurnal height of the baro-
meter, it is necessaiy to observe the height of the column of
mercury at several intervals during twenty-four hours, and to
take the mean of these observations : but this tedious process may,
to a great extent, be avoided ; for M. Du Bois Reymond has shown
that at noon the elevation of the mercury corresponds almost
exactly with the mean diurnal height.
* This instrument, altbongb fbDj described in a work of large circulation
more than twenty years ago, has lately been |>ut forward (perhapa igno-
rantly) as a new Inrention : it wonld form a carioas chapter ra the natural
history of icience to record in how many instances obsolete and not yery
practical oontrifmnoes bare . been again and again reproduced as new dia*
coreriee.
t Prof. Forbes' Report on Meteorology.
Height of the babouetex at great ALTrnTPxa. 271
492. The mean pressaie of the atmosphere is also subject to an
aDnnal oscillation, the amount of whicn, except for some parti-
cular places has not jet been ascertained. Within a zone, that
extenos probably to the parallel of 40** on either side of ihe equa-
tor, the ii^atest and least atmospheric pressures appear to corre-
spond with the greatest and least zenith distances of the sun.
Thns at Madras (lat. 13* 4' N.), the mean height of the baro-
meter in January is 0-21 inches greater than in July. At Calcutta
(Lfct. 22^** N.), the difference amounts to 0-62 inches. At the Gape
of Good Hope (lat. 34*" S.) the height is 0-29 greater in July than
in January.
493. The mean pressure of the atmosphere at the level of the
sea appears to vary with the latitude. The comparative heights
of the columns of mercury at 32** F. which are supported in Affe-
rent latitudes, expressed in English inches, are, according to the
moat trustworthy observations, as follows : —
Lat.
Height.
L»t.
Height.
Lat.
Height.
0'
10
20
30
29-930
29-976
30-064
30-108
40"
45
49
6li
30-019
30-000
29-978
29-951
64J"
60
64
67
29-926
29-803
29-606
29-673
494. Amon^ the many important uses of the barometer, must
be mentioned its application to the purpose of measuring heights.
As we ascend above the sea level the column of atmosphere
pressing on the mercuiy becomes lighter by the removal of the
subjacent stratum, and consequently the fluid metal falls in the
tube.
The increased rarity of the air, on ascending above the surface
of the earth, has been already mentioned (478) ; the following is a
table of the corresponding heights of the mercury in the barometer
at several elevations : —
Height abore
aea level.
Height of
barometer.
Height above
sea lereL
Height of
barometer.
Ofeet
6000 „
10000 „
15000 „
300
24-797
19-000
16-941
3 miles.
6 „
9 »
16 „
15-00
7-60
375
100
Hence the subsidence of mercury in the barometer, on ascending
mountains or other elevations, affords valuable data for calculating
their vertical height.
495. 8elf-regUtering Barometer, — ^A large portion of the time
expended in making and recording observations on the barometer
272
■tad other mcteorolo^cal instrameTita, hoB been EitTed hj Bppft-
retuB 80 constructed kb to recnrd tbeir TSriatioas bj some ftativ
matic process. Varioiig mechanical arraneemBiita haw been
derisea bj Dollond, Krei), and others, in which the grsvily of
the displaced column of mercury is made to act upon a pencil,
vhich marlia a sheet of paper, moving uniformly in its own plana
2* clock-work. The morementA of the pencil, correspanding with
Me of the column of mercury, aro in a direction perpeadiciilaT
to that of the paper ; conseqaently, by the combined moremenls of
the pencil and paper, an irregular lin&or curve is traced, of which
the abscissfB represent time, and the ordinates, the corresponding
variations. The delicacy of tbe indications will, however, evi-
dently depend an the smallnesB of the amoimt of friction in the
apparatus itself, whicb, in all arrangemeata of pencil-tracing, must
necessarily be considerable.
The photographio method of registration, which was first aac-
cessliilly applied bv the present ecutor of this treatise te the mag-
netic Lnstrumen ta, has been convenientlyeitended to the barometeT,
especially since the arti&:ial illummation and photugraphio
apparatus necessarr for the registration of the balanced magneto-
meter (see Magnetism), sec^e equally for the registration of the
baromeler.
XA, Fig. 3S9, is the BeK-regiBlering barometer;
IV-3^ BB,tbe upper and lower ends of a cyphon baro'
f the mercniy,
irns; this has
and RB this is
e movement of
OTercome, the
Tom that caoie.
cing for a few
lent to perhaps
the Tjjth or
inch of mer-
cury, has fre-
qoentlfbeen
recorded.
SELr-REOISTBftINO BABOMKTEB. 273
with a small aDerture, through which a pencil of li^ht passes. By
the length of ieTsrage, the indications are four times the actual
variation of the column.
o is a plate on which the tube rests, which is raised or lowered
by a screw, in order to bring the arm into its mean or horizontal
position when the apparatus is daily set to work.
B, a stand supporting a gas-burner.
I, the register line described on photographic paper by the
pencil of light transmitted by the screen, f ; which will rise and
fall with the column of mercury.
K is a tube, with a plano-convex prismatic lens at each end of it,
placed at the back of the burner ; tnrough this a pencil of light is
conducted in the direction indicated by the dotted line, and de-
scribes the base-line, l. By this arrangement two pencils are
derived from the name source of light, which fall perpendicularly
on two remote points of the paper.
M N is a stand supporting a cylindrical lenn, through which the
two pencils of light pass, and are brought to a focus.
9 IS a brass frame which supports a turn-table on three hori-
zontal and three vertical rollers : a pin projects vertically from
the centre of the turn-table, which enters a hole in the centre of
the cap of t, the cylinder resting on the turn-table, round which
the photographio paper is placed. The turn-table is carried roond
by the hour hand of a chronometer, placed concentrically beneath
it. The paper is covered by a second cylinder concentric with the
first, in oraer to prevent its becoming dry (which greatly impairs
its sensibility), during the twenty-four hours that the apparatus is
designed to remain in action.
A blackened zinc case is placed over the cylinders when in
actual operation, to prevent any li^ht from falling on the paper,
except tne two pencils that descnDe the register and the base
line, from which the variations of the register are measuied. In
order to avoid confusion, this is omitted in the figure, as well as
another case of the same material, which covers the whole of the
apparatus, to protect it from dust, and the sensitive paper from any
stray rays of light.
496. The Aneroid Barometer, — A compendious and portable
inatrument, capable of showing approximately the barometric
changes, had long been a desideratum, until the invention of the
aneroid barometer. The original instrument, invented by M. Vedy,
consists of an exhausted eUstic metallic chamber, the expansiou
and contraction of which, due to changes of atmo^^pheric pressure,
are multiplied by a combination of levers, and indicated on a dial.
The vacuam-chamber a, Fig. 330, is flat and circular, havine its
top and bottom corrugated in concentric circles, to render them
more elastic. In the best constructed aneroids the top of the
chamber is, in a certain degree, heldnip in opposition to the pressure
of the atmosphere by the elasticity of a folded lamina of spring-
T
whicli is raacbed fnim
Fif.3K. an npertnre in tbe
lottom of the e*ae,
the indei-erroT may
be corrected whenever
such IB fonnd to exist :
sod it maj here be re-
ninrked that amall in-
dei-errorB nilt occiai-
ooally anw, ontil hj A
little time and use the
numerous nioieable
parts of tbe inatrument
hare aaaumed their
pennanent bearingi;
wheu, hoireTer, it is
duly reasoned, it ma;,
if originallf veil coii-
struclcd, be carried
about with ordinary
care in travelling,
without unde^^MDg aoy eoaaible changp.
The folded spring, a, is fimilj connected with a atud on tha
centre of the TBcuajn-cbBmber(which has bern carefully exhaurted
b; an air-pomp, and the aperture soldered up), and rises and falls
irilh it in obedience to atmospheric presaurs. An arm. d, is at-
tached lo tha Bpriug, at the further extremity orvbich the aclunl
morementa of B are considerablT amplified. The end of D is con-
nected by a link (15T) wilh a abort arm proceeding from B lran».
verse bar, f, which is moveable on its anis. Along aim, proceeding
upwards from F, ia attached by its ezlremitv, o, to the ead of h
Kteel chain (similar to the fusee-chain of an LDglish watch), which
is wound round a small pulley on the aiis of the hand or index.
A spiral balance-apring, attached by one end to the pnllej, and by
the Other to the frame-work, opposes the pull on the chain at it,
and cause* the index to retreat when the chain is relaxed.
Since the elasticity of metals is diminished by elevation of tem-
perature, Ihatchangeof lemperatnrowill toaamall extent pn>dac«
the same change in tho indications of the ioBtninient aa increased
preaaure; this source of error mny, however, be obviated by attach-
ing by numerous rivets a slip of brass, ■, to the steel arm, d, which
thus compenaale iho change ofelaalicity of tba vacoum-ch amber,
ariaing trom the same cause. The dimensions of the slip of braas
must be determined by actual trial.
275
Mr, J. Browning (by vhom prolulily the most •ccanlle inalni-
inents of tbia kind bive b««n constructed) bu detennined bj'
eipeiimeal tbat tc render the moTementB of the top of thti vacuum-
chamber aiuform, doe proportiona muet exist IietKeen tbc diameter
oT the chamber, Hud lbs widtb and deplh or tba ramigations.
This ma ascertained by filing an exhauited chamber and a
micnscope on the aame stand, and having substituted a cord and
weight for the apring, n, examining by <be micruacope the Tertical
moTemeuts of [be aland on the top of the chamber. The aam«
iDgeuiEiOB mechanician haa paid great attention to the graduation
of his inatrnmenla, which, to beeiact, nmat bo a matter of trial
wilb each iuatruraent: it ia enclosed in the receiver of an air-
(lurnp, wliich freely communicaten with another receiver contain-
ing a atsndard barometer, and aa the exhaustion proceeds, the
podlion of tbe index corretpondiog to each successive inch of the
1 is carefullj marked, and the variations of the
naduation uniformly distributed by a dividing-engine.
Browning haa recently conatruoted an aneroid baronieteT, tbe
dial of which ia 2} feet in diameter, and the scale conaaqnently
amplified about 15 timL_.
197. Another kind of aneroid barometer haa been devised by
U. Bourdon. In ihia the vacuo m-chamber is a circular coil of
flattened tube, a b. Fig. 331, attached to the bottom of the case
Sa clamp, c. The free enJi
tba lube are connected by HJ-SSl.
two linka with a croaa-arm, d
on the axis of tbe aegmeni of
a wheel, f, in gear with a
S'nion on the axis of ihe in-
!X, I. The back-laab of the
pinion is pnvented by a spiral
balance-spring, B. one end of
vhich is alUcbed lo the
pinion, and tbe other end to
the buttom of the case. The
eUcct of increased pressure
on the vacuom-tubo will be lo
flatlen the tnbe, and to in-
ends thus approaching each
other, will communicale move-
aent through the croaa-bar, D,
to the segment, p. and thence
to the index. Diminished pressure will of courae act in (he
conlrary direction. The graduation of tliis instrument, like that
of the preceding, must be determined by_ trial. It ia recommended
hy simplicity of coostmction, hut couaiderable index-error is so
nadily iadttced by even slight concuasion, that it ia by no nMsai
276 PKEUMATIC8.
trustworthy. How far this source of error, which probably arises
from flexure of the tube close to, or near the clamp, c, might bo
obviated bj supporting the tube inside and outside by taper
springs attached to the clamp, or by some other contrivance, time
must determine.
498. From the fact of density of the atmosphere diminishing as
we recede from the earth, we learn that gases increase in vohime,
as the pressure exerted on them is diminished in intensity. This
has been long recognised as the law of Marriotte, but was in fact
first discovered by Boyle in 1 662, and may be concisely stated
thus : the volumes of gases are in the inverse ratio of the pres^
sure^ which they support. The truth of this is readily demon-
strable : let some mercury be poured into a glass tube, a b c, Fig.
''332, having its end, c, closed, and, by gently inclining it, let the
fluid metal flow into the shorter leg, until it stunds at the same
level in bothj as up to the dotted line d e. The space c e will con-
sequently contain a certain bulk of atmospheric air,
Tig, 332. submitted to the ordinary pressure of the air thronc^h
^z:s the open tube, a. Let this air be compressed by
t If pouring mercury into A, until it stands at an eleva-
tion above the line, d e, eoual to the heiprht of the
barometer at the time ; tne air in c will thus be
submitted to a pressure equal to that of two atmo-
spheres ; one, that of the atmosphere itself pressing
on the mercury in a, the other, that of the column
of mercury in the tube above d, which in an avera^
state cf atmospheric pressure will be 30 inches m
length (484), and therefore equal to the pressnro
of one atmosphere ; it will then be found that the
air in c has been compressed to half its original
bnlk, for the surface of the mercury will have oeen
raised to half the heieht between b and c. This
I law has been verifief up to a pressure of twenty-
\^y seven atmospheres. A necessary consequence of the
law of Boyle is, that the density of gases is in.
proportion to the pressure to which they are escposed; and conse-
quently, under a pressure of 770 atmospheres, air would become
as dense as water, provided the law holds so far; it is probable,
however, that air would at a less intense pressure assume a liquid
form.
499. Boyle^s law may be shown to be true for diminished, as
well as for increased, pressures. Let a bent tube, a c, more than
31 inches long, closed at a, and having near the open end, c, a
bulb, B, of greater capacity than the rest of the tube, be filled with
mercury from the closed end, a, to the commencement of the
bulb, and communicate with two receivers, m, n, (not represented
in the figure,) of equal capacity, and each of which is supplied
with a stopKjock, Let one of these, as m, be exhausted by an air-
J>'
WBISnT or tHE AT1103PBEU. 277
pnmp, pravioaalj to being put in conneiion with tlis tuba. When
placed tan vertical poaition, let D be the point kt which the BUrfacs
of the mercury rests. If (he etop-cock between the tabe and it
be opened, the mercury wi)l remnia at n, since the
preiBiue of the air in nia the same asthatorthaatmo- '^' ^^■
sphere; if the stop-cock of n be now cloBed, aai Ih&t
of n opened, the meronry will fall conriderablj-, »i, for
iDEtanci', to E, since ihe pre^anre of tbe air in tn has
been much reduced b; eihaustion : the column db
there'.ore reprenonta the difference of prcsmrea in the
two receivera. Let both the stotHMckB be now opened,
and tbe preiauro eqoaliied in the two receiTera, when
it will be foanil that (he mercury will riae to a point,
p, midway between n aod E, provided the capacity of
the commnnicationa, &c., be amall, compared with
that of [he receivers. Hence it appears that the
denaity in either receiver being the mean of the
farmer densities, the pressure is the mean of the
former pressures ; and thua the truth of the law is
eatsblished. This experimeatal proof may be varied
by partially exbaostiiig both receiven, hut in conri-
dersbly diBerent degrees, when it will always be
found tbat, when the prMaore is equalized, tbe point p will bisect
the interval db.
500. Uegnault basahowo, that under great nmiaare airceasea to
conform to Boyle's law. but that its compreBsibility increases with
increasing preaanre. The compresaibility of nitrogen U also aug-
mented by increasing preasure, b;it not to the same extent an nir.
The value of the ratio of presaure to denaity for ilry atmonpheric
air does not, hoirever, perceptibly change under the preaaure of a
column of niercnij nearly 90 feet high. It has also been ahonn
(478) that the expansibility of gases and vapoura is not unlimited.
Carbnnic acid and other gasea cnnfarm more closely to the
eeneral law as their temperature is raised. It appears probable,
IraiD the Rxperimenta of Faraday, tbat every gas may he mode to
Hsnme Ihe form of a liquid when eufBcientl; compreaeeil ; eape-
cially when at a very law temperature. When the condensation
of a gas is carried on nearly tu the point at which it- begins to
liqoefr, the ra^o of ila preaaure to its density at a given tempera-
tare IS no longer constant.
601. In consequence of the atmosphere, in avenes atatea, being
capable of supporting thirty inches of mercury, it la eaay to calcu-
latis the pressure upon each aquace inch of aurface expoaed to its
action by ascertaining the weight of a column of mercurj thirty
inches high, and one square inch in sectional area. Thia will be
found to De nearly equal to fiFtoen pounds, which is therefore
assunied as Ihe amount of preasnre on every square inch of surface
etpoicd to the atmosphere. This pressure corresponds very nearly
278
with that of a cotnmn oTitnioipheTic air five and > qnuter milM in
height, if of nnilbrm density equal to the me>a deositj >t tb« t*»-
tevel ; but, aa the densit; actually diminishei in progiortioD to tbe
height abore the level of the wo, the air reoUj eitenda to ■ much
greater eleTation (478).
If the anrfoce of an adolt be coBsidered as eqaal to SOOO Kiaat*
inches, the preE«ure exerted on hie bod; hy the atmonphere, in
equal to the enonnooB amount of 30,000 pounds, or nearly 14 tnit,
a fonw more than EuGGcient to cmBh him, were it not oppooed bj
il und contrary presauro of the oerironD and other floidi
g the caritie* and tiosuea of bii frame.
CtUX. The Fremira-aaugt. — Theamountofcompreaionofanen-
cloeed rolame of air boa (Vequently been emplojed aa a meaanra
of the compreaaing force; an iaitrnnient aSbnliaK thia indication ia
callad a preBaure-gauge. The tube repreKntea in Rg. 833 will
aoairer this pnrpoae veiy well for moderate pMwanroB, if it be filled
with air, and tbe mercury be then made to stand at the same leTel
ats and in the bulb. If then thetnbebe gradnated in aliqoot part*
of A B, as ), ^, I, Ac., the mercury will be raised to theaa diriHiona
by the presauro of 2, 3, i, &c., atmoapherea. At high pranuraa,
howerer, the scale becomes ineonveniantly oontrmoted for reoord-
ing amsll variations of preeapre. To remedy this in-
jtj. 39*. ojnvenienoe, Mr. Allan tuta proposed to employ m
conical tube, to which such a curved outlioe may b«
given, as in Fig, 334, that equal increments of presMira
maybe represented br equal apace* on the scale. The
pamcolor inslrument here represented ii designed for a
ateom-gaoge, the presaure of the aleam being eiertsd
on cold water contuned i[i a D shaped prUongatioD of
the tube beneath the gauge. The figures 10, 30, &c.,
repreaent pounds of pressure per square inch <rf' sorfacv.
As air U rapidly absorbed by water onder conaideTaMa
preasure,the indications voald speedily become (olsified,
unleat there were a second lateral tube with a stop-
cock at the bottom of it, for renewing the Bnp[dy of air
in the gange : this being reversed as at a. the cock
beneath the gange is also reversed aa at a (tbe ateam
having been turned off), and the water allowed to
aobside to (he sero point of the scale ; tbe atop-cock a ia then
closed, and the gau^e is adjusted for use. This inconvenienos of
the abaorption of air would be Id a great measare reioedied by
employing oil, and entirely so by the use of mercury. Id th«
S08. For Ihe purpose of examining the affitcts resulting frou
■lmD>pheTio pressure, an eihausting s^nge, or an air-pump, b»-
comes a oeceasary apparatus. These instruments are coostnwted
on the same principlsa : the fonner cousiats of a barrel, b c, Fig.
335, of metal, funuahed with a acrew at c, for the purpose c^ con-
nectinK it vith any appantiu required ; at * i* ■ Talve, opcDine
upward*. Tbe pistoD, e d, maret air-tight in iLe Uarrel, perforata
at E, and there fumiihed with a valve, also opeDiDg apvudB.
Thii ijHnge beins coonected, b; ineana of the ^,_ jgj^^
■crew c, with any clowd cavity, let E be drawn up
to B bj raining the handle D, and then depre^ud ;
the air encloaed between k and a will eacape
through ihe Talve a: on again eleTatiog the
piatoD E, the presiure on the valre, t, ia consider-
ablj dimiiiiihed, (hen the eUatic pressure of the
sir beneath a opeui it, aad the air panea inlu
the space between A and ■ , and on again da-
prrsamg the piston, this escapes thmagh the
Talve K, and ao on; tbe air in the vessel con-
nected with c hecoiuing each time more rarefied,
until the eiceas of preaaure of tbe air below A,
bejond that abore it, ia nn longer able to raise the
TaiTe, when tbe action ceaaea.
S04. As this process is eitremelf t«dioui, and
in proportion as the air becomes more mreSed, the
external atmospbere, pressing ou the piston, ren-
ders It more laboriauitn elevate it, lliia ijringeb^is
given way to the air-pomp, conalructed with two sioiilar harreln
conoecled by a tube with a perforation in the centre of a perfectly
datptale of brasa, on which strong glass vesae Is. called r^cnum, are
fitted air-tight, by applying a little Kieoso to tbeir rima previouslj
gronnd tmly flat. By working the pistons by means of a cog-vheel
and two racks, tbe labour of eihnustion it much diminished, be-
cause tbe preasare of air on one piaton is counlcrbalHnced by
that on the other, since (he; move iii opposite directions. A, n,
'~ — ' ) two barrels comma ui eating by a lube with ibu
T%
- ., ir-pump
plate: this modiScation
of Ihe air-pnmp is due
to Hawkabee. In the
eariier machines, the
ImrTela were connected
directly with a Urge
globe, in which the
■nbatiuice to be ex
perimented npon was
In order that the
vacuQni once obtained
■nay be mnintainei
any required length of
• BajUr, Wotki, n
280 PNEUMATICS.
time, without any risk of leakage through an^ parts of tlie pumps,
it is desirable to possess the means of isolating the receiver from
the pomps : for this purpose the plate is generally raised from the
wooden oase, and supported by a large and well-made stop-cock,
as D, Fig. 336.
505. Smeaton's air-pump consists of a single barrel, similar to
Fig. 336, except that the piston-rod works air-tight through a
stuffing box in the cap of the cylinder, b, which is likewise sup-
plied with a valve opening outwards; consequently, on raising
the piston, the air above it is driven out through the valve in b,
Fig. 335, but, on depressing the piston, a partial vacuum is
formed, which relieves the valve at e from pressure, and allows
the passage of air from the chamber below to that above the
piston, when considerably rarefied ; consequently, the exhaustion
can be carried much further. Also, the piston s being relieved
from the pressure of the atmosphere, there is very little labour
required in working the pump.
506. It has already been stated (503) that the limit of action
of an ordinary air-pump is the power of the exhausted air to open
a valve between the receiver and the cylinder. This difficulty was
overcome in Cuthbcrt*s air-pump, in which, instead of a valve at
the l)Ottom of the cylinder, there were apertures of communi-
cation with the receiver, which the piuton just passed on reaching
the bottom of the cylinder, and with this instrument a better
vacuum can be obtained. The limit of action is now the capacity
of the residual space above the piston, into which the bulk of rare-
fied air that filled the cylinder must be compressed ; and unless
its pressure is then sufficiently ixbove that oi the atmosphere to
open the valve, none will escape, and the action ceases: this
residual space should consequently be reduced as much as possible.
Both these latter points of construction have been carefully at-
tended to in the air-pump of Mr. Grove, by which the most perfect
vacuum at present attainable by means of an air-pump may be
procured. In this instrument the solid piston is a truncated cone,
made to fit accurately a conical cavity at the end of the cylinder,
the aperture at the end of which is just large enough to be closed
by a valve which is reached by the end of the piston. If the end
of the cylinder be imniertied m a vessel of oil, and the valve be a
conical plug opened by the piston at itn extreme point of descent,
the smallest bubble will escape that is capable of rising spon-
taneously in oil. According to the original plan of Mr. Groye,
the valve is an ordinary silk valve, which is a little strip of oiled
silk, or thin gutta-percha, wider than the aperture, and tied tightly
over it Grove's pump is not a convenient instrument for com-
mencing the exhaustion, especially of a lai^ receiver, because
the upper surface of the piston is subject to the full pressure of tlie
air in the receiver; the most convenient form of apparatus, both
for ease in working, and for jjerfect exhaustion, is one in which
Orora'* and Hawksbee'e prineiplea an combined, a.a in Fig. 336,
in which OroTe'a pump, r, ii pliced obliquelj at tbe aide, and is
not intended to be used until the preraare of air in the receirer
ia reduceii to leas than lliat o{ one inch of niercor;.
507. Aa, bj meanii of these inBtrnmenlii, a coDstanC aliquot part
M1I7 of tbe residual sir ia abstracted bj eiwh atmke ol the piston,
tho air, iu a connected vesse!, is only eitremelj rarefied, never
becoming a perfect vacoam, it ia (requenllj doHrabte to measure
the degree of rarefaction of the included air ; for this purpose, ths
open top of a barometer tube ia consected with the eihanated
Teasel, its lower end being plunged in mercniy. On eihnnating
the air, Ihe mercury is fnrced up inio the tube by tbe pressure of
tbe atmosphere on the mercury at its lower end: and the nearer
its heiglit corresponila to that of the barometAr at the time of tbe
experiment, the nearer the air in the rcL-eiver approaches to b
•late of perfect eihaurtion.
608. In place of a tube immersed in a vessel of mercury, tho
•«)AonAni«|l<'ig.337,iBmorecommon1yemplayed: this
conaiats of a small stout glass tube, a d, bent twice on ^' '''■
its«lf at B and 0, the end n being cemented into a
•crewed cap, by which it is attached ia the air-pump,
mnd connected with the caTity of the receixer : for this
piirpose a second smaller plaie is Ireqaently placed be-
nind the larger one.
The tnbe is closed at a, and the portion a b filled
with mercury. When the pressure of air in the r
ceiver beenmes less than that due to the column A 1
tbe mercury descends in a h and rises in b c ; and tt
two surfdoes approach the same level in the tv
branchea a b, B c, in proportion aa the vacDum beoom<
more perfect.
509. When the density of ilie sir is required Pig, S3e.
Ia be increased, ths condensing syringe, the con-
Terse of the eihsustlng syringe (303), is employed.
This consist* of a brass barrel, (Qmished at a with
a valve opening downwards ; a perforation ia made |
in the side of the barrel at A, just low enough to
■Low the piaton to passaboTB ' "
lyringe on a strong metallic Teasel, ond raising n
aboTB the openine at a, all the space between b
and s becomes filled with ur, and, on depressing
the piston, this ia forced through the toItb b, into
tbe Teasel screwed on 0. On again raising b, air
esanot escape through e, because the valve opens
downwards; and on depressing the piston, a ^v'-h
portion is forced through e into the vessel, and
thus the condensation of several volumes of air into
» small balk may be effected.
282
PNEUMATICS.
Fiff.93».
The limit of action of this instrument is the amoant of pressure
that the hand is capable of exerting on the handle of the piston.
Since the pressure of the compressed air upon the piston is propor-
tional to its area, it is evident that the conaensation may be carried
further with a piston of small diameter than with a larger one.
510. The air-gun affords an example of the practical application
of the condenser, by which several atmospheres are condensed into
a spherical ball, or cylindrical vessel, wnich is attached to the
barrel. Those uf the beat construction are furnished with two
condensers, the smaller of which may be used when further con-
densation by the larger is impracticable.
511. By means of the preceding machines, many highly interest-
ing experiments, illustrating the general properties of the gaseoos
flmds, may be performed. The following are examples of these :
Illustrating Atmospheric Pressure.
A. Place in close contact the two brass hemi-
spheres, A, B, the edges of which have been accu-
rately ground together, and connect them, by means
of the screw c, with the hole in the centre of tho
air-pump plate (504) ; exhaust the air from their
interior, close the stop-cock b, remove them from
the air-pump, and screw on the stand f, or more
conveniently for the purpose of forcible traction, or
suspending a scale to be loaded with weights,
another ring similar to the upper one. On then
attempting to forcibly separate a from b, it will be
found nearly impossible, by any moderate exer-
tion of the strength of the arms, to effect this :
r^ T-1 for they will be pressed together by as manv times
js'U ) I fifteen pounds as there are square inches in the area
"^k pL^^ of the section. This apparatus is well known as the
*^Cy ^^ Magdeburg hemispheres, from its having been in-
vented by Otto de Guericke, burgomaster of that
town.
B. Pour some mercuiy into the .cup a, excavated
in the substance of a piece of wood screwed on to
the top of the receiver b, and place the whole on the
air-pump plate. On exhausting the air from b, the
mercury will be forced through the pores of the
wood into the receiver b, in the form of a metallic
shower, by the pressure of the external atmosphere.
On this principle, the minute capillary vessels of
animal structures, and especially the absorbents,
and the tubuli of the testicle, may sometii^es be
iinected with mercury : for this purpose a glass
tube drawn out to a fine point must be passed
through the cover of the receiver and the bottom of
h'ig. StlO.
BE818TAXCE OF THE ATMOSPHERE.
283
^.341.
tbe cap of mercuiy, and the fine poiot inflerted into one of the
larger Tessela, and suitably secored.
lUuitrating the Elastieiiy of Air.
C. Remove the jet b from the Teasel l. Fig. 341, and screw on
the condensinff syringe (509), having preri-
oasly hatf-fillea a with water ; on forcing air
into this vessel, it will babble np throagh the
water and rise on its sorfaoe, dd. After
working the piston for a few minates, close the
stop-cock, c, remove the syringe, and screw on
tbe jet, B. The condensed air will press upon
the surface of the water in a ; and on opening
the stop-cock, will force it oat in a jet, forming
a fbontain.
D. If the same vesiiel, without any con-
densation of air into it, be placed under a tall,
narrow receiver on the smaller plate of the
aiF-pomp, or on a separate plate, connected
by a tube and stop-cock with the receiver,
and the latter be exhausted, as soon as the
two receivers are connected by opening tiie
stop-cock, the water in the tabe b, being relieved from the pressure
of the atmosphere, will be raised in the form of a jet, by the un-
opposed pressure of the air in d.
£. Press together the sides of a bladder, so as to nearly empty
it of air, and tie it tightly at the neck ; place it under a receiver
on the air-pump plate, and exhaust the air: as soon as the pres-
sure of the air is removed from the surface of the bladder, the
elasticity of the small quantity left in it comes into pla;^, and the
air expanding distends the bladder. On ro-admitting air into the
receiver, the small quantity left in the bladder is compressed to its
fbrmer bulk, and the bladder appears as empty as at first.
F. Place a vessel of spring water under tne receiver of the air-
pomp, and exhaust the air ; as soon as the pressure of the atmo-
sphere is removed, the air dissolved in the water expands by its
elasticity, fonns large bubbles, and escapes from the water, thus
producing in the latter a state of effervescence.
G. On placing baked apples, raisins, or shrivelled fruit, under
the receiver of an air-pump, and removing tbe pressure of the
atmosphere, the air they contain expands, and dilating the integu-
ments of the fruit, gives them the appearance of ripe plumpness.
On re-admitting air into the receiver, this artificial and delusive
appearance vanishes, and the frait again becomes as shrivelled as
before the experiment.
H. Place a glass vessel half fall of hot water under the air-pump
receiver ; as soon as the air is exhausted, the water will begin to
284
PNEUMATICS.
Fig, 842.
boil Tiolently ; l)ecau8e the temperature at which water boils i»
coDsiderablj reduced by diminishing the pressure of the atmosphere
upon its surface. (See Chap. XXV.)
512. Benttance of the Atmosphere. — ^The slow and irregular
descent of a light body, as a leaf, or piece of paper, compared with
the rapid descent of a dense body, as a stone, is due to the gi'eater
resistance that the atmosphere oflers to the extended surface of
the light body ; this may oe shown by means of a thin lamina of
heavy matter, as a piece of tin-foil or gold-leaf, which, when rolled
up into a compact ball, will descend as rupidly as a stone.
The same may be shown by removing the atmosphere, without
altering the form of the light body. Let a coin and a feather be
placed on two small brass shelves, a, h^ attached to the cover of a
tall glass receiver, Fig. 342 ; these shelves move on hinges, and
are kept in a horizontal position by means of a brass key, c ; on
turning this key the shelves are released, and
the coin and feather fall, the former reaching
the plate d sooner than the latter, owing to the
resistance of the air. Let them be replaced, and
the air exhausted by an air-pump; when the
bodies are now released, they will be found to
reach the plate d at the same instant, thus
showing that gravitation acts alike on both
light and heavy bodies.
This apparatus acts better when the descent
of the shelves, a, &, is aided bv a light spring :
and if the shelves be placed one above the
other, and a coin and feather be placed on each,
they may be released in succession, and the
experiment may thus be repeated in the same
vacuum. This experiment may likewise be
shown by means of a long and large glass tube, closed at one
end, and ground at the other, to fit a small receiver plate. The
bodies will fall from one end to the other, on suddenly inverting
the tube, when exhausted.
513. Mareet^s Apparattu ^or Artificial Betpiration. — ^A modi-
fication of the air-pump, devised by Dr. Marcet, affords the most
convenient means of producing artificial respiration. This instru-
ment, like the air-pump, consists of two cylindera a, v, the pistons
of which are simultaneously moved in opposite directions by means
of racks, and a wheel turned by the double handle b c. At the
bottom of each cylinder are two pipes, d, e, and f, o, in which
are valves opening in the directions of the arrows, d and f
are both connected with a pipe, through which the artificial re-
spiration is to be effected. The valve in f is kept closed by a
spring sufficiently strong to overcome atmospheric pressure, and
is opened by the rising piston in v coming in contact with an
adjustable stop on the roa h, which passes air-tight through tho
pialon. This atop limiti tbe range of nmtion of both pistoni, and
conBcquenllf [he unoont of sir injected nt eitth atmkc. Wh«a
ihe pUtOD ID A is dvprened, and ^, gU,
that ia V raiaed, the air, which
had preriouslj entered a throueh
t, pasaes throagh d into the re-
apirstion tube, and the riaing
Qneath it. But vhen the latter
fhlun reaches tho stop, Ihs vaUe
m r in opened, and that in d
doaing, tbe already reapired air
enlcrs ihrongh P, and filli Ibe
lacuum in v. On roTerain|t the
action, that is, raising tho piston
in A. and depnming that in v,
frei'h air panes into a. throngh b,
and the expired air in v is dia-
chirged throagh a, until the pii-
tonfi reach tho top of A, and the
botlom of V, when the aboxe .
&U. It hu Wn .howd (498,9) that lh(
. . . ic preisure of the air at a
given temperature Tariea as ita denaity; hence, if p„ be thodentity
of the air at 0° C. nnder the preutirc II, then Ilnfip,, when ft is
coastsnt. If A be the altitude of tho column of u]crcui7 aupporteil
k tlie preanure of tho air, o, its denailj at 0* C, and g Hie force
of gratitj, then n = {r. a,. A,- hence,
g.a,.k = ^.p, (a)
it. 46°, the air being free Irom m
constant preasare, and p„pi, its densities at the
' cantigTside, aa indicated by a mercurial ther-
iccording to the eiperiment* of Magnna and
u, = (l + 0'00366ox()ti„
286 PKBUMATICB.
provided the temperatare lies between 0* and 100" C, or does flot
much exceed thoee limits.
Since the quantity of air is constant, po • % = P< - ^«i hence
^, = (1 + 0003666 x«)Pf I
and hence n=f4 . po=M (1 +0 003665 x t) p^
It is the result of observation, that at the level of the sea, in lat. I,
^=32-17237 (1 - 000256 x cos 2 Q feet ;
hence, substituting these values, and that of fi in (a), we obtain
the density of mercury at 0" C divided by the density of dry air
at t"* C. under the pressure of a column of mercury at 0' C, h
inches high, at the level of the sea in lat. {,
314688 1+0 003^65 x«
X
h 1— 000256 X cos 2 Z
516. If two vessels, of which the capacities are u and v, are
filled with the ffases a and b, at the same temperature and pres-
sure, and whicn do not act chemically on each other, and a com-
munication be opened between them, it will be found that in the
oourse of a short time a mixtiire of the gases will be equally dif-
fused through both vessels (51), and the temperature remaining
the same, the pressure will remain unaltered.
If a volume u of A, at the pressure h, be mixed with a volume
V of B, at the pressure n, then what is the volume to of the mixture
at a pressure p, the temperature remaining the same throughout?
M N
Under the pressure r, the volumes of a, b, will be — tt,-t7,re8pec-
f p
tively ; but if the gases, after being mixed, occupy a space equal
to the sum of their volumes before mixture, their pressure will
still be p ; tborefore, under the pressure p, their volume to will be
- tt + - 1? : therefore, p«7=Mtt + N».
If tt = o = 10, that is, if the original volumes were equal, and the
mixture be compressed into the same volume, then p=ii + n; or
thepreMiure of the mixture = the aum of the original preB9ures.
517. When a volatile liquid is introduced into a vessel contain-
ing air, precisely the san^e effects are produced as in vacuo, except
that the vapour is formed more slowly ; the quantity of liquid
finally converted into vapour is the same as if the vessel contained
no air. Let m be the pressure of the air before the introduction
of the liquid, and p the pressure that the same quantity of vapour
would exert if the vessel contained no air. The volumes of the
air, of the vapour, and of the mixture are the same ; therefore
(516), the pressure of the mixture = m +p.
PRBflBUnS OF VAPOUR.
287
518. If a small quantitj of any liquid capable of affording
vapoQT be introdncea into an exhausted vessel, it will be almost
instantly filled with vapour, the pressure and density of which are
found to depend only on its temperature, provided the whole of
the liquid be not converted into va|>our. If the space in which
the vapour exists be increased, a frebh portion of the liquid will
take the form of vapour : and if it be diminished, a portion of the
vapour will return to the liquid state; but the pressure and
density of the vapour will remain the same in either case, pro-
vided the temperature undergoes no change. If the temperature
be increased, a fresh portion of the liquid will be converted into
vapour, of which the pressure and densify will consequent] v be
increased ; and if the temperature be diminished, a portion of the
vapour will return to the liquid state, and its density and pressure
will be diminished. If the space be suflSciently increased, the
whole of the liquid will assume the form of vapour : under
these circumstances, the relations between the pressure, tem-
perature, and density of vapours are very nearly the same as for
air.
The pressures exerted by the vapours of varioos fluids in contact
with the fluids from which they nave been produced, have been
determined experimentally, and empirical formulsB have been con-
structed, whicn, within a certain range of temperature, express
the results of these experiments with considerable accuracy : yet
hitherto no law has been discovered by which the pressure at any
given temperature can be determined.
519. The pressure of the vapour of water at any temperature
from — 5° to 110** C. in millimetres of mercury at 0% as ODserved
by Magnus, is very accurately represented by the formula,
7'44ff6i
P = (4-525) 10 or log P = log 4525 + 2^69 +t.
The following table gives the corresponding values of t and Pat
various points of the centigrade scale :
t.
P.
t.
P.
*.
P.
t.
P.
-5*
... 8115
25'
... 23-582
55"
...117-378
85«»
... 432-295
0
... 4-525
30
...31-602
60
... 148-579
90
.. 524-775
5
... 6-471
35
...41-893
65
... 186-601
95
... 633*305
10
... 9126
40
...54-969
70
. 232-606
100
... 760-000
15
..12-677
45
...71-427
75
... 287-898
105
... 907-157
20
...17-396
50
...91-965
80
...353-926
110
...1077-261
In DaIton*s table, the degrees are those of Fahrcnheit^s scale, as
follows, the pressure at 80"" F. being the unit : —
288
FNEUMATICB.
t.
P.
t.
P.
t.
P.
t
P.
32r..
0-200
60'...
0-378
70'...
0-721
90 ..•
1-36
35 ...
0221
55 ...
0-443
75 ...
0851
93 #.«
148
40...
0-263
60 ...
0-524
80 ...
1-00
96 •••
1-63
45 ...
0-316
65 ...
. 0-616
85 ...
117
99 •>.
1-80
520. When the pressure of a flnid is expressed in " atmospheres/*
an atmosphere denotes, in the metrical system, the pressure
of a column of mercury at 0** C. which is 76 centimetres, or 29*9218
inches high, at the mean level of the sea in lat. 45° ; this, in round
numbers, is taken to be 15 lbs. upon each 8<^uare inch of surface.
If T be the temperature of steam, and p its pressure in atmo-
spheres, it is found that up to 224'' C, and probably much higher,
T = 100 + 64-29512logp+13-89479(logp)«
+ 2-909769 (log p)> + 0-1742634 (log p)<.
log 64*29512 = 1-8081780, log 1389479 = 1-1428520,
log 2-909769 = 0*4638586, log0-1742634= 12412062.
The following table, calculated from the preceding formula, shows
the temperature, by a mercurial thermometer with centigrade divi-
sions, or steam at various pressuras from 1 to 45 atmospheres.
p.
T.
p.
T.
P. T.
p. T.
1 .
..100 0**
5-5
... 156-8°
12 ... 190-0°
21 .. 217-3'
1-5
... 112 2
6
... 160-2
13 ... 193-7
22 ... 219-6
2
.. 121-4
6-5
... 168-5
14 ... 197-2
23 ... 221-9
2-5
... 128-8
7
... 166-5
15 ... 200-5
24 ... 224-2
3
.. 1351
75
... 169-4
16 ... 203-6
25 ... 226-3
3-5
... 140-6
8
... 1721
17 ... 206-6
30 ... 236-2
4 .
... 145-4
9
... 177-1
18 ... 209-4
35 ... 244-8
4-5
... 1491
10
.. 1816
19 ... 212-1
40 ... 252-6
5 .
.. 1531
11
... 186-0
20 ... 214-7
45 ... 259*5
Fig. 344.
521. Elastic fluids, or gases, in escaping
from lateral orifices, produce a similar reac-
tion against the opposite side, and correspond-
ing tendency to motion, as in the case of
denser fluids (430). lliis may be illustrated
by a very common toy, now made by all glass-
blowers, consisting of a globular vessel, b,
Fig. 344, of thin glass, resting on a pivot at
From the opposite sides of the vessel
A.
proceed two tubes, bent at right angles to a
radius near their terminations. When a
little water is placed in the vessel, and heat
applied by means of a spirit-lamp, it will
become converted into steam, ana give to
VELOCITT OF 18901X0 GAB. 289
the apparatus, on escaping from the Literal tahes, a rapid rotatoty
motion.
Elastic flaids also appear to obey the conditions analogous to the
th^jorem of Torricelli (426), when escaping under the influence of
pressure* from orifices, unless the difference between the external
and internal pressure be verj considerable, in which case they
offer some exception to this law.
It is also extremely probable that, like denser fluids, gases
undergo, when escaping from apertures, a contraction in the dia-
meter of the current ; the area of the section of this contraction
appears to be equal to that of the orifice through which the gas
is escaping multiplied by the decimal 0*61 or 0*62.
522. One veiy remarkable phenomenon, connected with the
escape of a current of air under considerable pressure, must not
be passed over silently. M. Clement Desormesf has obseryed,
that when an opening, about an inch in diameter, is made in the
side of a reservoir of compressed air, the latter rushes out vio-
lently ; and if a plate of metal or wood, 7 inches in diameter,
be pressed towards the opening, it will, after the first repulsive
action of the current of air is overcome, be apparently attracted,
rapidly oscillating within a short distance of tne opening, out of
which the air continues to be emitted with considerable force.
This curious circumstance was explained on the supposition
that the current of air, on escaping through the opening, expands
itself into a thin disc, to escape between the plate of wood,
or metal, and side of the reservoir; and that on issuing from
the circumference of the plate, this current carries off with it
radially the adjacent particles of air, and thus an external current
towards the centre of the plate is produced, which keeps it in its
position. It is not improbable that a slight pressure against the
outer surface of the plate may be thus produced, but the real ex-
planation of the fact is founded on what has been already shown
(432) respecting the motion of a fluid in a divergent tube : for the
space between the disc and side of the reservoir may be sup-
posed to be divided radially into an indefinite number of diver-
gent tubes, to any particle in either of which the same resolution
of its pressure may be applied.
This fact may be reaoil^ demonstrated by attaching a flat cir-
cular brass plate about 2 inches in diameter, with a nolo in the
centre, to the end of a piece of tube, about half an inch in dia-
* The fonowing formula is Bemomlli's expreseion for the veloci^ of an
MOftpiBg gM,
where V is the relocity of the gM ; j», the internal, and p', the external
preMore, and 2ife ie a ooeflOcient eqoal to 166610 for gases at the temperature
«f 3r F. . _
t Annalee de Chim. et de Fhys. xzzvi. p. 69.
U
290 FHBDMATICB.
meter, through which a current of either air or ^ter may be
discharged. On this plate place another of equal size, with a
projecting pin in the centre, to enter the hole in the former plate,
ana thus to prevent the latter sliding off laterallpr. It will t^en
be found not only that no amount of current will displace the
loose plate, but also that if the loose plate be placed downwards,
its weight will be sustained by the diverging current; and an
additional weight likewise, if the current be sufficiently strong.
A circular plate of the thickness of a shilling may be sustained
by the force of the breath alone.
523. When the air is put in motion, the currents produced are
denominated winds, and are tolerably uniform for a given space.
The following table* gives a view of the rapidity of currents pro-
ducing winoB of various forces ; the numbers representing the
velocities reckoned in feet per second : — .
1*64, scarcely perceptible wind;
3' 18, sensible breeze;
6*56, moderate wind ;
18*04, brisk wind ;
32*80, strong wind ;
65*70,. violent wind;
73*80, tempest;
88*56, violent tempest ;
11808, hurricane;
I 147*60, violent hurricane ;
the latter sufficiently powerful to tear up trees, and to produce the
most violent mechanical effects.
Marriotte has shown that a wind moving at the rate of 12*78
feet per second, impinges against a surface of 395*67 square inchea
with a pressure equal to 2696 grains, or more than 5^ ounces.
524. Anemometer$. — Instrnments designed to indicate the
pressure or velocity of the wind are called anemometers. One of
the oldest of these is Lindas anemometer. This instrument con-
sists of two vertical glass tubes about six inches long and half an
inch or more in diameter, connected by a bend at the bottom, and
one of the open ends bent down into a horizontal position. This
tube is partly filled with water, and so attached to a vane, that the
horizontal open mouth may always be turned towards the wind.
The pressure of the wind acting on the surface of the water in the
tube raises a corresponding column, the height of which is
measured by an attached scale.
By making a hole in that tube in which the water a«eefu2f, jnst
above its surface when at rest, the instrument will be rendered
self-registering of the maximum force or velocity between the
periods of observation : for it is evident that all that would have
been raised will have flowed out at the hole, and therefore that the
height of the column due to the greatest pressure will be twice the
interval between the surface of the fluid when at rest, and the hole.
Dr. Robinson's anemometer consists of a vertical revolving shaft,
* Ann. de Bnreaa det LoDgitudet pour 18S8.
AXEUOXETERB. 291
vith four horizontal arms, canring hemispherical cups at their
extremities, Fig. 345, soplaced that the diametral planes may
be radial and Tertical. These cops
are found to revolve with one-third ^' ^^'
the velocity of the tpind. A train
of wheel-work actuated by an end-
less screw (203) at the bottom of
the revolviiig shaft indicates on a
dial the niunber of revolutions, and
consequently the aggregate motion
of the wind during a given time,
firom which the mean or average velocity may readily be found.
525. Self-registering Anemometers. — Various apparatus for re-
eistering automatically the force and direction of the wind have
neen devised by Whewell, Ostler, Dollond, Ereil, and others, ill
all of which the changes are recorded by two pencils, the points of
which rest perpendicularly against a sheet of paper moving uni-
formly by clockwork, the motion of the pencils being at right
angles to that of the paper. One pencil is suitably connected with
the vane, indicating the direction of the wind ; and the other with
a moveable disc subject to the pressure of the wind, and reacted on
by a spring, showing its force, or velocity, either of these quan«
lities being a measure of the other.
A very simple and ingenioas form of self-registering anemometer
baa recently been devised by Mr. Casella. In this instrument the
shaft of a Bobinson's anemometer (524) passes through a hollow
shaft on which a vane is placed, consisting of two flat plates
placed at a small angle with each other. Tbe vane-shaft is in gear
with a revolving steel die, having an arrow on its surface. A strip
of paper psasing between rollers is impressed with flgures repre-
senting tne velocity, as i^hown by the rotations of the cup-shaft ;
and a spring-hammer, actuated by a clock, periodically impresses
on the paper the arrow in its existing position, and thus tbe direc-
tion of the wind at the time is recorded. The whole apparatus
IB simple and compact.*
526. It would be unjust to the memory of a sincere and earnest
labourer in the field of science not to notice very briefly the
remarkable system of forecasting the changes of wind and iveatfaer
instituted by the late Admiral Fitzroy. He has stated, in an
account of his researches, that broad and shallow currents gene-
rally prevail, and that these are constantly in more or less
circuitous, but mutually opposed, progress; sometimes side by
• It is SDgKested to the inventor that the regiiter should be read on the
raised nde of the paper, as the pofif ion of the anew is then more sharplj
defined : also, that tne arrow shoold be »nrronnded by a fixed rins showing
the fonr cardinal and four intermediate points of the compass, to oe simul-
t«i>eoasly embossed on the paper ; this would render tbe reading the direc-
tion of the wind more easy taa certain.— Ssitob.
U 2
292 FVEUlfATlCS.
side, bat in contrary directions, sometimes superposed, and always
having lateral as well as direct prog^ssion. It appears to be
well established that there is a general lateral translation of the
mass of atmosphere towards the east in the north temperate zone,
while northerly, southerly, or other currents are in very various
movement, the actual movement at any given point being there-
fore of a composite character. It appearSi also, (as was the
opinion of Espy, confirmed by those of Dove and Herschel,) that
winds always set from places of higher to those of lower barometric
pressure, and that the meeting of such winds produces a circuitous
or cyclonic effect. Hence it appears that by the approximate know-
ledge now possessed of the progression of atmospheric currents, of
their relative breadth, ana oi the revolving eddies frequently
existing between their edges or boundaries, and by a compariaon
of the pressure, temperature, and other characteristics of the
atmosphere at each of numerous stations more or less remote from
each other, the conditions of atmospheric change may be known
over a considerable area, (within probably a radial dintance of
500 miles round London,) and a highly nrobable conjecture may
be formed as to what changes are impenaing.
In some cases the pressure of aeriform fluids is employed in
doing^ mechanical work, aa in the steam-engine (455— -458) the
principle of which might perhaps have been more appropriately
explained in the present chapter. Of these applications of atmo-
spheric pressure, a few illustrations may here oe given.
527. The Pneumatic Lever. — ^In the present system of organ*
building, all the heavy work of the performer in opening the valves
of large pipes, and moving the large parts of the machmery, is ao-
complishea by the intervention of the pneumatic lever. This
contrivance consists of a small rectangular moveable board, which
is flexibly connected with another fixed board of the same size
(like the sides of a common pair of bellows), and communicating^
with the wind-chest by a small aperture, which is closed by a
valve. In large organ pipes, such as the pedal pipes, the aperture
in the wind-chest that supplies the pipe is of considerable size,
and a fatiguing effort of the nand is required to open it, in conse-
quence of the necessary internal pressure of air on the valve. But
if the valve be connected with the moveable board of the lever,
the effort of the finger is required to overcome only the resistance
of the small valve of the lever, which itself immediately opens the
large valve of the pipe. Some other internal movements of the
instrument are similarly facilitated which it is needless to de-
scribe in detail.
In referring to the construction of organs, it may here be men-
tioned that the water-engine (459) is now frequently employed to
work the bellows, the opening and shutting of the valves being
effected b^ the moveable board of the ¥rind-ch68t, when it has
nearly arrived at its extreme positions.
i^
i^rm
JLIB AKD GAS BNGIKE8.
293
528. The Compre89ed Air Engine.— li has been proposed to
work an engine, constracted similarly to the non-condensing steam-
engine (457), by means of compressed air instead of steam : but
inaamucb as it would require a greater amount of work to com-
press the air, than that rendered oj the machine, by the amount
of loss by friction, &c., the proceediop^ can scarcely ever be expe-
dient. It has also been proposed to impart expansive force to the
air by means of heat, but tnis has not hitherto been economically
achieved.
629. Lenoir'9 Oas Engine. — The vacuum produced in the
cylinder of the condensing steam-en^ne, by the condensation of
the steam, is in this ingenious contrivance produced by the com-
bustion of an explosive mixture of air^ and ordinary coal-gas,
ignited by an electric shock from an induction-coil apparatus
(Ch. XV.). A horiasontal engine working on this principle is
shown in Fig. 346, in which a is the electric apparatus, b, the
communicator of electricity at the proper moment, and c, c, the
terminals of the coil, between which the exploding discharge takes
place. The air is admitted to the slide-valve chamber at d, and
the gas at E ; f is the exhaust pipe, for removing the residue after
each explosion, and a is the inlet, and h the outlet pipe for cold
water, to prevent the cylinder from becoming over-heated by the .
successive explosions ; i is an indiarrubber pouch, intervening in
the gas supply-pipe, to equalize the pressure, and therefore the
Jinantity of gas delivered at each stroke. This machine has been
ound very serviceable, whenever from any cause the use of a
furnace and boiler is inexpedient, and where at the same time a
supply of gas is available. It is especially conveui(fnt where the
moQve power is required only at uncertain periods.
294
CHAPTER X.
AOOOSTICS; THE PBODUCTION, TKAV83fISSION| AND PEBCBPTION
OF BOUND.
530. When the air, or any other elastic bodj, is made to assume
a vibratory motion, consisting either of a single pulsation, or of
a series of oscillations or undulations (469) repeated with sufficient
frequency, a sound is produced. During the existence of such
motions, the molecular arrangement of the vibrating body becomes
altered, but acquires its normal state on their cessation. Thus, if
a copper ribbon, 9 feet long, 0'4 inch wide, and 0*04 thick, be
vibrated, its length will appear unaffected. Let a weight of 90 lbs.
be fixed to its lower extremity, and still no change occurs, but if
again made to vibrate. Its molecular arrangement will become per-
manently affected, as shown by its length becoming increased
6 or? inches. When these vibrations take place in an uniform
and regular manner, as when a harp-string is struck by the finger,
a perfect sound or tone is produced ; but if the vibrations take
place irregularly, and are not isochronous, or if a single impulsive
disturbance of the air take place, as in the explosion of a pistol, or
the crack of a whip, a noise alone ensues. 1 ne transmission of a
single pulse through the atmosphere may be thus shown experi>
mentally. Place norizontally a tube about 10 feet long and two
or three inches in diameter, with a conical termination at one end,
tapering down to half an inch, and near the mouth of the oone
place the flame of a lighted candle. If now an impulsive move-
ment of the air be produced at the open end of the pipe, as by-
striking together two pieces of board, or the covers of two bound
books, a sudden displacement of the flame will immediately after-
wards be perceived, and if the flame be rightly placed, it will be
extinguished ; or if a little smoke be produced m the mouth of the
'tube, as by a bit of smouldering brown paper, it will be forcibly-
ejected.
531. When isochronous f378) vibrations are excited with suffi-
cient rapidity in an elastic ood^, not less than 16, or according to
some, than 30, in a second of time, the resulting tone or note ia
transmitted by the excitation of fresh and similar movements in
surrounding bodies, and in the air, extending on every side, like
the gradually widening circular ripples surrounding the spot where
a falling drop of rain disturbs the surface of a pool of water (463).
THE PBODDOnOX OF SOUND. 295
These eventually impingpo upon the membrane of the tympanum
or dram of the ear ; wmch then assumes a vibratory movement.
From this membrane tremulons motions are excited in the fluid
-with which the lahfrinth of the ear is filled, through the medium
of the air included in the tympanic cavity, and of the delicate
chain of bones connecting the tympanum with a membrane cover-
ing an orifice in the complex cavity of the internal ear ; and which,
actinj^ on the auditory nerve, produce that sensation of sound,
vhicn we reoognise as a definite tone or note.
532. The inferior limit of the number of isochronous vibrations
capable of blending into a definite tone may be determined by ex-
periment. A ver^ convenient apparatus for this purpose consists of
a mandrel carrying four flat wooden rays at right angles to each
other, and in the same plane, so as to pass successively through
a parallel slit in a piece of wood placed radially to the mandrel,
a pulley on which is driven by a multiplying wheel, and band.
The entrance of each ray into the slit is marked by a peculiar loud
impulse ou the air. Some difference of opinion will, however, be
found to exist between different observers, as to the prebise velo-
city of revolution at which the impression of a continuous tone is
produced. This the writer has found to depend in a ereat measure
on the sharpness and distinctness of the individual impulses : for
example, if the slit be set exactly parallel to the edge of the enter-
ing ray, so that eveiy part of it may enter the slit simultaneonslv,
a sncoession of sharply defined rounds will be produced, which ao
not so readily blend into a low note, as when the slit is set a little
obKouely to the entering ray, so that it may enter the slit, so to
speM, more gradoally ; in which case the separate impulses lose
much of their sharpness, and the intervals between them are less
defined.
dSS. Whenever no material substance intervenes between the
vibrating body and the ear, no sound is heard. If a bell be placed
imder the receiver of an air-pump (502), and the apparatus be
shaken, the sound excited by the clapper striking the sides of the-
bell is distinctly heard. Let the air be exhausted from within the
receiver, and the bell again agitated, the clamper will be seen to
strike its sides, but no sound will be audible ; in consequence of no
elastic medium existing of sufficient density to convey tne sonorous
vibrations to the sides of the receiver. A convenient
apparatus for this experiment is a bell with a lever escape- ^' ^*' •
ment (258) within it, to the anchor of which the clapper "^^^
18 attached, Fig. 347. A pulley on the arbor of the
scape-wheel carries a string, one end of which is attached
to tne end of a rod working air-tight through the brass
cap of a tall receiver, and the ouer end to the under
sioe of the brass cap. On raising and lowering the rod,
the escapement will drive the olapper ; and as the bell
is connected by the string alone with the rigid materials of the
296 A0OUBTXC8.
lur-pnmp, yeir little vibration will be tbus contacted, and in a
tolerably gooa Tacunm scarcelj any sound is audible.
634. Trayellers, on ascending loflj mountains, have noticed the
extraordinary diminution of the intensity of sound, in consequence
of the rarefied st^te of the atmosphere at considerable elevations
above the level of the surface of tne earth (478) : Saussure found
that on the summit of Mont Blanc, the explosion of a pistol ap-
peared no louder than the ordinary sound of a cracker. And con-
versely the intensity of sound increases, on increasing the density
of the air surrounding the sonorous body ; thus sounds which are
of ordinary pitch in the free air,acQuire a painful degree of intensity,
if heard in a reservoir of condensed air, or in descending in a diving-
bell, in which the air is condensed by the upward pressure of the
water (397).
535. The intensity of sound, like that of attraction (31), dimi-
nishes in the inverse ratio of the square of the distance of the
sounding body. This law, however, applies with its fiiil force only
when opposing currents of air, or other obstacles, do not interfere ;
for the sound of a church-bell is inaudible, during a contrary wind,
at the distance of a few yards, while the sound of the cannonading
at Waterloo is said to have been heard at Dover; and the noise of
a sea-fight between the English and Dutch, in 1672, was heard at
Shrewsbury, a distance of 200 miles. In these cases the intensity
of the sound was no doubt preserved through these distances, by
the presence of aerial currents, moving in the directions in which
the sounds were heard.
536. From the researches of Dr. Derham, the intensity of sound
is modified by —
A. the direction and velocity of the wind ;
B. varieties in barometric pressure ;
c. chsnges in the temperature of the air ;
D. its hygrometric state ;
E. the original direction of the sound ;
F. the nature of the surface over which the sound passes.
Sound is heard with great distinctness over a considerable
space, in a frosty air undisturbed by winds or aerial currents.
Lieutenant Foster, in the third Polar Expedition of Captain Parry,
held a conversation with a man across the harbour of Port Bowen,
a distance of one mile and a quarter. .
537. Resonance. — The intensity of sound is found to be consi-
derably anemented, if the vibrations be confined in tubes or cavities
of any kind : of this we have a familiar illustration in the speak-
ing-tube, by which the voice is conveyed from one part of a build-
ing to another, freouently to a considerable distance, and by a
circuitous route. Tne stethoecope, an invaluable means of ascer-
taining the physical signs of pulmonary and other diseases, is
another application of the same principle. Biot found that the
■ligbtest vliUper vss haard thioDgh an iron pipe 3120 feet long.
If ibe handle ofa vibrating tunine-fork (Fig. 3A1) bereiteda«aiiut
the head, and one earbeclogedbjthe pBlmofthe liaQd,imm«diatetj
after Ihe umnd of the tDning-fork ceues to be heard, it* vibration
will immediately be perceived by the closed ear, even althonph
the fork real on a portion of the ikull coiitignoDB to Ihe oppoaite
ear. In this experiroent the vibrations transmitted through the
Bolid ilniclureB of the akull feebly affect the sentient o^an, nntil
they are amplified by reverberation in the closed cavity of the ei-
temal ear, and thus affect the tympBDom. The experiment
farther ebows that sonorous impreasions are transmitted te the
sentient organs principally, if not entirely, by the atmosphere, and
Dot by the boneaof the head, as some phy Biologists have snppoeed.
638. This lact of small sonnds beiog ampHfied by closing the
external cavity of the ear, and thus augmeDtin^ il« resonance, is
the principle of Wheatatone's mierophons, which consista oif a
Y ibaped metallic rod, the two upper ends of which are bent in
lowarda each other, and terminated by small fiat cironlar plates,
when these are placed over the two Burs, any feeble —^ ^^
■omid, tranamitted tbrangb the stem, wilt be more
Wadilj perceived, than if simply conveyed to the i
•ar throDgh the medium of the atmoophere.
539. The Jietonator of MelmhoUz.—Ttiii is a
transmitted throngh the atmospber
partdallj closed cavity (in this case, cylindrical) i, Fig.
S48, connected by a flexible tube with a small rennded
ivory tube, b, to be applied to Ihe ear. Any sound,
to which the cavity of a reciprocates, will by its uae
be considerably angmented, and thus more readily
perceived by the ear.
640. Sedpmcation of Soimd. — When the air is in ■ state of
■onoroas vibration, it excites similar movements in bodies with
which it is in contact, if they are susceptible of isochronoDS
vibratory movements. This may be shown by tuning two harp,
violin, or guitar-strings in unison : on causing one to sonnd, the
•ir Borrounding it assamea a vibratory movement, and, this being
propagated to the second string, cansea it to vibrate, and emit a
sound or tone, because each aerial pulse communicates motion to
the second string, and as the movements of both are by the snp-
podlioD isocbronooB, each aucceeding impnlss augments the effect
of the preceding one; tbis phenomenon is termed the recipromfion
of found. If an ordinary tuning-fork be held over the mouth of a
tube about 20 inches long, and turnisbed with a piston, it will be
found that in some position of the piston the tube will yield the
aame note as the tnniug fork, while if moved an inch m eitbsr
direction the tnbe will remain perfectly mute. Id this case, when
the tnbe reciprocates, each puliation of the air prodaced by a vibia-
998 AOOCBTICB.
tion of the fork must trayel down the tuhe, and being reflected
from the bottom, reach the fork just in time to be reinforced hy
its next vibration. In this experiment the resonance of the tnbe
is much louder if the open mouth be partially closed bj a cap
having an orifice in its centre. Instances have occurred of persons
who, bj modulating their voices, have excited vibration in glassee,
BO powerful as to overcome the cohesive attraction that held the
particles together, and consequently, to break them in pieces.
541. Waves on the surface of water, unless they differ very
freatly in sizej are capable of passing over each other without
eiug destroyed. And in a similar manner, in the case of the
waves of sound, or sonorous vibrations, excited by a crowded
orchestra, an attentive ear can readily distinguish the sound of
each particular instrument. These are individual applications of
the principle of the fuperposition ofimaU motions, which may
be thus enunciated generally : —
If the partidea of which a body is compoied are actuated hy
several small disturbing foreeSf thev will obey each u» the same
manner as if it existed alone ; and the motion of a partide in any
given direction is the algetiraic sum of the motions that would
result from the disturbing forces acting separaUly.
642. AU sounds, in traversing given distances, are propagated
with equal rapidity, passing through spaces proportional to the
times : this is evident from the fact that the music of a band is
correctly heard at anv distance at which it is audible, thus show-
ing that all the sounds reach the ear in their proper place, that is,
at equal intervals of time, whatever may be the length of their
undulations. Sir John Herschel* has shown that, in round
numbers, sounds of every intensity travel at the temperature of
62* F. at the rate of 1125 feet per second, equal to 9000 feet in
8 seconds, 12) miles per minute, or 765 miles in an hour. At a
freezing temperature and in perfectly dr^ air, the rapidity of the
propagation of sound is diminished; as it traverses 1092 feet, or
364yards in a second.
Tne velocity of sound obtained by theory is about 175 feet per
second less than that obtained by observation; this difference
depends on the effect of heat disengaged by compression of the
air by its own vibrations. The familiar contrivance for lighting a
bit of tinder, or amadou^ hj suddenly forcing a piston to the
bottom of a closed cvlinder, is a conspicuous example of the dis-
engagement of heat by the sudden compression of air. It must
not^ however, be supposed that the temperature of the body of air
is elevated by the passage of sound, for there is as much <ild pro-
duced at the points of rarefaction, as there is of heaJt at thoee of
condensation, and the augmentation of velocity is alike due to
both these causes ; both tending to accelerate the recoil of each dis-
placed particle, and therefore to augment the velocity of the wave,
* Bnc. Matiop. Art. Souiro.
VELOCITY OF SOUXD.
299
The exact fonnuU for determining v, the velocity of sound in
feet per secondi is
r=1090-8^(l +0003666. 0(1 +0-375g)
where t is the centigrade temperature, Y the pressure of vapour,
and n that of the air, at the time of observation.*
543. From more recent observations on the influence of tempe-
ratore on the velocity of sound in air, it appears that within wide
limits the velocity of sound is augmentea by 1*6 feet, for each
increment of 1** C.f
The velocities at the following temperatures have been ascer-
tained by observation : —
Tel.
Tel.
0 5C. .
. 1089
16-0 C. .
. 1112
2-0,, .
. 1092
26-6 „ .
. 1130
SO,, .
. 1100
544. By knowing the velocity of sound per second, a close ap-
proximation to a knowledge of the distance of a sonorous body
may frequently be obtained. Since light travels with an enormous
velocity as compared with sound, the distance in feet from the
source of an explosion may be ascertained by observing the number
of seconds elapsing between the appearance of a flash of light, and
the instant when the sound, produced simultaneously with such
flash, ia heard ; then multiplying this number by 9000, and di-
viding the product by 8. The following is an example of this : —
A flash of lightning is seen 12 seconds before tne thunder is
heard ; what is the distance of the cloud \^ere the explosion
occarred?
12 X 9000 = 108,000 -f- 8 » 13,500 feet.
545. Sound travels through different bodies with very different
degrees of velocity : the following velocities have been observed : —
Bobitanoe.
Carbonic acid
Olefiant gas
Oxygen . .
Air . . .
Hydrogen .
Turpentine .
Alcohol '. .
Sea water .
Temp.
Vel.
0*C.
858
i>
1030
i>
1040
II
1092
11
41640
24
3976
20 .
4218
II
4768
Sabetanoe.
Biver water
II
II
Lead . .
Gold . .
Copper
Steel wire
Iron . .
11
Temp.
15' C.
60
18
II
II
II
II
200
Vel.
4714
5657
4030
5717
11666
15470
16822
15483
« Miller's HydroaUtict, p. 00.
t C. denotes the oentignde scale.
300 ACOUSTICS.
It ma^ here be observed that while the velocity of sound is in-
creased m water, it is decreased in iron by increase of heat ; this
might have been anticipated, from the well-kuown diminution of
elasticity in metals, by elevation of temperature.
It may also be remarked that in gases at the same tension, but
of different densities, sound travels more rapidly as the specific
gravity becomes less ; thus a diminishing scale of velocities is met
with in hydrogen, air, and carbonic acio. Similarly if the tension
in both cases be equal, sound will travel more rapidly in a warm
atmosphere than in a cold one.
It is equally a law established by observation, that in gaseous
fluids of tne same density, but of different tension, the Telocity of
sound increases with the tension. This may be readily ascer-
tained, by measuring the velocity of sound in air contained in a
closed cavity when heated : the Telocity will be found to increase
with the temperature.
646. The Telocity of sound in wood has been observed to be
Tery different in three directions ; in the longitudinal it is more
than double that in any other direction ; and of the two transverse
directions — Tiz., radial, or euross the layers, and tangential, or
parallel to the layers, that in the radial cUrection is the greater of
the two. This has been ascertained in seTeral kinds of wood : —
Long.
Bad.
Tang.
Pine . .
. 10900 . .
. 4611 . .
. 2605
Oak .
. . 12622 . .
. . 6036 . .
. . 4229
Elm .
. . 13516 . .
. . 4665 .
. . 3324
Poplar
. . 14060 .
. . 4600 .
. . 3444
Acacia
. . 16467 .
. . 4840 .
. . 4436
It has been observed by Prof. T^ndall that the law of transmis-
sion of heat in wood corresponds m a great measure with that of
sound.
647. Sound is not transmitted with equal facility through all
media: thus, various gaseous mixtures assume sonorous vibra-
tions with extreme difficulty. The sound of a bell under a re-
ceiver full of hydroeen gas is, according to the experiments of Dr.
Priestley and Sir John Leslie, scarcely louder than when placed
under an exhausted receiver (604). When hydrogen is respired,
the voice of the person undergpes a curious change, being rendered
extremely feeble and raised in pitch, as might be expected, from
the lungs and windpipe being filled with a rarefied medium.
The fiEkcilit^ of transmission of sounds is, like their velocity,
ereater in fluids than in gases, and still mater in elastic solids.
If a musical box be attached to one end of a series of firmly united
deal rods, 100 feet or more in length, the sound will be distinctly
heard by an ear placed close to, or in contact with the other end,
when it would be <|uite inaudible without the intervention of the
rods. If the sounding-board of a Tiolin or guitar be now placed in
lOOXD. . 301
eantact with the other end, and perpendicnlar to the direction of
(he rod, the musical KMiDdswill be greatly dereloped, aad the tones
are a singalar mixture of those at the boi and iriHtruiaaDt. This
eiperiment viU aacceed beat at loog distances, if the rod be ana-
pended bj threads or strings, so that the looeitudinal Tibratiuns
(3IS) a( the rod majr not be airesced br the contact of soUd
matter. A fina cooneiion of tbe consecutiTe piccea of the rod is
essential to the sQccen of this eiperimenl, la otherwise (he vibra-
tioDs are considerably weakened by their tranamiasion from one
piece to the other, and become imperceptible at a comparatiTely
short diatince. Similarly the intenention of any portiooa of
eUstic or yielding matter between two adjacent pieces of rigid
matter is fonnd materially to interfere viUi tbe traDsmiasion of
648. Sounds generated in air are indistinctlT hsard by a person
immeraed in water; bat if excited in that fluid, the/ are conveyed
to a coDuderable distance with facility. M. Colladon heard Ibe
aonnd of a bell Mmck under vat«r acroai the whole breadth of
tbe Lake of Geneva, a distance of nine miles ; thia aound ap-
peued to pass through the water with a trelocity of 4708 feet per
549. Inierferenix of Sound. — Two sets of aonorous ribrations
of equal intensity, encountering each other in oppoiite phases of
TibrAtion, will mterfirt, and become inotuBJly checked ; and thus
aiknce will be produced by the conflict of two sounds. To under-
stand this intarl'etence of Bon'>rous vibrations, let us suppose tbat
two aeries of vibnitioDa, occurring simattaneonsly, are superposed
on eacb other (Ml] aa a, b. Fig. 349, so related that some gireo
number of nhrations iu the series a, 1! for example, may coincide
iu dmation with one more, or one lesa, aa 13, inn; aJaolet the two
aeriea be in opposite phases at the points \ and n, then after the
pninosed nnmbera of vibrations have taken place, they will again
be in opposite phaaea (369) at the same point, as at □. But at
the poiut D, midway between a and a, one aeriea is half ■ vibra-
tion ID advance of the other, and they will therefore be at that
point, HI tAe tame phatt; consequently, if the two series be of
equal intensity, they will neutralize each other at A and c, and
grestly inteDsify each other at n (496), and a compound aeries of
undulstioDS, B, periodically increasing and dimilliBhing in in-
302 . ACOUSTICS.
tensity will result. For the sake of illastration, the vibrations
are here supposed to be transverse (376) ; but it must be borne in
mind that sonorous vibrations in the atmosphere are, for the most
Eart, longitudinal (376). This result may be readily exhibited
y opening at the same time any two ac^acent notes in the bass
of the organ, when a curious pulsating sound will be heard, to
which the term " heat '' has been appropriately applied. When
the beats recur at sufficiently short mtervals to produce on the
ear the impression of a continuous sound, as in f (Fig. 349), a
new note is heard which is necessarily lower than either of those
which conspire to produce it ; this is known as the Orave Har-
monic.
A curious experiment by Biot illustrates this point: — If an ob-
stacle be so placed in the way of a vibrating string, that it shall
be struck by the middle point of the string after each semi-vibra-
tion, the note thus produced will be a fifth below the fundamental
note of the string, whioh is the note that will result from its entire
and uninterrupted vibration.
550. Another kind of pulsating or intermittent sonnd (not
however to be confounded with the " beat " resulting from inter-
ference) will be produced, if sonorous undulations be successively
accelerated and retarded in batches : this may be effected by the
apparatus of M. Mach, which consists of a hollow arm attached
radially to a hollow rotating spindle, through which it can receive
a constant current of air, to actuate a free reed (666) placed at its
extremity. The rotating arm must be counteipoised. When
this apparatus is rotated, if the ear be placed anywhere in the
axis of rotation, a continuous note will be heard ; but if the ear
be placed in the plane of rotation, the vibrations of the reed will
be accelerated on its approaching, and retarded on its receding
from the ear, and a Kyand of alternately higher and lower pitch
will be perceived.
551. Flumographi.-^'ilLaiy varieties of compound, or resaltant,
vibrations have been automaticallv delineated by means of an in-
genious apparatus designed by MM. Scott and Koonig. This may
be briefly aescribed as a larpe conical drum, one head being much
smaller than the other, when two or more different sets of
vibrations impinge on the larger drum-head, the resultant series
is transmittea b^ the intervening air to the smaller, which actuates
by mesns of a simple and appropriate mechanism, a tracing point
that rests on the surface of a cylinder, so that the motion of the
tracing-point may be parallel to the axis of the cylinder. The
cylinder rotates hy clockwork, and is surrounded by paper, the
surface of which is covered by finely levigated lamp-biacK. An
extremely small amount of force is required to remove the lamp-
black sufficiently to render the trace visible ; this is subsequently
fixed by damping the- pnper, when a sufficient quantity of the
lamp-black adheres to the size with which the paper has been
COMPOUND ▲GOnsnC CUHVE8. 303
prepared ; the paper is Bubsequentlj vamigfaed, to preserve the
Imce. As the direction of the continaous rotation of the cylinder
is at right angles to that of the alternate motion of the tracer,
when both motioDS coincide, an undulating line is traced on the
paper, yerj similar to those represented in Fig. 349. The re-
sultant vibrations arising from the consonance of notes at the
various mnsical intervals are extremely interesting.
552. Another equally ingenious form of phonograph has been de-
▼ised by MM. Lissajous and Desains. In this the vibrations of one
or more large diapasons are commnoicated to a lever carrying at
its extremity a fine tracing-point, moving, as in the preceding,
parallel to the axis of the rotating cylinder, similarly prepared.
If two or more independent vibratory movements be simulta-
neonsly communicated to the tracing-point during the rotation of
the cylinder, it is evident that the resultant curve, representing
the combined motions, will be traced out. Some very curious
resnltant corves have also been traced in which the movements
were at right angles to each other. A very copious and interest-
ing series of tracings was exhibited by M. Koenig, at the Inter-
national Exhibition of 1862: amongst these were the actual
vibrations of the human tympanum (or drum of the ear), and even
those of the membrane of the fenestra ovaUs were found to have
been delineated.
553. Instruments have likewise been constructed for the optical .
demonstration of these acoustic curves. For this purpose a small
plane metallic speculum is attached to the end of one branch of
each diapason, and a counterpoise to the other. A small bright
pencil of light, as that transmitted through a small hole in a
copper chimney covering a naphtha-lamp, failing on the speculum
18 reflected on to a screen; or still better, a small pencil of
electric li^ht is employed. If the diapason be vertical, when
made to vibrate, the spot of light on the screen becomes a short
vertical line : when this is received on a mirror capable of rotating
on a vertical axis, and thence thrown on to the screen, the rota-
tion of the mirror will resolve the line into a horizontal series of
imdnlations. If the pencil of light from the first diapason be
received on the mirror of a second placed horizontally, and thence
reflected, fall on the screen, when both are thrown into vibration,
very beautiful and ever-changing resultant curves will be deli-
neated. The simplest of these are produced when the two periods
of vibratioTi are as 2 : 1 ; when either differs very slightly from
this ratio, the series of curves represented in Fig. 256 will be
Eiodnoed. When the ratio of vibrations is that of larger num-
ers, as 2:8, 4:5, &c., more complicated figures wfll result.
When the dissonance is a little greater than that just mentioned
(which can be readily effected by a slight alteration of one of the
weights appended to the diapasons) a very carious undulating
dhange of the figures takes place.
304 ACOUSTICS.
Various acoustic apparatus have been constructed, in which
electro-maguets have been employed to actuate of control the
vibrations of diapasons : but want of space forbids the detailed
description of these and many other objects of great interest to
the earnest investigator of physical truth.
554. The result of intenerence may be shown by vibrating a
common tuning-fork or diapason, and
^'^' holding it over the mouth of a
cylindrical vessel, a, Fig. 350, of a
suitable length, that the air con-
tained in it may assume synchronous
vibrations, and produce the same
note. Then hold a second similar
cylinder in the direction of b, at
nght angles to a, as shown in the
figure, and immediately the musical
tone previously heara will cease;
withdraw b, the tone reappears ; replace it, and it again disap-
pears, and so on. These curious phenomena arise from the
mutual interference of the sonorous vibrations excited in the air
contained in the two glass vessels.
The following experiment by Prof. Wheatstone presents a re-
markable example of interference. Let the handle of a vibrating
tuning-fork, held obliquely, rest on the surface of a table : as long
as it remains at rest, a loud resonance of the table is audible ; but
if the tuning-fork be moved parallel to itself along the surface of
the table, the resonance of the table immediately ceases, from the
perpetual interference of the planes of vibration with each other*
The instant the tuning-fork stops, the resonance bursts out again
in a very striking manner. If the tuning-fork be held vertically,
the planes of vibration coincide, and the resonance is not inter-
rupted bv moving it.
555. Again, when a tuning-fork vibrates, its branches alter-
nately recede from and approach each other, as shown
^.361. ]yy tijQ dotted lines in Fig. 351, both communicating their
ms movements to the air, and producing a musical sound.
\| // Let a fork, whilst vibrating, be held upright about a foot
\ / from the ear, and slowly turned round. It will be found
that when both branches are equidistant from the ear, or
in the same direction from it, a distinct tone is heard,
whilst in all intermediate positions scarcely any sound
I can be detected. This is explained by the fact, that when
J the branches coincide, or are equidistant from the ear, the
A waves of sound combine their effects, whilst in all inter-
H mediate positions, as they reach the ear in different phases,
they interfere, and produce total or partial silence.
A similar result of interference is obtained by attaching a
tuning-fork to any rotating mandrel, so that the length of the fork
IZA1CPLB8 OF IHTSRrERENCB. 305
may coincide with the axis of motion : if the fork he made to
vibrate, no sound will be heard, so long as it continnes to rotate,
bat will become andible the instant tbat the rotation ceases.
This result is best shown b^ placing a reciprocating cavity, as ▲,
Fig. 350, beneath the rotatmg tnning-fork.
556. The passage of sound through heterogeneous media com-
posed of substances of different degrees of elasticity, is effected
with difficulty ; for in passing from a less to a more elastic por-
tion, sonorous waves of different intensities are excited, which,
partly being reflected (464), and partly from mutual interference
(466), become broken up, as it were, into numerous secondary
vibrations ; and thus the sound, which eventually reaches the ear,
will be not only of less intensity, but of a different quality from
the true one.
If some portion of a mixed medium be capable of conducting
sound more rapidly than others, some vibrations will reach the
ear before the others, and a confused false sound will alone be
heard. We have an example of these facts in a glass vessel filled
with carbonic acid ; this, when struck, instead of emitting the full
tone proper to it, will merely produce an irregular flat sound : here
the medium in which the vessel is immersed, the air, is of very
different density and conducting power from that with which the
glass is filled, and accordingly, vibrations of different intensities
are excited, which, probably, by their interference deaden the
proper tone of the giass yessel. Humboldt explains the fact of
sounds being more readily audible at night than in the day, by the
greater homogeneity of the atmosphere at that time : in the day-
time its density is constantly changing by partial variations of
temperature.
557. The comparatiye conducting power of different media for
sound was well iuustrated by an experiment made by Biot. This
philosopher fixed a bell at the end of a long iron tube ; on striking
It, two consecutive sounds were heard by an observer at the oppo-
site end, one conducted by the iron itself, the other by the air in
its interior. The well-known double report of a fowling-piece,
fired at a distance, probably arises from a similar cause, the sound
of the explosion bemg condncted to the ear, unequally by the air,
and the masses of vapour floating in it.
iKmilarly, if the ear be placed near the surface of a rock which
is beine blasted at a considerable distance, a distinct double re-
port is heard, the first transmitted through the substance of the
to^ the second, through the atmosphere.
A curious phenomenon of this kind was once observed by the
writer, at the Boyal Observatory. Greenwich, during the firine
of the Tower guns, at a time when the atmosphere was loaded
with a dense stratum of vapour. Each report was deadened, but
preceded by four or five smaUer reports at equal intervals, and
then a longer interval} thus : - - - -
z
806 AcouanoB.
658. The BOond-wtTes in their traDemission throngh aiij me*
diuiD, or from one medimn to another, will alwaja maintain the
same direction : thoe if a thin disc of wood be fixed transrerBeljr
at the end of a rod through which Tibrations are travelling loon-
tudinally, the eonod transmitted throngh the air to the ear will
be greatly augmented, because the Tibrations maintaining their
direction become normal in the disc, and therefore act upon the
air by a much extended moving Burface.*
If one end of a vibrating rod be fixed perpendicularly in the
Bide of a vessel in the form of a rectangular parallelopiped full of
water, and the rod be made to vibrate first vertically, and then
longitudinally, corresponding movements will be observed on
the surface of the fluid in contact with the agitated side of the
vessel ; and if a glass plate strewed with sand be freely suspended
by threads in the water, the movemente of the particles will indi-
cfkto vibrations in the fluid agreeing in direction with those of the
rod.
If a thin lamina of deal be placed horizontally between two
parallel portions of the
'V* ^**« ^ same chord under equal
tension, Fig. 852, and
strewed with sand, the
particles will indicate
either normal or tan-
gential vibrations in the lamina, according to the direction in
which the chords have been excited by a viulin-bow.
Again, if several small horizontal lamine of wood be separated,
and also firmly connected,
J^. 8«8. by small blocks of light
/ / vrood at their centres, and
— — a vibrating chord be a^
tached to one of the series,
Fig. 853, similar vibra-
tions in all will be indi-
cated by sand strewed on
their surfaces.
559. In obedience to this law of continuity of direction in the
propagation of sound, sonorous undulations must reach in their
original directions the complex recipient and percipient structurea
of the internal ear. Now it is a remarkable fact, that in all orders
of the vertebrate kingdom one portion of these structures is met
with in much the same perfection of development, even though
other complex auditory structures, met witn in all the higher
orders, are either wholly absent, or represented by less complex
organs ; this is the system of semicircular canals, consisting of
* Thia prindple hM been siiooenfblly applied by the preeeiit editor totba
deteetion of nnaU Teaieal oaloali or fregmento, in ladiTlaiiftlB salTering front
their presenoe \ eepedally wibeeqoent to the pnetioe of ttlhotiity.
RBFLICTIOS OP 80UHD. 307
three curved tabes, more or leas exactly Bemicireiilar, Ijing tn-
tHMriaUjf in three planes exactly perpendicular to each other, filled
with a conttnuouB fluid, and opening into a common cavity ; the
memfarane lining these tubes oeing freely supplied with branches
of the auditory nerve. It scaroely admits of a doubt that when a
•oond-wave is propagated in its original direction to these canals,
each is influenoea by a portion of it resolved in the direction of its
own plane, and the relative intensity of the resolved portions will
depend on the direction of the undulation ; and thus oy means of
the relative force of the impressions made on the nerves of the
three canals, an impression of direction is instinctively conveyed
to the sensorium, or common recipient of sensuous impressions.
The mechanism of the auditory apparatus is most simple in
those orders that are destitute of vocal organs, the fish-tnoe for
example ; but for these a perception of the direction of impending
danger is as important as for the higher orders, and hence the
perfect development of their semicircular canals.
560. £e/e(ium of Sound. — Sonorous vibrations, on impinging
Qpon a pliuie surface, are reflected from it in such a manner, that
the angles of incidence and reflection are equal, in the same
manner as in the case of the collision of an elastic body against a
plane surfiice (296) : the velocity and intensity of the sound- con-
tinuing the same aSfter as before reflection (see, also, 464).
561. When a sound is reflected, and reaches the ear altera
certain interval, an echo is produced ; for this to be perfect, the
observer must be at a certain distance from the reflecting sprface;
and the syllable will be repeated once or several times, according
to the number of reflecting surfaces presented by the body against
which the sound impinges. The reflecting plane must be at a
greater distance to affora polysyllabic echoes. ' At Woodstock is
oue of this kind, repeating from seventeen to twenty syllables.
A striking and beautiful effect of echo is producea in certain
localities by the Swiss mountaineers, who contrive to sing their
Banz des Yaches in such time, that the reflected notes form an
agreeable accompaniment to the air itself.
When sound is reflected between parallel planes, at a propei'
distance from each other, multiplied echoes are produced, repeat-
ingsyllables an almost indefinite number of times.
The blow of a stick or hammer aeainst one side of a parallel
fissure in a rock is sometimes found to produce the sound of a
beU :* in this case the repetition of the first sound, by successive
reflections, is sufficiently rapid to produce the impression of a con-
tinuous and definite tone.
It appears frx)m the exj^riments of M. Colladon, that if sonorous
undulations excited within a fluid impinge very obliquely on its
sur&oe, they do not emerge, but are tnterwiUy refUctedf as wiU
sabseqnently appear to be the case with regard to fight.
* The B«Opro«k a» Tnnlnndge WeUi le a weU>known example.
Z 2
rti.
IM.
v/v
-/V
1/
\il
1 ^
1 1
&G3. Sound ia reflected by corred nufuMi in
of aiif bard polinhed mb-
■tsDce, ind at r in tbe focoa
(Ch. XIX.) of A, lei a low
•oimd aa a whitper, be
Qtterod. llie loiianHH vi-
bntiniu that excited wUL
in reaching a, be reBcclad
n the direction of a nriea
of liiiei parallel to thoae clrawc in (he Ggnm to n, from the concaira
surface of which thej will coDvei]^ to & faeni at f', and be dii-
tinctl? aodible lo an obeerrer ntuated there. IT a «klch bo
plai^d at I, and the ear at r', the ticking will be distinctlj besnl,
although inaudible at anj' other point id the Ticinitj of b. In a
eimilar manner, any ■ound in an elliptic chamber, uttered in oob
of tbe foci of the eUipie, will be audible to aa ebaerver plaenl in
tbe other rocua, whilet penoni placed miilwa; will not be able to
hear h (465).
If the lubaUnce againet which the lound impingea be eoft and
jieldiog, it will be much diminiehed in iutennly; tfaiu, wbilat
Toicei are beard iu a remarkublj aoDorona muasr in loftj aptrt*
menta with large polished walla, they almoat ceaae to be audibk
in chambera bung with tapeatry, from the aouoroua ribratioaa
becoming checked or abeorbed, on impinging againit thia aoft and
jielding mWerial.
5«3. Rtfraciion of Sound.— \t will nbrnquenthr be abowo
(Ch. XJX.) that when a ra^ of light reachra obliquely the oommcu
aurftce of two media of different deumliei, it ii n^atMd at bent
out of ita original course ; preciael; the aune efiect ii prodnced
with regard to aonoraua undulationa, placed under aimilar ciRnmi-
stanoei. Moreorer, the ratio of the ainea of the angtei of inci-
dence and refraction ii found to be the aame ai that c^the Tcloci-
tiea of aound in the respectife media, Theae (acta mar be readilr
demonstrated li; means of the apparatua of H. HiTech. Thu
Km uc conaista of a rectanaolar boi i » c. Fig, S55,
"'' in which is encloaea a source of Bflund oif ani-
I form pitch, conaiating of a diapason, or tnnii^-
fork, Biruck at will bj a Bpring.hamoier, v\
front of this is a cyhndnul chunbcr dkfo,
of which the external larfaoe, ef, is ■ •nr-
tical plane oblique to ibe axis of tlie chamber,
both ends of which are cloand by a thin mem-
brane : it is rnmisbsd with two atopcocka fbr
the admissioa and exit of any gaaeoua or
other fluid, to be eiperimsnted on. A
of Helmholta (539), in uniaou wiUi th< "
•^ — ■^y'
■BFRACnOV OP 80UVD. M
tacbed to an aim moving on an axia perpendicolar to the plane of
the membnine, b f, ao as to move parallel to, and acitwa the centre
of, iu sorface. The tube df, and resonator are enoioeed in a
padded case, through a hole in the side of which the flexible
tube passes, in order to isolate the refracted sound, and the
resonator is moved by a handle projecting externally, by which
its position is shown ; and it is thus moved from side to side, until
the position is ascertained, in which the refracted sound is heard
most distinctly. Let ab coincide with the axis of the chamber,
and 6 e be the direction of the refracted undulations, then if the
cbaoaber be filled with a gas denser than air, ( e will be deflected
Uncardi the base of the prism, fo; but if with a gas of lest
demdty, as hydrogen, the deflection will be in the contrary direo-
tion, towards d e.
564. The Aooustie Lens of S<mdhaui. — ^This instrument pre*
sents another means of demonstrating the refraction of sound ;
it consists of a large brass ring, to the sides of which two thin
laminae of caoutchouc are so attached that the cavity between
them may bo air-tight The ring is furnished with a stopcock
for the admission and retention of any kind of gas* If the mem-
branes be distended to the form of a lens by carbonic acid, and a
body emitting feeble sounds, as a watch, be placed in the axis of
the lens, at a distance of about one-and-a>half times its radius of
curvature from the surface, and the ear be carried along the
axis on the contrair side of the lens, some point will be found, at
which the tick wiu be distinctlv audible ; but if the lens be re^
moved, not the faintest sound will be heard, until it is replaced.
A much simpler form of apparatus may be extemporised, Fig.
356, which shows this experiment exceedingly well. This consbts
Fig. 3S6.
of one of the thin caoutchouc globes, used as children's toys, in*
flated with carbonic acid.
If the globe be inflated with hydrogen, no focus can be dift*
covered ; in fact, it then becomes analogous to the concave lens
in optics (Ch. XIX.), and the divergence of the incident rays is
increased, instead of their converging to the ear.
It most here be remarked that these facts present a forcible
confirmation of the undulatory or dynamical theory of light and
heat, to be hereafter investigated.
565. Injlexion of Sound, — Sounds excited in air are distinctly
audible to persons cut off from rectilinear communication with the
sonorons body by any obstacle, as a projecting wall, although with
some diminution of intensity. This is precisely analogous to thd
TMnlt obterred vhsii luidnlationa on the inrfaco of water en-
ooiint«r Ui AMrturt (4ST), after psBsiag through which they
•pread Utenll;. In wntar, however, M. CoIUdon foand that tfao
piMenoe of a wtll or nick projecting between the eu- and the
BOanding bod;, nearly rendered the sound inaudible, M thoogh *
kind of " aeotulie thadou" bid been produced by the vaU.
In thii case, owing to the phjsical properties nf the medium,
niDch less lateral eiteniion M the nndalationi takes place. It
will snbseqne at); appear (ChBp.XX.)_ thatintheTibralionsof the
hypothetical medium, ether, producing the impreaaion of light,
•nalogous plienomena of inflexion areobserred ; aa alsothose of in-
terference (549—555), reflection (560, 1, 2), and refraction (663, 4).
566. The aama tone or note is always produced by the same
number of vibrationa in a pven time, no matter what may be the
means by which, or the medium in whioh, the Tibralinns may hare
been Kenemled. The relatione between the tune and the nnmberof
equalsuccesidve impulses producing it are well illuMrated by the
BirenofCaiguanldeLatDnr, Fi^. 357. This
Ifg.KT. inaCnunenC consists of a rotatiDg metallic
plate. A, pierced obliquely in a concentric
rin^ of equidistant holes, which stand over
a circle of the same number of equidistant
holes made obliqnely in th£ contrary direc-
tion, through a plate in ibe base, B, reiy
near to which the plate i rotalea. A cBvily
beneath Ibis piste receives either air or
' ir under pressure, from h euitabie vessel
which [he inetrommt ii inserted by the
projecting piece, e. The spindle, c, on which
is Gied, terminates above iu an endiesa
^rew (203). which may, by means of the
ud, D, be (brown into or out nf gear with
n wheel, on the axis of which the index, r,
isplaced. Thiswhsel, ateachcompletc revolution, carries forwanl
one tooth'of a second wheel, on the aiia of which the index a is
plaoad. Bv means of these wheels, when in gear, the number of
rotations of ^ may he counted. The apertures, a, in a, and b in
B, being pierced obliquely thni— j« > the atteam of fluid iwaiug
from the aperture b impinging on the side of a, will tend to pmdnce
rotation in a, and a thas passing on, tho flow from b will be
■topped until the next apertare comes opposite to it, when another
Ettle jet will issue fh>m a, and another rotator; impulse will be
given. The apertures being equidistant, a constantly accelerated
BDCoesfinn of impiiloes at ench coincidence of the apertures, and
% aound rising gradually from a low drone to one of mtenae shriU-
neia will be produced. If any given note is required to be main-
tained for a ehort lime, for the purpose of counting the number of
impulses producing it, the wheels most be put in gear, and tlw
MUnOAL VOTM. 811
pmmiTe of the cnrrent of flaid regnlsted bj a stop-cock, so as just
to overcome the resbtances of the apparatus itself, when the velo-
city of rotation will remain constant. The number of rotations of
the disc, multiplied hj the number of apertures, will give the
number of impulses or vibrations in a given time, producing the
note in question. The name of *' Siren was eiven to this instru-
ment because it is capable of uttering melodious sounds either
in air or in water ; bat^ except for the sake of demonstrating
this fact, a current of air is always employed for purposes of expe-
riment.
Several modifications of this apparatus have been constructed :
in the Siren of Seebeck a large aisc rotates by clockwork, in this
are many series of concentric and equidistant apertures, correspond-
ing numerically to the several notes of the gamut, also several
double series, the numbers being in the ratio of the harmonious
intervals. This is actuated by a current of air from a tube, or
several tubes, placed in front of anv required series of holes.
The Siren of Helmholtz is a reanplication of that of Gaignard
de Latour ; there being two superposed, and the two discs at op>
posite ends of the same axis. There are means of slowly rotating
the upper chamber b, by means of which a beat (549) is produce^
the coincidences of the upper series of apertures being slightly
decelerated or retarded, according to the direction of the slow rota-
tion. There are also four series of apertures in each rotating disc in
the numerical proportion of the notes of the common chord in
music (574), namely, 4 : 5 : 6 : 8 ; to any or all of which the current
of air may be admitted at will by means of stops.
567. Sounds of the same pUeh — that is, produced by the same
number of ribrations in a given time — ^may differ materially in
their character, so far as tiie timbre^ or quality of tone, is con-
cerned. Quite independently of the number of ribrations producing
them. Thus it is notorious that two players, on drawing the same
bow across the same string, will produce tones of very different cha-
racter, although of the same pitch. . It seem^ probable, from the
researches of Dr. Young, that the timbre depends on the manner
in which the string ribrates, and the curve which it describes.
By reflecting a ray of light from the shining surface of a ribrating
string. Dr. Young was enabled to observe some of these curves.
Vide Fig. 257, p. 200.
568. When a sound is produced bv ribrations sufficiently re-
Silar to constitute a musical tone (531 ;, it is termed a note; and to
stinguish one note from another, a series of terms is applied to
them. These, in this country, are taken from the alphabet, the
first seven letters being used to designate particular notes. On
the Continent, the seven syllables, tft, re, mi, /a, sol, la^ st, are
usually preferred. These notes constitute what is termed, the
DiaUmic scale, or gamut. A note is said to be sharper than
another when it is produced by a larger number, and to be graver
or flatter than another, when by a smaller number of vibrations
312
▲C0DST1C8.
in a given time. The gravest audible muneal sound is produced
by about twenty, and the sharpest by about 12,000 vibrations in «
second. This, however, is subject to great latitude, for, as Dr.
Wollaston long since showed, many sounds at either extreme of
the scale, utterly inaudible to some persons, are distinctly per-
ceived by others. The chirp of the cricket, and also that of the
grasshopper, are produced by such a rapid succession of vibrations,
that to many persons they can scarcely be appreciated as musical
sounds, and to some they are totally inaudible. A fine ear is able
to recognise as a distinct sound, a peculiar hissing noise made by
a body completing 24,000 distinct vibrations in a second. M.
Savart has, bv means of a series of very interesting experiments,
shown the high probability of there scarcelv being any definite
limit to the audibility of sounds, provided tney are sufficiently
loud. By means of a rapidly rotatory coeged wheel so arranged
that each tooth should strike a piece of cara, or quill, he found that
12,000 strokes per second on the card produced a sound perfectly
audible as a musical sound of high pi ten.
569. M. Biot * has calculated toe lengths of sonorous waves
produced by different numbers of vibrations in a given time. The
results of his observations are shown in the following table, in
which the first column represents the number
of vibrations in one second, and the second,
the corresponding approximate length of the
wave in French feet.
These sounds are identical with those
produced bv an organ-pipe open at bfith
ends, and of the same length as that of the
sonorous wave, given in this column.
This is probably the utmost range of
sounds audible by the human ear; it com-
prises eleven octaves.
It will be seen from this table that M.
Biot assumes the velocity of sound to be
1024 French feet per second; a velocity
less than that previously assumed (542).
570. An assemblage of eight consecutive notes is termed an octavO)
16
64 ft.
32
32 „
64
16 „
128
8„
256
4„
512
2 „
1024
1 »
2048
6 in.
4096
8 „
8192
14,1
12288
1 M
24576
in
thus:— C D E F G A B C
•^
m
and one octave is said to be higher or lower than another, accord-
ing as the notes it contains are produced by a greater or smaller
number of vibrations in a given time.
A note of any octave is produced by a certain number of vibra-
tions, which are twice as numerous as in the corresponding note
of the next lower, and are half as numerous as in the correspond-
ing note of the next higher octave.
* PrMB de Physique, i. 367.
TABUTIORS OF PITCH.
813
The following table shows the Continental names of the notes,
their English synonyms, the relative lengths of the wares, and
numbera of vibrations producing them, expressed in fractions and
Continental
BngUik
Lengtiuof
Nomben of
Yibrations
namea.
nMnes.
WATflS.
Tibntiont.
in a second.
ut
C
1 or 180
lor 24
261
re
D
t„ 160
1 „ 27
294
mi
£
i u 144
t„ 30
' 326
fa
F
i » 135
*» 32
348
sol
G
i « 120
i» 36
391
la
A
*,, 108
J„ 40
435
•
81
B
Ai, »6
Vn 45
489
Ut
Ci
i„ 90
2 „ 48
522
in the lowest integers, as well as the relative numbers of vibrations
in a second, taking the French btandard, 435, as concert-pitch.
The octave is that which occupies the lower lines of the treble in
ordinary music, and is represented above.
571. In the various cities and countries of Europe, in which
music has been much cultivated, and even in the same locality at
diflfereot epochs, there has been, and is, a considerable difference in
the pitch, or diapason, as it is frequently called, that is, in the
oomber of vibrations per second that constitute some given note,
as, for example, the A or 2a on the second space of the treble.
The assumed tone or note in any particular locality is there called
the concert-pitch. The following tables (from the report of the
French commission, appointed in 1858 to aetermine and establish
in France a uniform diapason) show the numerical relation of the
diapasons in various places in Europe, and thoir differences from
the then existing concert-pitch in Paris : and the variations in
Paris, Berlin, and St. Petersburg, at different epochs, according to
the most trustworthy observers.
looattty.
Vibrs.
Di&.
Locality.
Vibw.
Dxflb.
Brussels . .
London, c* .
455-5
455-2
452-5
4520
451-8
451-5
449-7
4481
448-0
+ 7-5
+ 7-2
+ 4-5
+ 40
+ 3-8
+ 3-5
+ 1-7
+ 0-1
Marseilles
Pesth . . .
Turin . . .
Brunswick .
Stuttgardt .
Toulouse, of .
h .
Carlsruhe . .
London, a
447-0
446-0
444-8
443-5
4430
442-5
437 0
435-0
434-0
- 10
— 2-0
— 3-2
- 4-6
— 50
- 5-5
-110
-130
-140
Lille '. '. .
Berlin , . .
St. Petersburg
Prague . . .
Munich . .
Paris . . .
^ -^ . _ ■ I _L
* ▲ u Meeers. Broadwood's pitoh« considered as beet suited to the Toioej
1, tbeir eoocert-pitch ; and c, Uke Philharmonic concert-pitch,
t a n the pitoh at the Theatre, and 6, at the Conservatoire.
814
JkOOUSTlOS.
Obterven.
Bate.
Tibn.
ObMrren. ' Date.
Vibn.
PIXIS.
Sauveur . .
1700
1713
1810
1823
1830
1839
1858
404-0
406-8
423*0
431-3
435*7
4410
448*0
BEBLIN.
Marbure . .
Nieprecnt . .
• •
1762
1806
1830
1868
1796:
1858
421-8
430*5
440-0
451-8
436-0
4515
Drouet . .
FiBcher . .
Drouet . . .
Delazenne
Lissajous . .
ST. PETKKflBURQ.
Sarti . • .
Lissajoua . .
572. In consequence of the practical ioconyeniences to ma-
Bicians that arise from this eztensiye diveraity of diapaw^ns, and
especially to vocalists, from the high pitch recently adopted in
many places, the commission recommended that the number 4S5
be adopted in France as that of the normal diapaaoUf and this
recommendation has since been carried into effect by the French
government. It is much to be desired that other European nations
should adopt the same uniform diapason.
It may here be remarked that the meaning of the term " vibra-
tion ** is different in France from what it is in this country and in
Germany. What has been previously (369) represented as^ one
complete vibration or undulation, the French consider to consist of
two vibrations: French acoustic numbers must therefore be di-
vided by two, to reduce them to English or German measure,
and vice versA.
573. The Tonometer ofSeheibler. — ^This apparatus is extremely
useful in precise determinations of the pitcn of musical sounds.
It consists of a series of 65 diapasons arranged in rows, from C to
C^ each of which differs from the preceding by four vibrations ;
A practised ear will not only assign to any given note its place
between two consecutive diapasons, but also give by estimation a
near approach to its position in the interval : and thus the num-
ber of vibrations in the note in question may be inferred : this
number may also be accurately determined by the frequency of
the recurrence of beats, each of which shows a difference of one
vibration.
574. The perception of a simple^ musical tone has been aptly
compared by Euler* to the visual impression of a series of dots
equidistant nom each other, thus,
If the intervals between these dots be greater or smaller, tlie
tone produced will be lower or higher (568). It can scarcely be
doubted that the perception of a smgle tone by the ear is analo-
Sous to the appreciation by vision of such a set of equidistant
ots ; thos enabling us to represent to the eye, in a certain degree,
what the ear perceives on hearing sound. If the distances between
the dots be not equal, or if they be irregularly scattered, thej
* Letters to a Gennaa Prinoste, voL L let. 4»
HUMEBIOAL KATI08 OP YIBIUTIOHB. 815
vonld represent a confoned noiBe, inconsistent with harmony.
When two tones, each produced by the same number of yibrationsi
strike the ear simultaneously, they appear to blend, forming a
tmiaon; which may be represented by two lines of equidistant
dots, thus, :::::::::
When the di£ferenoe between the number of yibrations pro*
dndng any two notes is in a simple ratio, so that the ear readily
diacoTers the relation existing between them, a concord is pro-
duced. Bnt i^ from the absence of this simple ratio, this relation
cannot be detected, a ditcord is said to result. The following are
some of the most important concords.
I. The octave represented by 2, because the higher note is gene-
rated b^ twice as many vibrations as the graver one, corresponding
to the interval of the two Cs or ut% : this concord is termed an
octave, because in the musical succession of notes (570)^ C, is the
eighth note from G, and the vibrations of C and C, are in tne pro-
portion of 1 : 2.
II. The>Sf<A, (, the higher note being produced by three vibra-
tions, whilst the lower is produced by two in the same time, cor-
responding to the interval of 0 to G, or t«< and 8ol: this concord
is termed a fifth, because the latter note is the fifth from the
former : a similar explanation applies to the numerical names of the
other concords.
III. The /bvr<A, or 4, the higher sound produced by four, and
the graver by three, vibrations m a g^ven time : this corresponds
to the interval of C to F, or ut io fa,
IV. The fnajor third, or f , corresponding to the interval C to £,
or tci to mi,
V. The nwMT third, or f , is the interval E to G, or mi to sol.
These concords are represented in the following diagrams ; the
upper line in each representing the higher, and the lower, the
graver, note. Those vibrations which occur simultaneously, and
therefore intensify each other, are connected by vertical lines.
Octave, 2 : 1. Fifth, 3 : 2.
frrrrf {v}v)v{v
Fourth, 4 : 3. M^'or Thiid, 6 : 4.
\v:\v:\:v V:::\:v:\'.
BCnor Third, 6 : 6.
|:v;:l:v::I
The fundamental note, with the third, fifth, and octave above
it are called the chord of that note.
SI 6 ACOUSTICS.
575. If, when a string is yibrating, it be partially checked, by
touching it in the centre, its two halves will vibrate twice as nt-
pidly as the entire string, each producing the same note of the next
ni^her octave. Hiis sometimes occurs spontaneously, producing a
series of harmome Bounds^ which are cnaracterized bv a remark-
able sweetness of tone ; the same effect may be readily produced
in the strings of the harp, violoncello, or guitar, by lightly touch-
ing them at a nodal point (373), while they are vibrating. Any
number of these points, or nodeSi may exist in the same string,
at which the string maintains its position of rest. Let the
dotted line A B, ilg. 358, repre-
^' ^ sent the direction of a stretcned
r. string, and after it has been
'^'v.^ " ^ ^K^~~ — ~~^j» made to vibrate, let it be
touched with the finger at o,
then the two halves, A c, c b, will begin to vibrate twice as rapidly
as the entire string a b did, but in contrary directions ; each end
pulling equally in opposite directions from c, this point will be
found to remain at rest.
576. When chords are made to vibrate in a transverse direction,
as by drawing a bow across a violin-string, or by drawing a harp-
string with the fingers, the following phenomena are observed.
A. Chords of the same diameter, and equally stretched, have
the number of vibrations in a given time in the inverse ratio of
their lengths. Thus, a chord, performing thirty-two vibrations in
a second, will, if shortened to one-half, or vibrating with one node,
produce sixty-four, and if to one-third, or vibrating with two nodes,
ninety-six vibrations in the same time. This may be conveniently
demonstrated by a
JV« 369. simple apparatus con-
^ ^^ ^ ^^.^ sisting ot a flat piece
J" A/ /J^ 'B/^ of wood, AB, Fig. 359,
i {/ to the end a, of which
AC D B a violin-string is at-
tached by a pin. The
chord, after passing over a pulley at the end b, is carried round a
tightenine-pin at a. It is evident that the two portions of the
chord will be of the same tension and thickness. By means of
shifting bridges, c, d, &c., the number of vibrations in portions of
different length, as indicated by the tones produced, will be found
to be in the inverse ratio of those lengths.
B; Chords of the same length and degree of tension, have the
number of their vibrations in the inverse ratio of their diameters.
This may be readily shown by the apparatus. Fig. 359, if for the
uniform chord we substitute one of two parts, one of which consists
of four strands of the other loosely twisted together. Since the
number of vibrations also varies inversely as the length, it follows
that when the length is inversely as the diameter, w number of
TIBSATI0K8 OF CH0B08. 817
Tibrations, or in other words the pitch, will remain constant, if the
proposition be tme. When two bndges are placed under the thicker
chord, comprising half the length between those under the sincle
chord (the tension being necessarily the same), the two portions be-
tween the bridges will be found to siye the same note. If the thicker
chord consist of two strands only, &en the lengths must be taken as
1 : v^2, that is, as the side of a square to its diagonal, in order to
yield the same note.
C. Since the number of vibrations here evidently de^nds,
ecEferw paribtu, on the quantity of matter vibrating, a similar
result would be obtained, if, for the chord of double or quadruple
sectional area, were substituted one of the same diameter, but of
double or quadruple density. But as the section is proportional
to the square of tbe diameter, it follows from B that the number of
vibrations in chords of the same leng^, diameter, and tension will
be in the inverse ratio of the square roots of their densities.
D. Chords of the same diameter, length, and density have their
Dumber of vibrations in the ratio of the square roots of their tensions;
ex. gr^ a harp-string stretched with a tension of one, will i>roduc6
a certain number of vibrations ; with a tension of four, it will pro-
duce twice as many ; and with one of nine, three times as many,
in the same snace of time. This may be shown by an apparatus,
Fig. 360, similar to the preceding, except that the chord, after
passing round the
pulley at B, is attached ^^ ^.880.
to a block containing ^^^
two sheaves (1S4);
there is also a pulley
running on a pin at
the end a. Another
piece of the same chord attached to the fixed stud at a passes suc-
cessively over one of the sheaves at n, the fixed pulley at a, the
other sheaf at n, and the adjusting pin at a. It is manifest from
the constniction that the tensions of the two portions of the chord
will be always as 4 : 1. By the bridges c, d, e, any lengths may
be intercepted ; and it will be found that the length of that portion
of the ^hord, which is under quadruple tension, must be just half
the corresponding length of the other portion, in order to produce
the same note.
•It may here be remarked, that an analytical investigation of the
motion of a vibrating chord firat gave rise to the solution of a
partial differential equation ; from which it appears, that if a chord,
of which the length is l, the semidiameter or radius, r, and the
density, <2, be stretched by a weight w, the time of an oscillation,
.v^=
and if K be the nnnberof *il>i*tianain* gireD time, tlMU
titm which cxprtMon the Uwt a, a, c^ d maj be immertiately
deJDced.
If tbe weight wconiut or> Iflogth lofthe Btme chord, then
•nd mbatitDtiiig this valiw in (a), we obtaio
. .'-^'■^' ..
which is the fbrm in which the reiult was orieintllj obtuned.
llie troth dF this rorninU was tested by ^Veber, b? meiJia of a
cotton thread 51 feet 2 inchrs long, whicli weighed 864 graio* :
the fDllowiug timei of vibration were detenained both b; olcnl*-
tion, and by acconle obsemtion : —
ObKrred time . . 46-00" I CaknUted time . 46011"'
„ „ . . 84-72 , a5S4«
„ . . 16-25 I „ „ . . 17-48S
It maj likewise b« demonstrated that the intenntT of a toand
is proporlioDal to the square of the amplitode of the Titiration
proouciDg it; thns, if a chord prodnce ■ certain sound with a
Kiren bmadth of eicnrsioo, when the excaroras are twica as
broad, tbe sound will be four times louder than it was before.
5TT. M. iUelde baa isgeiuausly illustrated the laws of Tibratine
chords, by producing inaudible but virible vibrations of considerab^
amplltnda. For this puipose a chord is attached to one eitremitT
td a fixed diapaaon cl laif^ size, a. Fig. 361, and paadng hori-
K».sm.
^.Honne. B19
■ootelly over » pnllcT. a, in ilrctcbed br & veight pUceil id th«
Kale, c, ftttacbM to tbe end of the cbora. The fivme fapporting
the palle; B mny be placed at tnj required disUnce fnnn a. Ths
ribrktioDB of toe cliapaMti are exdtea bj drawing a baat-viol-bov
aetiiee tbe side of ihe bnQcbet, in th$ direction of the choni If,
for example, it be deaired to d(iiTHiiiBtrat« the lav, D |A76), let a thin
chord, aa a. piece of ailk, be atretched bj a suitable weight, and
let the diitance of tbe eiippart, b, be >o adjusted, that the chord
will vibnte io ita entire length, aa a,b; then let (he etretch-
ing' weight be redaced to one-fbnrtb, when it will be found
that tbe choni will nbrate with one no-le at ita middle point, e :
or (he pnltej, b, mnat be made to coincide with the point, c, in
order that the inlerrening chord mar vi bra(e in ita entire length.
If the wpjght be redooed to one-ninth, the chord will tibrate with
3 nodal points, and w on. In thii eiperiment it ia desirable to
oonnleipoise Uie acale by a imall weight attached to a thread
paaeing over the pulley.
In order to demOBitrate the law, B, let the diatsnca, a B,
Elg. 362, be ascertaiDed at which the cboid will reciprocate; then
attach to B a qnadmple piece of the ume choni, iatretched by the
aame weight, and let the pnlleybe placed al c, Bc being one half of
A B ; it will now be (bund that the coropoand chord will vibrate
with one nodal point, not however in its middle hnt at b ; thiu
ptOTing that AS and bc vibrate synch rononaly. If the added
diord, B F, he double instead of qnadrnple, then tbe length of e r
most be equal to the side ofaaqoBre of which D B is the diagonal,
and the node will now be found at K.
The law, A, may be readily domonstrated by employing two
diapeaons, one of which is an octave abore the other, end there-
lore vibrates in half tbe lime. If the chord when attached to the
lower, vibrates in its whole length, then when it in nttaohed to the
higher, with the same tension, it will vibrate in half the length,
with a node at e. Fig. 361.
When the chord is placed in the plane of vibration of the dia-
paaon, »■ in Pig. 361, one vibration of the chord carreapoiids in
time with two of the diapason. For suppoee the diapason to he
excited by withdrawing some bard anhstance, as a piece of wood,
(a little thicker than the interval,) from between its branches ;
before its removal, the branch » will have been presBed towards b,
andtlnu relaxed the chord towards b. The first movement of the
820 ACOUBTIGS.
brancb, a, will pull the chord up to the straight line between a
and B, and the acquired energy of its particles will canj them op
towards a, daring the return towards b, or second semi-vibration,
of A. The second negative motion of a (from right tolefl, vide note^
p. 45) will again pull the chord straight, and the energy of its
particles will carry them down towai^s b, during the second poixitiye
motion of a, when one oscillation of tlie chord will have been
completed. This may be proved by turning the diapason round
through 90", so as to bring the chord perpendicular to the plane
of vibration of the diapason : it will now reciprocate with a nodo
at its middle point, or its length must be reduced by one half;
one vibration of the chord now corresponding with each one of the
diapason. To the double ratio of vibrations here described there is
some analogy in the reduplication of motion by link-work (264).
Illustrations of these interesting physical laws might be almost
indefinitely multiplied, but enough nave been detailed to complete
their practical demonstration.
578. The Vibratians of Bods, — These may be either transverse,
or longitudinal ; and the rods may be either fixed, or merely restiup,
or free, at one or both ends : their motions have been investigated by
Chladni, but the analysis is difficult, and the results not altogether
satisfactory.
When rods of any elastic material fixed at one end (370), are
made to vibrate, they produce sonorous vibrations varying in
number in the inverse ratio of the sqnares of their lengths, and in
the direct ratio of their diameters. These rods, like strings, may
vibrate entire, or in nodal subdivisions.
Another particular case affords a curious result : when a rod,
resting at both ends, is made to vibrate in two or more nodal sub-
divisions, the number of vibrations in a given time will bo as the
sqpaare of the number of subdivisions ; that is, when vibrating in
two or three parts, the number of vibrations will be four or nine
times those of the entire rod. This result shows the fallacy
of assuming any
^'^' ^ d»nV>ri foundation
01 the elementary
principles of har-
mony trom the laws
of governing the
vibrations of a
chord.
Fig. 363 repre-
sents convement
forms of apparatus
for exhibiting the
effects of vibrating rods, either resting at one or both ends,^ or
clamped at any nodal points. The standards, A, b, o, are ac(ju8-
tible, and fixed by a wedge, passing through a mortice. In the
A
▼IBKATX0H8 OF TUBES. 821
standards b, c, the rod is clamped by a wedge in the angle of a
triangular aperture.
A musical instrument has been oonBtmcted of a series of thin
wooden rods fixed vertically into a base, and of such lengths that
their longitudinal vibrations produce the series of musical notes.
They are excited by drawing them through the fingers previously
rubbed with powdered resin. Beyond being a Scientific curiosity,
the instrument is of no practical value.
579. When sonorous vibrations are excited by blowing into
tubes, the higher notes are, ccgteris paribttSf produced by the
shorter tabes. Sounds thus excited are produced by the alter-
nate condensations and expansions of the successive layers of the
column of air contained in the tube.^ The following are some of
the more important facts connected with the relation between the
sound evolved, the length, and the open or closed state of the ex-
tremities of the tubes employed.
A. In parallel sided tubes, dosed or open alxke^ the number of vi-
brations IS, as in chords, in the inverse ratio of the length of the tube.
B. In a cylindrical or prismatic tube, onea at both ende^ the
sound is the same as that produced by a cylindrical tube closed at
one end, and one half its length.
C. In a cylindrical or prismatic tube, closed at hoik ends, the
sound is the same as in a tube open at both ends.
D. Nodes, or points at rest, in the included column of air, are
observed in the case of the vibrations of tubes as in vibrating
chords (373), or rods (375).
580. In the simplest mode of vibration of a tube open at both
ends, there is a node in the middle point of the length of the tube,
to and from which the equidistant layers of air in the ends of the
tube approach and recede simultaneously.
This may be demonstrated by an apparatus ^* ***•
due to rrof. Wheatstone, consisting of an
annular tube mounted on a stand, the two
halves of which are jointed together at
A. If the comer of a square vibrating^ plate
of glass, to which the column of air m the
tube is capable of reciprocating (540), be
placed between the open ends of the tube
(which are separated by an interval of half
an inch) no resonance is produced, the im-
pulses being in the same direction at both
ends of the tube, and thus mutually destroy- ^
ing each other. If, now, the moutns of the
tube be separated from each other by a distance equal to the side
of the glass plate, and the plate he made to vibrate in its first
mode (a. Fig. 367), if the adjacent comers (which are always in
opposite phases of^ vibration) be placed over the months of the
tttbe^ the impulses being now in contrary directions at the ends^
Y
oFthe tabs, reKt on ewh other ti the noda in the middle, uid a
loud resonance immedist«lf reinlti.
If a tube be stoppad at botb endi, the oitremitieB are DOdal
points, and the greatest Tibratoir motioa ii in the centre, like the
vibrating Wring, Fig. 263. If the length of the slopped tobe be
the ume as that of a tube open at both endi, the length of the
wave is evidentlj the etuae, but ita holTei are joined together in a
reTersedpoeition .
681. Ine conditioas of Tibrations of the air in tabee hare been
ingeniously demonatrsted by anparatne deeigned by M. R. Kcenip.*
One of these conaiatB of a wooden oi^n pipe, A c. Fig. 36&, banag
apertores at one aide, at the half and quertera of its length.
These apertures are covered by tbin latnime of coontchonc, over
which are placed saucer-shaped
Hj. 3M. metallic chambers, a, b, c. Each of
these chambers is supplied vitb gaa
from a common aonrce, and has a
email tube proceeding front the
centre, carrying a burner with avety
noBll nperture. This, being a pipe
open at bnth ends, will, when sound-
ing its lowest note, have a node at
its centre b, whero eonseqnenlly iuc-
cessive condensation b aiid rarefac-
tions will take place. If the jets be
lighted, the pressure of gas in the
cbsmbers being redaced to a loir
point by a stop-cock, directly the
pipe is sounded, Ihe jot at a will be extinguished, and a slight
tremulous movement will be perceived at a and a. This happsDS
in oonsequence of the diminished internal preseore on the mem-
brane at B, during the phaae of rarebction ; atmoepberic preaaare
on the orifice of the jet then stopping the escape of gas. The
changes of pressure at a and c l^ing veiy much less than at B,
are insufficient to canse the extinction of those jets, and merelj
^e slight tremor ia perceived. If bj a greater pressure on the
bellows, the pipe is made to speak the ocCace above its ftmda-
mental note, the wave-length will be reduced one half, and there
will be two nodes at a and c, the lamina of air beneaUi B having
now only a backward and forward motion, withont any change of
density ; consequently the jets at a and c will now b« extin-
gnishsd, and that at b wilt remain motiooteas.
The other spparitna consists of two pipes having similarljr
arranged ga»-jets at their middle points, d, b, only. The jets
F, a, are furnished with longer tubes, so that they may be placed
one under the other, and in front of a mirror capable Ol revolving
on its veiiica] stem, H. For the purpose of vatying the expen-
*SMnts these pipes are closed at the lop, and have lateral oriBoBs,
* Foffgendo ff, Annai^B, eixU.
VIBRATIOXB OF PLATES. 323
capable of being more or less closed bj sliding doors k, l, by wbich
the pitch of each may be regulated. When these pipes are in
exact unison, it will be found (contrary to what might nave been
anticipated) that when both are made to speak, the sound pro-
duced is much less loud than when either is made to speak alone.
The reason of this apparent paradox is, that the columns of air
are always simultaneously in opposite phases of vibration in the
two pipes ; (hat being the mode of vibration in which the pressure
of the air in the neighbourhood of the two embouchures is ren-
dered most uniform. This fact may be readily proved by lighting
the two jets, and leaving the flames a little too large for extinc-
tion, when the pipes speak. A considerable fluttering of both
flames will now oe perceived, and if the mirror be now made to
rotate on its axis, h, the images of the two flames will be
resolved into serrated lines of light, in which the phases of eleva
tion of one will correspond vertically with the phases of depression
of the other ; thus proving that the phase of condensation at the
middle of either pipe is 8>*nchronous with the phase of rarefaction
in the other. By varying the pitch of either pipe, so as to pro-
duce a beat, also by employing two pipes, having intervals of
a third, fourth, fifth, or octave, and further by making the tubes
from D and e to unite in feeding one jet, a number of curious and
interesting results may be obtained, for which the reader must be
referred to the original memoir.
582. The vibrations of the column of air in tubes may be ex-
cited in various different ways, and the quality of the tone will
depend partly on the material of which the pipe is composed, and
partly on the mode of excitation.
1. By blowing obliquely into the open end of the tuBe, as in the
Pandean pipe.
2. By directing a current of air into an embouchure, or aperture,
at or near the closed end, as in the flute, and in organ-pipes not
fumished with reeds.
3. By a small flame of hydrogen gas.
4. By the vibration of the lips placed against a small cup-shaped
cavity, at the smaller end of an open conical tube, as in all kinds
of horns and trumpets.
5. By thin vibrating laminae of wood or metal, called reeds :
these are of two kinds, one of which covers a small aperture like
a valve, and vibrates against the edges of it, the others vibrate in
an aperture which they nearly fill, but do not touch ; the latter are
called fre^ reeds. Tne brass reeds of all varieties of reed organ-
S'pes, and the wooden reeds of the clarionet and hautboy, are of
e former class, while those of all kinds of harmoniums, concer-
tinas, and accordions belong to the latter.
683. VUbratums of Plates. — ^Vibrations are readily excited in
elastic plates by friction, or by striking them, and sounds are
eyolved ; the plates dividing themselves into vibrating portions,
separated by nodal points of rest, arranged in lines. The position
y 2
324
ACOUSTICS.
of these lines of rest is beautifully shown b^ scattering sand on
the plates, and vibrating them ; the sand will assume a carious
rapid movement, and be thrown off the vibrating portions, upon
the nodai lines^ or lines of rest. If a square or rectang^ar plate
_ of glass be grasped in the cen-
^. 8M. ^^ ^j^. ^ gjjj^jj hand-vice, Fig.
366, sand scattered over its
surface, and the bow of a violin
drawn rapidly across its edge
close to one of its angles, the sand will be thrown into the position
shown in a, Fig. 367. If the bow be applied to the middle of
1^.867.
KO
one of the sides, the sand will be arranged as in b. If the plate
be held near one of its angles, and the bow applied as before, the
sand will be arranged as in c.
Prof. Wheatstone, in a paper on this subject, published in
the Fhilos3phical Transactions, has calculated a large number of
these acoustical figures, by the principle of the super-position of
small motions (541), a great many of which have oeen obtained
experimentally by himself and others.
684. From a series of highly interesting experiments* on this
subject by Faraday, it appears evident that while the accumu-
lation of sand or any other coarse and heavy powder, on the
nodal lines arises from its being, as it were, jerked off from the
vibrating portions, the vibrations excite currenta of air over the
agitated portions, which entangle any light powder scattered on
the plate : if it be a very light one, as lyeopodium, it will be
caught up by the aerial currents, and will collect chiefiy on the
most agitated portions of the plate,
instead of on the quiescent portions,
and appear animated with a curious
vortex-tike motion. If the plate be
vibrated in highly rarefied air, the
lycopodium will lie collected on the
nodal lines, like the sand when
vibrated in ordinary states of atmo-
spheric pressure ; and if the plate be
covered with sand, and made to
vibrate in a much denser medium,
as water, the sand wiU be under the
PhiL Tnuw. 1831.
Fig, 968,
ftr-«-=<^W
VIBRATIOHS OF MBlfBBANES. 326
same conditions, reladvelj to the medium, as the iycopodium in air,
and will be collected chieflj on the most agitated portions of the
plate. Thus, the lines a, b, Fig. 368, represent the position of the
sand when the plate is yibrated in air, and of the lycopodium,
when in vacuo ; and the triangles, c, e, d, f, the parts or inter-
nodal spaces where the sand is collected when the plate is vibrated
in water, and the lyoopodium, when in air.
585. The vibrations of a membrane may be well exhibited by
stretching a piece of bladder over the mouth of a funnel, and
passing a horse-hair, retained by a knot, through its centre; by
drawinff the hair through the fingers, previously nibbed over with
resin, the membrane whI be made to vibrate, and if sand or lyoo-
podium be scattered on its surface, a symmetrical arrangement of
the heavy particles at the nodal lines, as well as the accumula-
tion of the light particles at the centres of vibration may be
observed.
If a thin membrane be stretched on a triangular, square, or
circular wooden frame, and strewed with sand, its vibrations will
be readily excited by holding a tuning-fork, bell, or other vibrating
body near it, and they will be indicated by the motion of the
particles of sand. French tracing-paper, or still better, thin
parchment-paper (prepared by immersion in strong sulphuric
acid), damped a little, and attached by paste to the edges of the
frame, will answer very well for these experi- .. „^
d86. A very delicate mode of detecting acous-
tic vibration lias been described by Strehlke : *
be scatters some lycopodium on water, so as
to cover its surface with the thinnest possible
layer, which is best effected by agitating the
fluid in a box, the inside of which has^ been
mbbed over with the powder. On placing a
drop of this on a vibrating plate, the particles
of lycopodium begin to revolve in the water,
dividing into two or more currents, if the sono-
rous vibrations be intense, as shown at A b, Fig. O-
369. If a drop be placed on each side of a
nodal line, or line of rest (583), c d, these intes-
tine motions occur, but in opposite directions.
587. The evolution of musical sounds during the cooling of
heated metals, observed by Mr. Trevelyan and others, is extremely
curious. These phenomena are best observed by using the th&rmo'
phone. This consists of a bar of copper five inches long, and about
naif an inch thick, grooved in such a manner that its transverse
section resembles c. Fig. 370. A piece of thick iron wire, about
eight inches long, is fixed in one end for a handle. On heating
the copper bar, and resting its convex surface on the edge of
* Poggendorff, Aanalen, xl. p. 146.
326 ACOUSTICS.
a block of lead, as at b, it will begin to vibrate strongly, and soon
afterwards evolve musical sounds, usuallv be^nning like the drone
of the bagpipes, and rising to a loud plaintive swell, like that of
the ^olian narp, and then falling in the most fitful manner.
These wild and irregular
"*^»870. sounds continue until the
y— ^. — ^^ temperature of the block of
( ] C lead and copper-bar are nearly
Ly^VJ eoualized. They are evolved
with the greatest shrillness
when a small channel is filed
out in the back of the bar, as
shown in c, or a similar chan-
nel excavated in the surface
of the leaden block on which
it rests. It is necessary that the surfaces of the metals employed
should be quite clean ; and that their powers of conducting heat
should be as different as possible ; hence, copper and lead succeed
the best, as the conducting power of the former for heat, according
to Despretz, is 398, and that of the latter 179-6.
PRODUCnON OF VOCAL BOUNDS.
588. The sounds emitted by the lower orders of the animal
kingdom are not strictly vogoL : the hum of insects, and of the
humming-bird arises from the successive impulses of the wings on
the atmoRphere being sufficiently rspid to produce the impression
of a continuous tone ; the hiss of tne serpent is but the sudden
expulsion of air from the sacculated lungs through a narrow fissure ;
and the acute chirp of the cricket and grasshopper is produced by
the friction of the legs against the rough integument, just as an
elevated tone is produced by passing a stick rapidly idong a row
of railins^s. True vocal sounds are produced only By mammals
and biras ; the vocal organ being at the upper end of the wind-
pipe in the former, and at the lower end in the latter order. The
vocal organs are most fully developed in the higher classes of
mammals, in which the essential part of the organ couHists of
two parallel folds of elastic mucous membrane, with a highly
elastic chorQ extending through, and supporting, their free mar-
p;ins. These parts, called the vocal chords, when suitably placed
in apposition and stretched by the muscular apparatus of the
larynXf emit a sound when air is driven between them by an
eflbrt of expiration. * This may be readily demonstrated by at>
taching the iresh larynx with a portion of the windpipe of an ox
or sheep to a small pair of organ-oellows, when, by a little easy
manipulation, the peculiar intonation of the animal may be readily
produced.
The mode in which vibration arises will be best understood bv
a reference to the diagram, Fig. 371, in which a is a narrow slit
PBODUCnOK OF VOCAL 80UND8.
327
in the wind-cbest of the or^an bellows, and b, c, two small wooden
frames comprising three sides of a rectangle, to which pieces of
yerj thin leather are attached. When these frames are placed
m a converging position, as at a, resting on the sides of the slit,
the issuing current of air will cause «. j...
the free edges of the membrane to *^' '
become convex outwards, as at b, c ;
but if they be placed in a divergent
position, as at d, the issuing cur-
rent of air acts as in a divergent
tube (435), and ^Bspecially if the
roace between me ends of the
frames be closed by the finder
and thumb) the membranes will become convex inward». If,
now, the firames be placed in a nearly parallel position, the free
maigina will have an equal tendency to become convex, or con-
caye towards each other, and will consequently assume both posi-
tions alternately, and be thrown into yibration.
The actual conditions of yibration of the vocal chords may be
more aptly illustrated by the following simple apparatus. Let
▲ B c be a small transversely-oval piece of wood, coming up to a
thin narrow ed^ at c, having an aperture through it, and a tubular
stem for insertion into an aperture in the wind-chest.* A thin
bit of leather is bound on to this at ab; having
two free margins at c d ; and two small pieces of
cork are attached to the corners at d, for the pur-
pose of regulating, by the fin^r and thumb of each
Sand, the position and tension of these artificial
vocal chords, which may be readily made to emit a
sound when placed in a parallel position, and a
current of air forced between tbem : and the pitch
of the sound will be found to be regulated by the
tension. If the upper edges of the bits of cork at d
be turned outwaros, so as to separate the edaee of
the membranes, no sound is produced when lower parts of their
surface are brought into apposition ; it is thus that the actual
glottis is closed whenever the production of sound is not in-
tended. This and many other interesting phenomena relating to
vocalisation may be readily studied by inspection, bv means of the
laryngoscope, an instrument for the ocular examination of the
lanmx, by means of reflected light.
J n the production of the notes of the human voice, the length
of the reciprocating tubular cavity, that surmounts the larynx,
constitutes an element : and it may be readily observed that the
larynx is raised when high, and depressed when low notes are
produced. In singing, two distinct qualities of tone are recog-
* A very oonvenienfe form of bellowt for tbit and other aoonrtical ozperi*
ziMiits may be obtained firom Menrs. Elliott, Stnmd, London.
1»V. 372.
■»•_
V
828 Acousncs.
niaed — the {oil, or chest-voice, and the faUetto^ or head-Toice :
and mftny sineers can produce some notes in either quality of
tone. The differeqce of these is, that in the chest-yoice the vocal
chords are thrown into vihration, while in the head-voice vibra-
tions are excited only in the reciprocating cavity, as in organ-
pipes having an embonchare.
689. Vowel-Sounds. — Professor WilHs has shown, in a paper
published in the Cambridge Philosophical Transactions, that
vowel-sounds may be produced either by partially closing a conical
cavity, excited bv a reed placed at its apex, or by a column of air
in a tube excited by a reed at its closed end, the particular vowel-
sound depending on the length of the tube. As tne reciprocating
cavity is flpradually lengthened, the vowel sounds may do recog-
nised in tne following order: — 55 (n), 6, l^ e, i. As the pipe is
further lengthened, a point is reached at which the discord be-
tween the notes of the reed and pipe is such that, neither being
able to coerce the vibrations of the other, no definite tone, bnt
only a harsh grating sound is produced. After a little further
elongation, the same series of sounds is reproduced in an inverted
order, bnt with somewhat less distinctness; and still further
elongation will reproduce the first series of vowel sounds.
In human articulation, the vowel-sound depends on the form
and configuration of the oral cavity. It has been advanced by
Helmholtz that the production of a vowel-sound depends on the
superposition of some high harmonic on the fundamental sound.
590. The last-mentioned phenomena are illustrations of the
theory of forced vibrations ; which assumes that, if$uffieient time
has eiapiedfrom the commeneemerU of motion to oUow the iniHal
periodic disturbances to have been destroyed byfriction, imp&rfect
eUasticity^ and other causes^ the resultant motion^ if any^ wiu be
vibratory and isochronous. This theory is applicaUe to luminous
as well as to sonorous vibrations : it is likewise the foundation of
Laplace's theory of the tides (503).
Refebbncbs.
To no single work on the subject of acoustics can the student
refer with so much advantage, as to Sir John HerschePs mono-
graph on Sound, in the Encyclopaedia Metropolitana, which embraces
all the leading points of this subject. The writings of Young,
Chladni, Savart, Weber, Seebeck, Helmholtz, Koenig, Lissajous,
and others will also supply much valuable information. The
subject is more geometrically treated by Newton, in the Principia,
torn. ii. § 8.
829
CHAPTER XI.
XAGNETISM.
591. The property of attracting pieces of iron, possessed by
certain ferraginons ores, has been long known ; and the ores them-
selves have been termed moffneta, from Magnesia, a town of Lvdia,
near which they were stated by the ancients to abomid. Pliny
states that these ores were in his time termed ferrum vivunif or
quick-iron. In England the term loadstone has long been ap-
Eiied to the magnetic oxide of iron. All the phenomena exhibited
J such magnets, including their action on iron, cobalt, and nickel,
and other metals which appear to obey their attractive influence,
have been collated, and the important science of Magnetism
foanded upon them.
Not only do ores of iron possess magnetic properties, bat masses
of that metal which have been placed in contact with them, or
have been submitted to the effects of certain mechanical actions,
generaUy present the phenomena of magnetism. The magnetic
ores constituting what are termed natural — and the latter, arti-
ficial magnets : but the phenomena presented by both kinds are
of precisely the same nature. Nor must the great natural magnet,
the mass of the earth, be overlooked, which has been shown to act
on the magnetic needle precisely in the same manner as a com-
pound artificial magnet would do, composed of two bars placed at
an angle with each other, and suitably arranged as to magnitude,
position, and distance. The investigation of the magnetic pheno-
mena manifested by the earth, their magnitude, direction, and
changes, constitutes the object of the science of terrestrial mag-
netism.
Magnetism will subsequently be shown to be inseparably con-
nected with other forms or phases of potential energy, viz., elec-
tricity, light, and heat; it must therefore be a cosmical, and not
merely a terrestrial pr(»perty of matter : and this view is strongly
confirmed by the recent researches of General Sabine, for it wiU
appear in the sequel of this chapter, that certain periodic changes
in terrestrial magnetism may be respectively identified with cor-
responding periodic cotimical phenomena.
Li the examination of magnetic phenomena, magnetized bars of
steel are ^nerally employed: if straight, they are called bar-
magnets; if smafi and tapered towards the points, magnetic
needles ; and if bent into the shape of a letter U, horse-shoe magnets.
592. If a magnet be dipped m iron filings, it will attract them,
caiuing them to ftilliere to its snriaoe, but nnrqnall]' in different
parta; being collected in sbonduice at the ends, and nearljr
abiient from the intermediate portiona. This is beHt seeo by
placing & aheet of cardboard over
"!'■ "■ tbe two polea of a horae-aboo
magnot, FiE, ZT6, scattering; iron
fllJD^ on its suriiLce, and then
' ta^piBg the paatebnard lijihtlj
, witn the nail : the iilingB will ar-
' range themnetres in linea diverg-
^ ing from the poles of the m^net
J in cnr»es, whilst the outline of
' both ends, n, i, of the bar will bo
well defined by the iron filings, as
ahonn in the fignre. K the card-
board be now laid on the aide of the magnet, the direction of the
magnetic curves, in a plane perpondicular to the fomer, latj be
observed. The eilremitiea of the magnet, in which the nMgnetio
action appeara thna to be concentrated, are tensed polf. The
grenteat inlenmtj of action ia not found to be eiacttf at the ends
of the magnet, but at pointa a little dialant from them, represent-
ing centrea Iram which the magnetic force appeara to radiate.
The ourrea formed hj the iron filing* may be conveniently re-
garded aa pointing out the existence of mmnetic Una o/Jbree,
Bod the apace, a, between the two polea has been termed tbe nag-
nelicjield, or space where the two polar foreea mutually react.
The inHuence of the lines of force traTeraing the magnetic Geld
on certain Iradies, will preaentlj fall onder our notioe.
693. Let ■ magnetic bar be sospended by a thread attached to
ita centre, or supported on a
Sv. S7*. pi,ot_ jg K a, Fig. 374, so aa
to be free to move in a hori-
EOotal plane ; it will be
fonnd (o asaumoi after a few
oacitlations, a conatant po-
sition, K it be moved from
this portion, the bar icill
return to it as noon as the
diaturbing force is removed.
One of the poles (592) of the
bar will be found to point
V ' — J constantlytowBrdetbenorth,
"~— '—'^ and the other towards th«
south. The forioer, h, in common language called the north,
and the latter, s, the south pole, of the magnetic bar.
53i. Let the loulh pole of a bar-magnet held in the hand be
brought Interally towards the north pole of a magnetic needle,
placed on a pivot ; immediately the needle will more toward* tho
POLES OP A MAGNET. 331
bar, being attracted by it. If the north pole of the magnet
presented to the needle be now substituted for the aotUh^ the north
pole of the moTeable magnet will fly ronnd to attain the greatest
possible distance from it, repnlsion having taken place ; and the
Boath pole will now be attracted. Henoe it appears that poles of
the same name repel, and those of opposite names attrcict each
other. That the attraction or repulsion is mutual may be proved
by using two magnets on pivots, mstead of one.
595. n hen a piece of iron is in proximity to a magnet, it as-
snmes magnetic properties. Present a piece
of soft iron, b, Fig. 375, towards (he south pole
of a bar-magnet, h s ; it instantly becomes at-
tracted. And if a second bar, c, be presented
to B, it will attract c, although less, strongly
than it is ttee(^attracted by the magnetic bar ;
proving that b assumes magnetic properties
nnder the influence of the bar h s. Gradually
slide Ns ofi*B, and instantly the magnetic pro-
perties of the latter will vanish, and the bar c
will fall from it. The influence exerted by
H 8 on B, is termed induetionf because it induces magnetic pro<
perties in the bar ; retaining them in it, whilst they remain in
approximation. If the end of b be dipped in iron filings whilst in
contact with h s, thej will adhere to it, and arrange themselves in
curved lines. And m the experiment before mentioned (592), in
which iron filings were arranged in curved lines, under tne influ-
ence of a magnet placed beneath them, the filings became mag-
netic by induction^ each single particle acting on its neighbour
like a little magnet on a pivot (593), attracting or repelling it
according to circumstances.
596. Whenever the pole of a magnet induces magnetism in a
bar of iron, the end of the latter
nearest either pole will acquire ^' ^?*
properties of the opposite kind
Fig, 876.
y
-S"
Jf
c
If
to it. Thus, if the iron bar, a b.
Fie. 376, be brought near n s, it
will become magnetic by induction (595), the end a becoming
the north, and b the south pole, provided the v^^ 3^^^
end 8 of the magnet were a south, and n a ' *
north, pole. If the magnet n s be brought in I
contact with the middle of a bar of iron, a b, '*
Fig. 377, the centre, c, will become a north
pole, and the ends, a, b, both south poles. And
if the pole of a magnet be placed in the centre
of a circular piece of sheet-iron, the whole cir
jr
cumference will assume magnetic properties of
the same kind as that of the pole of the magnet, whilst the centre
with which it is in contact will assume an opposite polarity.
332
UAONETlflM.
Fig. 379.
C
K
I
i
1
697. If a magnet, irs, Fig. 878, be broken in half in the
centre, the half 8 will not be foand to possess all sonthern, and n
all northern, polarity, as might be expected, bnt both portions
will become perfect magnets, each
rig, 378. ^^ ^Yi% fractured ends exhibiting a
polar state, as perfect as the entire
magnet. The fractured end, /, be-
coming a south, and n' a north
pole ; although at this middle point, where a' and n' join, no
magnetism could, before breaking it, be detected, nor if the broken
ends be again placed in close apposition. A strin of spring-steel,
hardened and magnetised, answers very well for tnis expenment.
598. If either of the pieces n tl^ or n' s, be afirain subdivided, the
same phenomena will occur : and the same will be repeated as far
as the subdivision may be carried. This shows that me ma^etic
energy, chiefly manifested near the poles of a magnet, is an
aggregate effect of the energy inherent in each particle. A bar-
magnet, then, AB, Fig. 379,
may be considered to consist
of an agg^gate of molecular
ma^ets, e, <2, e, &c., all of
r which have their axes of fK>-
larity similarly arranged ; in
the middle of the bar, c,^ tho
polar enerey of the particles
will be controlled and neutralized by equal and opposite energies
on either side of them ; but on receding from the centre towards
either end, the polar energy of each particle is less and less
neutralized by the polarity of the particles beyond it ; and thus
the agercgate of molecular energy will be manifested at each end
of the bar.
599. An extension of this reasoning will explain why a steel
ring may be converted into a magnet by passing it over the
pole of a powerful magnet, without its exerting any attractive
influence on iron, or exhibiting any other phenomenon charao-
teristic of free magnetism ; for here every portion of the ring being
oontinuovB^ the energy of one pole of each molecule is held dii-
guised by the energy of the opposite kind, on the opposed side of
the next molecule of steel in tne series. On breaking such a ring
in half, the terminations of the fractured portions will be found to
present energetic magnetic polarity, the portions which disguised
their polar state having been removed. And thus ever^ fragment
of the fractured ring will be a perfect magnet, a fact so interesting
and extraordinary that the Abbe Haiiy wittily termed magnets
let polypes du rigne mineral.
600. /The phenomena of induction and consequent attraction
may be explained on the hypothesis that the permanent polarity
of the molecules of a magnet is capable of inducing a similar
y^ J
poUritr in tbe moleculea Id b bar of mimagnetjBed steel, or soft
iron, placed in close proiimilj with it. Thus, if the unmagnetiaed
bar be placed parallel to the tnagnet, the polar arrangement of
their molecules u represented b; the parallel rowe in Fig. 379, but
if placed end to end, that armngemeut will be tjpified Gy the two
halTeB formed bj a transverse diTJBioD of tbe same figure. Tbe
only diflereiice betweeu tbe Bleel and soft-iroD bars is, that in the
former the induced magnetism is more or less permanent, and in
tbe latter, Iranaitory.
It IB not improbable that the phenomena of dunaaffnetiim (to ha
inbaequentlj considered) may be sirnilarlj eipUired bj the hypo-
thesis tbBl a magnet is capable of inducing oppotite polarity, and
consequent repidaon (394), in the moleculea of the diamngnelic
601. The ilaria^'t Corapaa. — A macnetic needle properly
balanced upon a pivot (593), conelittiles the active agent in the
well-known mariner's compass. This invaluable instrument was
used in Euro^ in 1180, according to a sntiricnl poem of Gay of
Provence, entitled "La Bible," in which it was minutely described.
It is tolerably certain that it was known to the Chinese in a rude
and imperTect form, nnder the name of Tchi-tian, or chariot of the
aoQtb, about 2600 years before the Christian era, Miirco Faolo
was the first European navigator who applied the compass-needle
to tbe practical and important purposes for which it is now con-
stantly used, in bis return to Europe from the East Indies in I26tl.
Tbis important property of a magnetic needle pointing always in
a constant direction has Wen variously accaonted for ; thus Cardan
baa supposed that a star lodged in tbe coneleilation of Ursa Major
attracts the needle, whilst otbers with more probability have sup-
posed the earth to be, or to contain an enormous magnet, the poles
ofwbicb nearly correspond with the geographical poles of tbe globe.
602. Tbe mariner's compass, as now con-
gtmcted, consists of a poised needle at-
tached to a card, on which the cardinal
Eints [as they are called) of the compass,
_ W, N, and 8, and the intermediate
points are marked. This is enclosed in a
case resting on a pedestal called a " bin-
nacle-'' These are canstructed in a variety
of forms; Fig. 380 is a design of one of the
best kind, made in brass by Mr. Browning.
The compass is contained in the vase A, and
viewed tbrough a glazed window, a. Two
lamps, o and d, are placed ahove for the
purpose of illuminating the card at night.
It has been aBcertained (as will subsequently
be pointed out) that by using two needles
parallel to. And at a certain distance froDl,
331 UAOKKTISH.
tho N. S. line, instend of a aingla nna cobciding with it, an
important source of error may be obviated : for this purpOM they
muBt bo to placed, aB to intereect the cirouinferaiica of the card
at a distance of 30° on either aide of the N. 8. line.
In ths firsl-class compasBfls made hj Mr. Browning, the two
needles are called dippiag-ntedlet, being sopported on a trans-
verse aiia passing through their oantres of gravity, and trealy
n «,, moving verticslly in sHls in tho
"S- ™'- card, which arrangement is sop-
I posed to dimiuish some of the
a to vhlch the compase IB
e. The distance B A, Kg. 381,
hich one of the needles iutcr-
.thecircumrerenoeisaU". The
rests as usual by a small cup-
,.ed catity in a Bmnll piece of
agate on a ateol point, c, but the agate-cup instead of being in
the p)Bne of the card, is raised consida-
Mb.SW, rablj above it by means of a bridge, b,
I and thus the tendency of ihe card to
oBcillaCion in a vertical plans is much
diminished. This form of compass has
been much approved of by competent
authority.
In the Admiralty standard compaiefonr
parallel fixed needles are employed, as
shown in Fig. 382 ; these are so placed th^t
the distance h a, may be 15°, and x s, 45* ;
the two are therefore at a moan distance
of 30° from the N. S. line,
603. J)eviation of the Cornet. — From what has been alieady
stated (595) respecting the induction of magnetism on ban of
iron, it will readily be uaderslood that the masses of iron osed
in the construction even of wooden ships most always have been
8 source of disturbance to the compass-needle, by innnencing it«
podtioD due to the action of the earth's magnetism alone, and
therefore falsifying its indication of the geographical direction in
which the ship is moving. It must also be evident that this error
of position, called the decialioa of ihe compaii, is immeasurably
increased m vessels built wholly of iron, and especially in those
that are also armour-plated.
A knowledge of the properties of the compass, which has ex-
isted in China for probably more than 4000 years, appears not to
have penetrated into Europe more than fire or sii centuries ago,
and little was dooe towards perfecting the theory of the compass
until the commencement of the present century. In the voyage
of Captain Flinders to Australia in IBOl, he observed, as omera
had done before him, that the ditectiou of the compass-needle fnt-
quently deviated cnnsidonbly from the known variation doe to
DEVIATIOH OP THE COMPASS. 335
the geograpbical position of the ship ; bat not satisfied with
ascribing this apparentlv capricious change to errors of construc-
tion of the compass, as had previously been done, he proceeded to
investigate the causes of the phenomenon, and soon observed that
the error was most considerable when his ship's head pointed
about east or west, and that it entirely disappeared wnen the
line of the keel coincided with the magnetic meridian ; it ap-
peared, therefore, that the north end of the needle was drawn
towards the ship's bows by some unknown force, which he natu-
rally supposed to be the south pole of some unknown magnet
between tne biDuacIe and the bows. But Flinders found that in
Bass' Straits the phenomena were entirely reversed ; the sotUh end
of the needle was now drawn towards the bows, as the north end
had been in the northern hemisphere. From these facts he
readily inferred that the disturbing cause was magnetism induced
by the earth in the stanchions and bolts in the ship's hull ; and
that the disturbing intiuence might be counteracted by upright
stanchions placed abaft the binnacle ; and for this particular error
no better remedy has been devised.
The phenomena of deviation in wooden ships wera further eluci-
dated by the observations of Captains Scoresby and Sabine, during
their voyages of Arctic discovery, but to the present Astronomer
Royal, and to Mr. Archibald Smith, the elaboration of the theory
of compass-deviation in its present complete state is almost en-
tirely due.
604. The extensive introduction of iron in ship-building has
introduced sources of error not only vastly greater in degree, but
also differing totally in kind, from those previously observed.
While the errors detected and corrected b^ Capt. Flinders rarely
exceeded 2** or 3^ in some iron ships a deviation of 50° or 60° has
been observed ; and the existing errors are alwavs largely in-
creased, whenever the ship Keehf or rolls from the action of the wind
or waves. The variation in the nature of the disturbing causes
depends on the materials of which, and the direction in which, an
iron ship has been built. This result may be thus illustrated :
a bar of veiy soft iron placed vertically may be freely magnetized
by induction from the earth, and if reversed in position may be
demagnetized, and its polarity reversed in a few seconds ; while
a bar of hardened steel is scarcely at all susceptible of the earth's
induction. But a bar of hard iron partakes of both these quali-
ties (of soft iron and steel); and although not very susceptible of
spontaneous induction, a lar^e amount of induced magnetism may
be hammered into it, which it requires an equal amount of ham-
mering to subdue, or to reverse ; and this is precisely analogous
to what takes place in iron ships, and in proportion as the iron
partakes of the quality of steel, the induced magnetism once ham-
mered in, cannot by any change of position, or by concussion, be
entirely eradicated. That portion of the induced magnetism
336 MAOKETISll.
which no subsequent mechanical violence will destroy has been
called the permanent magnetism of a ship ; while that portion
which, althou);h not susceptible of remoyal by merely reversing
the position of the hull, may nevertheless be beaten or shaken out
of it, has been called wh^oermanent magnetism, both differing in
their effects on the needle from the induced, or soft-iron mi^«
netism, which varies with the position of the ship on the earth b
surface, and with the direction of her course.
In the year 1839 an extensive Hcries of experiments was made by
Mr. Airy on two small iron vessels, from which he was led to con-
clude that the source of disturbance was ftlmost entirely the perma-
nent and sub-permanent magnetism of the vessel (which of course
concur in the direction of theirdisturbinginfluence), and thatif these
were compensated, the temporary magnetism of position might be
neglected. This compensation he effected by two permanent
magnets placed in the neighbourhood of the compass, one fora
ana aft, the other in a transverse position, the magnitude and
position of which must in each case be determined by experiment,
guided by the results of analytical investigation. But the diffi-
culties of compass-adjustment are increased by another cause not
vet alluded to. Liarge horizontal masses of iron, such as deck-
beams, engine- shafts, &c., are introduced, and these disturb the
compass in a manner totally different from the upright stanchions,
&c., previously alluded to. There are, therefore, three distinct
disturbing influences to be combated : —
1. Vertical induction ;
2. Horizontal induction ;
3. Permanent and sub-permanent magnetism.
Theory and experiment concur in showing that the influence of
horizontal induction vanishes when the ship is placed in either
direction in the magnetic meridian, or perpendicular to it, and
that it is a maximum at the four intermediate points ; this dis-
turbance has therefore been called quadrantaL
Vertical induction will cease to affect the needle only when the
keel is iu the magnetic meridian, and the maximum effect will be
produced when the keel is across that line : permanent magnetism
vrill cease to act onlv when the needle is parallel to the magnetic
axis of the ship, which may or may not coincide with its longitu-
dinal section ; and the same influence will be a maximum when
the needle is perpendicular to the ship*s magnetic axis : the devia-
tions arising from these causes are hence caUed aemieircuiar.
605. The qnadrantal may always be readily discriminated
from the semicircular errors, and both horizontal and vertical in-
duction may be compensated by corresponding^ elon^ted masses
of soft iron suitably disposed ; the latter especiallv m some ships
18 partially compensated by the iron masts, if the compass be
suitably pUced. As, however, some difficulty and inconvenience
OOMPBHBATIOH BT MAGHBTS. 337
attends the correction of qnadrantal deviation by mames of soft-
iron, it occurred to Mr. JByans, and has been demonstrated in a
paper by Mr. A. Smith and himself,* that the correction might
De made by the reciprocal action of two compasses, placed at a
distance of from 18 to 24 inches from each other, as in the
ordinary double binnacle. For this purpose, the two needles
must be of equal power, and as their correcting influence bears an
inTcrse ratio to the earth's horizontal force, it is necessary, for
accurate correction in high latitudes, that the interval between
the two needles should be capable of adjustment.
The semicircular error, arising from permanent and sub-perma-
nent magnetism, may be compensated by magnets, after Mr.
Airy*8 pum, but it is not always easy to riu^htly apportion the
semicircular error between the vertical induction, and the perma-
nent magnetism; and, moreover, any discrimination between
permanent and sub-permanent magnetism is absolutely impossible.
It must, therefore, be borne in mind that, supposing all semi-
circular deviations to be accurately and completely corrected by
permanent magnets, after Mr. Airy's plan, still the residual
▼ertical-induction-error will be doubled m a southern latitude,
and that arising from sub-permanent maffnetism (frequently veiy
considerable) will be doubled as soon as that sub-permanent mag-
ne tism is beaten out of the ship by the impact of the waves, or
still more rapidly and effectually knocked out of her by collision
with a pier-head, another vessel, or a sunken rock.
In fact, the efficient correction of the compass by permanent
ma^ets would be utterly hopeless, were it not the result of ex-
perience that, after a certain amount of buffeting, say after the
ordinaiy casualties of a twelve months* voyage, sub-permanent
magnetism entirely disappears, and the vessel appears to assume
its permanent magnetic condition.
In a new ship, therefore, magnet-compensation is a sadly broken
reed to rest on, as experience has unfortunately shown. A new
iron ship, the TayZeur, of 2000 tons burden, sailed from Liver-
pool with emigrants early in 1854. She had, as usual, been
gwung^f and evinced very large compass errors, amounting in
one position to 60**. Like all very large errors, this was undoubt-
edly due to the inherent— or transient — ^magnetism of the ship.
A compensation by magnets was applied in the usual manner,
and the compass-readings were fairly correct. She, however,
experienced severe weather in going down Channel, and within
two days after leaving port, was wrecked, with groat loss of life,
• Phil. TnoB. leOB.
t Swinging a Bhip is brioging her head round to all points of the oompass
in soooeflsioiK for the purpose of ob«erTin|( the errors of her oompaasee.
It maj here be remarked that this process is mudi faoilitated by the use of
the " Feloms" of Ur, Browoing : which oonsista of a oompass-oard with a
sight attached to it, but without a needle, and moving in asimath with
saffljientlj stiff firiction.
z
SS8 MAexEnsM.
on the coast of Ireland. No reasonable donbt can now be enter-
tained that in her first encounter ¥dth the waves, she had parted
with much of her snb-permanent magnetism, and that the applied
correction had become worse than useless, — a delusion and a snare.
It was hardly believed at the time that so rapid a magnetic change
could have taken place, but the sad catastrophe led to careral
observation on the loss of magnetism in ships. The ill-fated
BoycU Charter sailed on her first voyage with an error of 20** in
her standard compass (which was uncorrected). All but about 3*
of this error disappeared in her first voyage, and the steerage-
compass, which had been compensated, came home with an error of
22^, due entirely to the over-action of the compensating mafi;net.
On the contrary, a striking example of the magnetic stability
which a ship acquires after some years of service, was afforded
by the Adventure. She once struck on a rock with so much force
as to tear awa^ a portion of her outer iron skin ; yet, on careful
examination, it was found that this violence had produced
scarcely^ any sensible effect on her permanent magnetism.
The inherent difiSculties of mecnauical compensation have led
Mr. Archibald Smith and some others to prefer a careful observa-
tion and tabulation of a ship's compass-errors, to their mechanical
compensation :^^oubtle88 both plans have their advantages and
disadvantages, the balance of which it must be left to practical
mariners to detennine.
Analysis and experiment have disclosed another source of
error due to the proximity of the compensating magnet, to which
the name of eexttrntal deviation has oeen applied, on account of
its occurring periodically at the ^th parts of the circumference ;
this error may however oe entirely obviated by the arrangement
of two or four compass needles, already described (60^ : and
another, termed octantal deviation, due to the proximity oi a soft-
iron coinpensator, is by the same means eliminated.
606. The effects of horizontal and vertical induction may per-
haps be better apprehended by the aid of a diagram. Let the
transverse bar ab, Fig. 383, placed transversely beneath the
Fig. 9S3.
binnacle, represent the aggregate of transverse masses of iron,
and either the single bar c d, or the two c e, f d, that of lonnta-
dinal masses. The qnadrantal deviation would be produced by
cither of these types, but there is this important difference be-
w^
BRSOBS IN AHMOUB-PLATED 8HIPfl,
339
tween tbem, that the continuous bar, c d, would always diminish
the dirvctive force, while the separated bars c b, f d, would in
crease it, if their combined inflaence were greater than that of a b.
If the effect due to vertical iron be that of a bar o h, its in-
flnence on the heeling error due to the combined effects of a b
and 6 H will be readily understood. If o be beneath a b, then g,
and the end of a b that is raised by heeling, will consmre in pro-
ducing a heeling error to windwara, because they wul then be
both on the windward side of a vertical plane passing through
the compass ; but if o be above a b, then in heeling they will be on
omorite sides of the same vertical, and the heeling error will be the
difference of the actions of a b and g h ; and if the influence of g H
predominate, the heeling error will be to leeward. Compasses on
the upper-decks of iron ships, especially if built head to N., are of
the type a, and there is a large heeling error to windward, but
the main-deck compasses, and particularly those of armour-plated
ahipe, are generally of the type b, and their heeling error is fire*
qnently to leeward.
607. The more recent investigations by Messrs. A. Smith and
Evans of the magnetic conditions of the armour-plated ships of the
Boyal Navy,* have shown that the introduction of armour-plating,
and tiie great increase in the amount and thickness of iron used m
the construction of modem ships of war, have greatly increased
the amount of the deviations previously considered, and have
given importance to two sources of error not hitherto regarded,
viz., the cUmintUion of directive force, and the heeling error. The
observations made confirm the conclasion that the semicircular
deviation in an iron ship is chiefly due to the attraction of the
north pole of the needle to that part of the ship which was south
in building, this direction in armour-plated ships being modified
by the direction in which the ship was plated. The following is
an approximate value of the semicircular deviations in British
waters in the Warrior^ Blatk Prince, and Defence, and of the
proportions due respectively to vertical inductiqu, and to perma-
nent magnetism : —
S0miciro. deT.
Vert. ind.
Perm. mag.
Warrior . . .
Black Prince .
Defence . . .
-244°
+ 23
+ 26i
+ 12'*
+ 23
+ 14i
-36r
0
+ 11J
The great difference in the last values between the Warrior and
Blade Prince depends on the fact of the former having been built
head to N., and the latter, head to S.
In the iron-built armour-plated ships the quadrantal deviation
* Phil. Trans. 1866.
z 2
340
IfAONETISU.
becomes very laree, greatly exceeding what has been fonnd in'
other vessels. This, however, is not to be attributed to the
armour in all cases, as it has been determined both by theory and
observation, that in plated wooden ships, with the compass in a
central position, the effect of the armonr is to diminish tlie qnad-
rantal deviation.
The following table shows the larg^ amoont of qnadrantal de-
viation in four armour-plated iron ships : —
Compua.
Wurior.
BUe. PriDo«.
Defcnoe.
BaosUaee.
Standard •
Steering -
Main-deck
8'*2r
U 66
11 43
7'* 38'
10 32
13 06
7*00'
10 16
14 35
6M7'
8 28
14 00
In the following armour-plated wooden ships the ouadrantal
deviation of the standard compass was diminished to tne respeo*
tive amounts : —
Royal Oak, 3' 0^ ; Caledonia, 2* 67' ;
Ocean, 2*31'; Prince Consort, 2* IS*.
The diminution of direction force, producing sluggishness of
the needle, is also remarkable ; in the main-deck comnasses of
some of the armour-plated iron ships the mean direction force
scarcely exceeds 0'7 of that exerted on the same needle on shore.
In the same vessels the amount of heeling error is also very
considerable, averaging about l** for each degree of heel, or lateral
inclination ; but greater in those that have ueen built head to N.
Thus, in the Warrior ^ it amounted to 1** 49' for each degree of
heel. This error may be corrected by means of a vertical magnet.
On the contrary, in the armour-plated wooden ships, the heeling
error is very small, and generally to leeward ; thus, for each degree
of heel the amount is — Royal Oak, 7' to windward ;
Prince Consort, 8' to leeward ; Ocean, 15' to leeward.
The more important practical conclusions drawn by Messrs. A.
Smith and Evans are, that the best position for building an iron
ship is head to 8.; that if armour-plated, the plating should
be laid on in a position the reverse of that in which the hull
was built ; that there should be as little iron as possible within
the space of a cone formed by the revolution of a line passing
through the needle, and forming an angle of 64'' 46' with the
vertical ; and that it is very hazaraous to compensate the compass
of a nevf ship by permanent magnets. It also appears that the
effect of iron masts is to increase the qnadrantal error.
The scope of this treatise forbids a full analytical investigation
of this all-important subject, but it is hoped that enough has been
written to render its leading features intelligible. The more ad-
vanced student, and the naval officer, will find the subject fully
•^^■^K ■ - V . — »^*- ■ - ■ - . ■■'-.•-».■-_ ,iif
TEBRE8TBIAL UAOKETI8U. 341
treated in all its bearings in the last edition of the " Admiralty
Manual on Compass Deviation."
608. It appears that, admitting the existence of bat one mag-
netic pole in either hemisphere, it is difficult to explain the pheno-
mena noticed bj different observers. In the northern hemisphere,
two poles or centres of attraction have been distinctly made out,
one m Siberia at 102** E. long., and to the north of 60** N. lat. ; the
other, and apparently far the more important, is situated about
96"* 40^ W. long., and 73** 14' N. lat. ; these two poles are about 200^
of long, apart, measured across Qreenland and Norway. The two
southern poles are supposed to be located, one near Cape' Horn,
and the other to the south of Australia. Frof. Hansteen supports
this notion of the existence of two magnetic poles in each hemi-
sphere ; he however places them rather differently. On the other
hand, a high authoritT, iVof. Gauss, from more recent researches,
is induced to conteua for the existence of a single pole in each
hemisphere. Faraday has, howeyer, expressed an opinion, that
the hypothesis of the existence of magnetic poles at the geo-
graphical poles is unsatisfactory, and that the phenomena of the
dipping needle lead to one of two things ; either that the mag-
netism of the earth is simply the result of the induction of electric
cmrcnts — or, if a terrestrial magnet really exist (which is highly
improbable), its poles must be close together, near the earth's centre.
609. The compass-needle does not point exactly north and
soath, a circumstance generally supposed to have been first ob-
served by Columbus iu his earliest voyage of discoyery in 1492 ;
and consequently, the magnetic meridian, or plane bisecting the
earth in the direction of the needle, does not coincide with the
geographic meridian. The magnetic meridian is not constant,
sometimes bein^ on the east, and sometimes on the west of the
geographic meridian; this difference is i^. 38i.
termedthe magnetic declination. Thus, 1
if AB, Fig. 384, represent the geo^phic ^^ — I — -v^^
meridian, v s will represent the direction /^ \ x.
ines \ \ y
683, ^^^J^
lagnetism,
first paper on the isogenic tines, or lines
of equal declination, in the year 1683,
and a general map, comprising all that
was then known on the sul^ject, in 1701. He supposed the exist-
ence of four magnetic poles (a conjecture whicn has been con-
firmed by subsequent observation), one in each hemisphere sta-
tionary ; the other two, the northern to the east, and the southern
to the west, subject to a periodic revolution in about 700 years.
On certain portions ot the earth's surface the magnetic and
842
MAGNETISM.
geograpbic meridians appear to coiDcide, as in some parts of
North America, the nortb-eastem point of South America, the
western part of Austrah'a, &c. These places are connected by an
imaginary irregular cnrred line, termea the line of no variaHon.
This line appears to move proffressively over the surface of the
globe ; it passed through London in 1660, in which ^ear the
needle there pointed exactly to the north, and in 1663 it passed
through Pans. In its westward course it has lately traversed
Amenca. These have been termed by Prof. August, agonic
lines, and two of them are supposed to exist, one in the western
hemisphere termed the American agoiMy and another in the
eastern, or Asiatic cigone. They both intersect the geographic
meridians at different angles. Prof. Gauss is induced to believe
that there exists a greater and lesser agone ; the greater em-
bracing the globe like a meridian, passing through the magnetic
pole, and dividing the earth into an eastern and western magnetic
nemisphere. Of these the former will embrace Australia, Arabia,
Persia, and Russia ; the latter including the eastern parts of North
and South America. The lesser agone of Gauss forms an oval,
and runs through Eastern Siberia and China. The mean decli-
nation in Europe is about 17^ W., increasing towards the west,
and decreasing towards the east. The following table shows the
amount of declination in London and Paris at different epochs.
Year.
Beolia.
Year.
Beelin.
YAUS. 1
1580
1634
1660
1670
1690
1720
1740
1750
1770
1780
IP 16' B.
4 6 1.
0 8 —
2 30w.
6 Ow. I
14 17 w.
17 Ow.|
17 48 w.
21 9w.
28 17 w.
1780
1800
1810
1818
1820
1830
1841
1850
1860
1865
23''39'w.
24 8 w.
24 11 w.
24 30w.
24 22w.
24 2w.
23 16 w.
22 23w.
21 14 w.
20 82 w.
Year.
Deolin.
Year.
BecIin.
1680
1618
1663
1678
1700
1767
1785
ir86'i.
8 Ox.
0 0-
1 SOW.
8 10 w.
19 16 w.
22 Ow.
1805
1813
1817
1822
1826
1830
1866
22** 6'w.
22 28 w.
22 19 w.
22 Uw.
22 12 w.
22 «w.
18 44 W.
610. Prof. Ben wick found the declination to amount to 5" 28^ W.
at New York in 1837. At London the needle at present points
about 204** west of the true north pole, the maximum variation
having been attained in 1318, when it amounted to 24** SO'. Largo
variations have been observed by the Chevalier de Langle be-
tween Greenland and Labrador, amounting to 45° W.: and by
Captain Cook, in lat. 60'' S. and long. 92'' 66' E., where the
variation amounted to 43** 6', east of the geographic meridian.
611. A magnetic needle, if movable on a horizontal axis pass-
ing through its centre of gravity, does not remain horizontal ; its
north pole io our hemisphere dipping considerably; and in the
southern hemisphere the opposite pole inclines ; this is termed the
dip or indinatton of the needle, and a needle thus suspended i|i
termed a dipping-needle. The inclination of the magnetic needle
nrcusATiov. 843
WM discovered by Robert Nonnan, FIff, 885.
in the 16th centary, who found it to ^f"^ ""^v.
smoont to 72". Let a b. Fig. 386, /\ | >.
be a needle balanced on its hori- ^ \ '• \
Bontal axis, o, and placed in the / \ | \
magnetic meridian; in England, ^/ \ J.^
then, instead of remaining horizon- ^\
tally, as d s, it dips or inclines to- \
wards the north, its north pole V
forming an angle, bob, of nearly x,^
68 J* with the horizontal line d e. ^ ^
612. A line traced over the earth's surface through the various
points at which the dip becomes evanescent is called the mag-
netic equator. It does not coincide at more than two points with
the geographical equator, from which it diflbrs as the magnetic
do from the geographical meridians. The magnetic eouator is
tolerably regular for a part onlv of its course, and may oe repre-
sented by a part of a great circle inclined at an angle of from 12^
to 13** to the geograpnic etjuator, which it intersects in at least
two points ; one near the island of St Thomas in long. 8* £.,
another in the PaciBc in long. 142*^ £. In the southern hemi-
g»here, however, especially between the Sandwich and Friendly
lands, this line presents numerous irregular and sinuous curves
Hke the magnetic meridian. The magnetic equator is an irregular
doable curve to which the term of adinie line has been applied ;
it, like the agonie lines (609), appears to be undergoing a pro-
greuive motion, which, as far as observations have been made, is
in a direction from east to west. Lines connecting places where
the inclinations correspond are called iaodinie, or isoclinal. These
have during the last twenty years become more nearly parallel to
the meridians.
The greatest inclinations of the needle ever observed, were by
Captain Cook, who, in lat. 60°40' S. observed it to be TSMa';
Captain Phipps, in 1773, in lat. 79" 44' N. found it to be as great
as 82* 9' ; and Sir James Roes, in 1831, in the vicinity of Hudson's
Bay, in lat. 70** 5' 17" N.. found the dipping needle to be within
one minute of being perrectly vertical.
613. Prof. Krafit, of St, Petersburg, announced in 1809, a veiy
simple law governing the amount of the dip, at different parts
of toe earth s surface, which has been confirmed by the later
researches of M. Biot, viz., if we measure the latitude of any place
from the magnetic equator, and calculate its tangent, it wul be
found exactly equal to half the tangent of the dip at that par-
ticular locality.
614. The inclination or dip of the needle undergoes a secular
change, but bv no means to so great an extent as the declination,
as shown by the following table : —
344
MA0SSTI8M.
Tear.
Inolin.
Tear.
Indin.
TAMia.
1680
1723
1773
1786
1790
1800
1818
73*30'
74 42
72 19
72 8
71 53
71 25
70 34
1828
1830
1840
1850
1860
1865
69*47'
69 38
69 12
68 51
68 21
68 2
Yaar.
Inelin.
Tear.
Inelin.
1798
1810
1818
1824
1826
69*61'
68 50
68 35
68 7
68 0
1829
1831
1835
1840
1865
67*41'
67 40
67 24
67 7
65 58
The diminution of the magnetic dip has heen going on in
London for the last half centniy with great regularity, at the rate
of about 2*81' annually. From the observations of M. Quetelet,
it appears that the angles of inclination and declination seem in
Europe to be undeigoing a tolerably constant gradual diminution ;
these angles at Brussels were found by this philosopher to be of
the following values : —
Tear.
Inelin.
DecUn.
Tear.
Inolin.
Deolin.
1827
68* 56' 5"
22* 28' 8"
1838
68*26' 1"
22* 3' 7"
1830
68 51 7
22 25 8
1839
68 22 4
21 53 6
1832
68 49 1
22 19 0
1841
68 16 2
21 38 3
1834
68 38 4
22 15 2
1860
67 30 8
19 23 5
1836
68 32 2
22 7 6
1865
67 19 9
18 50 6
615.^ Besides these gradual changes, which are termed aecular
varitUionaf of two of the magnetic elements, the declination and
the dip or inclination, various other periodic variations have been
detected by the careful reduction of large series of observations,
which Gen. Sabine has clearly shown to be of cosmical origin,
that is, arising from causes external to the earth itself.*
These magnetic variations may be arranged in two classes : one
comprising those that are uniibrmly periodic, continuous, and
comparatively small in extent ; the other, those that occur at un-
certain intervals, and are equally uncertain both in magnitude
and duration, but yet manifesting certain periodic characters. To
the latter class the term disturb<meeSf and when violent, that
of magnetic BtorttiM has been applied. These arise from causes
that are at present very imperfectly understood.
Of the first class the most considerable is the iolar-diumal
tHKriaHoiif that which, with small climatic differences, is repeated
on each succeeding solar day .f The solar-diurnal variation of de-
* The Beade Leotnre, Cambridge, IMS. By Mqor^en. BaWae, P.B.& At.
t A day in astrunomical lanynafe ia the mean period between two rae*
oeeaive tranBita of the meridian : tnu, a tolar day, a Unar day, a tidtMul
DIUBXAL TABIATION. 345
clination has been folly inTestigated ; the ohsenrations are made
on a freely snapeuded har-ma^et, called a declination-magnet,
or deelinameter. This Yariation is alike in direction all oyer
Europe ; the north pole mores towards the west from snnrise to
about an homr after noon, when it retrogrades towards the east,
until eight o'clock in the evening, after which it remains nearly
Btationaiy until sunrise. The terms East and West, here used,
most be understood to reJer to the moffnetie, not to the geo-
araphical meridians : between the nuu^netic and terrestrial poles,
for example, geographical East would be magnetic West, and viee
veraA; and in surrounding regions, the two meridians will be in-
clined at all angles to each other. The amplitude of this yariation
differs considerably in different parts of the earth, and also in dif-
ferent months of the year, but the nuizimum yariation precedes
the maximum temperature ; in London the variation of declination
attains, in June and July,19''6 ; and in December, 7''6. In Paris,
its maximum is as iu London in Jone and Jul;, and amounts to
firom 13' to 16', falling in December to 8' or KK. In the northern
ports of Europe and America, the diurnal variations are more
considerable, but less regular ; at the magnetic equator (612) they
diaapi>ear entirely ; and on the south of that line they re-appear
in an inverted order.
Science has been greatly indebted to the ingenious researches
of Faraday, for a nighly probable explanation of the pheno-
mena of solar-diumal magnetic variation. This acute phi-
losopher having observed that oxygen gas exhibited magnetic
Properties, thence inferred that the diurnal variations might be
ue to the varying intensity of this foive. It has long been known
that an artificial magnet loses a portion of its force by elevation
of temperature, which it regains, either partially or entirely,
according to circumstances, as the temperature is again reduced.
And the same changes take place in the magnetic intensity of
the atmosphere, whioh is due to the oxygen it contains. In our
latitude, atmospheric ma^etism will evidently be weakened east-
ward of the meridian dunng the momine hours, by the solar rays,
and consequently, as it is observed to be, the deviation is then
towards the west ; and the contrary must for the same reasons
happen during the afternoon : also, as the atmosphere does not
immediately part with its absorbed heat, the heated mass of air
mnst lag somewhat behind the sun, hence the change of deviation
from west to east does not occur exactly at noon, but at some short
period afterwards. The observed average quiescence of the de-
clination maenet during the nocturnal hours, is an evident con-
sequence of the same hypothesis. The observed absence of diurnal
variation in the neighbourhood of the magnetic equator is equally
day, an tifmify different periocU of time. The h ^x^n of an attronomical (in
•ootndisunotioii to ft eimi) daj are nambered eonsecvtively from d^, it* oom-
it at noon } thus 6 A.if. in dvil reokoning will be, aatronomii^Uy, 16^.
346 IfAOMBTIBK.
a necesBaTy result ; for the atmoflphere will be eqaallj affected by
the sun's rajs north and south of a magnet there sitoated : con-
Bequentlj, two equal deflecting forces, in opposite directions, will
neutralize each otner ; while in the southern hemisphere, the sontli
pole of the magnet will be similarlj influenced with the north pole
u our hemisphere ; and consequentlj, the diurnal movements will
be there exactly contrary to ours.
If two bar-magnets be laid on a table, with their opposite poles
towards, and at a small distance from, each other, and a very
pmall magnet be placed anywhere in the magnetic fleld (554), the
whole being covered with a sheet of card-board, and strewed with
iron-filings, the distortion of the magnetic curves by the small
magnet will rouffhl^ illustrate the distortion of the terrestrial
curves by atmospneno magnetism.
616. The solar-diurnal variation of declination has been found by
Gen. Sabine to be subject to a verv remarkable semi-annual in-
equality. The means of the diurnal variations at each solar hoar
have been taken from April to September, and from October to
Mareh, and tabulated in the form of curves. In the summer curve a
maximum, but in the winter curve a minimum occurs at from 19*^
to 20^; and in the summer curve a minimum, but in the winter
one a maximum, at from 1^ to 2^. At seven magnetic stations
from which the observations have been reduced, the contour of the
curves is extremely similar : at two stations, namely, Kew and
Toronto, a small secondary maximum and minimum occurs be-
tween 5^ and 7^. These semi-annual curves are at each station bo
remarkably similar to each other, that if one were turned over on
the other about the line of mean position, they would almoet
exactly coincide ; one curve may in fact be said to be the reflected
image of the other. The extreme amount of this inequality does
not exceed 4'.
617. Lunar-ditamat Variation of Dedi7iation.-—'Bjf a Judicious
exclusion of " disturbed" observations, and by an elimination of the
Bolar-diumal change. Gen. Sabioe has succeeded in establishing
the reality of lunar influence on magnetic declination ; an influence
not less remarkable for its extreme regularity than for its minute*
ness, its amount not exceeding 20". The curve presents a re*
markably qrmmetrical double progression in each lunar day,
having two easterly and two westerly maxima at very nearly
equal alternate intervals of the hour-cirole ;* and in this respect it
closely resembles the great tide-wave of the ocean : may it not be
due to the atmospheric tide-wavcj which must at the same periods
alternately increase and dimimsh the amount of atmospheric
magnetism on opposite sides of the meridian? At Pekin, some
degrees nearer to the equator, this variation is zero 84^ earlier
than at Kew. In the southern hemisphere, at the Cape of Good
Hope, which is nearer to the equator tnan Hobarton by about the
• FULTrMM.1868.
DIBTUBBJUICB TABIATXOVS. 347
same amoant, it is also 2\^ earlier ; and at St. Helena, which is still
nearer the equator, it is 3i^ earlier than at Hobarton.^ There is
no trace of a decennial period in this ▼ariation. A similar pro-
gression was snhseqnentljr discoTered, by the same indefatigable
observer, in the lunar-diurnal yariation of the horizontal and ver-
^taX components of the earth's magnetic force.*
618. Annual Variation of Total Force»—Then is a Pinall an-
Bual variation both of the total magnetic force and of the dip ; both
have their greatest value at the winter, and their least at the
Bummer solstice. As this variation is alike in both hemispheres,
in which the seasons are opposite, it must necessarily depend
on the sun*s distance from the earth, and not upon changes of
temperature.
619. The force by which the north pole of the dipping-needle is
directed obliquely downwards, may be conceived to be compounded
of two forces (284), one acting horizontally, and the other verti-
cally ; by the former of which acting alone, the needle would as-
some a horizontal, and by the latter, a vertical position. In this
conntry, the proportion of the vertical to the horizontal force is
nearly as 2 : 1. As the instrumental means of measuring small
variations of these two components are more available than those
for measuring directly the aip and total force, the values of these
magnetic elements are frequently inferred from a comparison of
the components mentioned.
620. disturbance Variations, — Some remarkable facts have
been developed by a separate investigation of the second class of
magnetic pnenomena Mfore alluded to. It appears from the re-
duction of the observations ou declination made at Toronto and
Hobarton in the years 1843-4-5, by Gen. Sabine,f that both an
annual and a diurnal law may be discovered in the larger mag-
netic disturbances, called shocks and storms. There appears to
be a maximum in summer, and a minimum in winter, and to a
certain extent there is a correspondence between easterly distur-
bances at Toronto, and westerly at Hobarton. Easterly distur-
bances predominate in l^orth America, and westerly disturbances
in Nortnem Asia. The aggregate amount of disturbance is small
in the Tropics, it is augmented in mean latitudes, but by no means
in the ratio of the increase of latitude, as there is a large dispro-
portion in different meridians ; for example, the hourly observa-
tions made at Point Barrow during 17 months in 1852-4, show
an amount of disturbance unparalleled elsewhere, even in the
highest latitudes. Here, as elsewhere, a correspondence has been
observed between the occurrences of Aurora Borealis, and mag-
netic disturbance : it is recorded that Aurora was seen during at
least one-third of the hourly observations then made. It has
since appeared that the mean diurnal disturbance-variation of
declination is remarkably similar at all magnetic stations, the
• FhlL Truu. 1866. t Fkil. Trans. 1851.
548 MAONBTI8M.
distarbed obBer^ations of which have been reduced. It may here
be remarked that the "disturbed*' observations have been taken
to be those that show a variation from the mean position exceed-
iuff a certain fixed amount, which depends on the amount of local
solar diurnal variation. It was then (in 1861) stated to be pro-
bable that if the larp^r disturbances were eliminated, the residual
diurnal variation might prQbablj appear as a single progression
with but one maximum and one minimum in uie twenty-four
hours; and this anticipation has subsequently been verified.
When in the disturbed observations made at Kew and Hobar-
ton (separated from each other by nearly half the circumference
of the earth), the easterly and westerly deflexions are separated,
and curves laid down representing the local-hourly means, a very
near approximation is observed between both pairs of curves.
They all, and especially those showing the easterly deflexions,
present but little variation during the morning and mid-day
nours, from 18^ to 3^ ; there is then a steady advance towards a
mirUmum in westerly, and towards a maximum in easterly deflexions,
from 3^ to 1 1^ and then an equally steady retreat during the
night to 18''. At Nertchiusk in Siberia the curve of westerly
deflexions corresponds very closely with that of easterly deflexions
at Kew and Hobarton, but the minimum occurs at 9^ and the
maximum at 21^, just twelve hours later. May not this depend
upon the providential presence of Aurora during the long and
dreary nights of that inhospitable region?
621. One of the most singular and unexpected results of the
reduction of the disturbance-variations of declination has been the
discovery of a periodical change in the annual amount of disturb-
ance ; and a v^ry remarkable coincidence of natural phenomena,
between which there is no apparent connexion, has been revealed
by the independent researches of Gen. Sabine, and M. Schwabe :
the former naving reduced the lar^r magnetic disturbances ob-
served at Toronto, and other colonial observatories, during a con-
aiderable period, found that their frequency evinced a gradual
increase, and subsequent diminution, during a cycle of nearly
eleven years ; and exactly the same cvcle, having simultaneous
maxima and minima, has been assigned by the latter to the vary-
ing frequencv of the observed occurrence of spots on the sun's
disc. Prof. Lament has assigned a duration of 10*43 years, and
Prof. Wolf one of U'll vears, to these solar periods. These wholly
independent investigations were published at nearly the same
time.
622. The magnetic changes that occur in any given locality
may be comprised under six different heads, independently of the
sulidivision already made, into ordinary and extraordinary varia-
tions, namely, those which deflect the declinometer to the east,
and those which deflect it to the west ; those which increase, and
those which decrease the inclination, or dip ; and those which in-
increase, or which decrease the intensity of magnetic force. The
OBSBBTATIOH OF MAaHBTIC YARIATIOKB. 84S^
remits of the researches above mentioned appear in some measure
tb confirm the anticipations of M. Gauss, one of the ablest pioneers
in opening out the path of magnetic investigation, that the phe-
nomena arranged under each of the above heads would be found
to be governed b^ independent laws. M. Gauss has truly re-
marked that it will be a triumph of science should we at some
future time succeed in unfolding the intricacies of the phenomena
of magnetic variation, in separating the individual forces of which
thej are the component result, and in assigning the source and
the measure of each.
The instruments by which the changes of the magnetic ele-
ments are observed are the declinometer; the bifilar, or horizontal
force, magnetometer ; and the balanced, or vertical force, magne*
tometer. The declinometer consists of a bar-magnet freely sus-
pended by a long bundle of untwisted silk fibres : the variations
of the position of this magnet correspond with those of the vertical
plane in which the earth's force is exerted.
The bifilar is a similar bar-magnet, suspended by two nearly
Srallel bundles of fibres, separated by a small interval. The
nble point of suspension is twisted round until the bar assumes
a position exactly perpendicular to the magnetic meridian, in
which it will then be retained by the opposition of two equal
forces — the gravity of the bar and its appendages tending to un-
twist the suspension skeins, while the horizontiO component of the
earth's force tends equally to turn the bar in the opposite direc-
tion, and to increase the twist. As the former of these forces re-
mains constant, it is clear that any variation of the latter will
produce corresponding changes in the position of equilibrium of the
magnet : and it is by the observation of these changes of position
that the variations of horizontal magnetic force are determined.
The balanced magnetometer is a bar-magnet, very delicately
poised on knife-edges, resting usually on planes of agate, so as
to move in a vertical plane like the beam ofa balance (113). And
Uke the balance, this instnunent is most perfect, when a single
knife^dge rests on a single plane. To dimmish the influence of
the twisting tendency of the magnetic force, it is better that the
middle portion of the knife-edge be cut away. This instrument
is placed at right angles to the magnetic meridian, and is main^
tained in a horizontal position by a weight, or, more correctly
speaking, bv a portion of its own gravity, which coimteracts the
tendency of the earth's vertical force to place the magnet in a
vertic4i^l position. As the counterpoise remains constant, it
follows that any changes in the amonnt of vertical force will be
indicated by corresponding changes in the position of the magnet ;
which latter have been made a subject of observation.
623. The method formerly adopted for observing the indications
of these instruments has been that of viewing, through a fixed
telescope, the divisions of a fixed scale reflected by a plane mirror
attached to each magnet: or by viewing a finely-divided scale
360 IfAOHBTlSM.
on glass, placed at the further end of the bar, in the focoa
of a lens placed at the nearer end. But by this system of obsef>
vation an imperfect knowledge of the nature of magnetic changes
has been obtained; and as it has been deemed necessary, in
magnetic observatories, that the observations of the various in-
struments should be made at intervals of at furthest two hours, by
night as well as by day (many important changes being even
then lost sight of), this laborious duty has devmved upon the
assistants ; hence some means of enabling these instruments to
record their own changes was long an acknowledged desideratum
in physical science. With the aid of photography, this much-
desired object has been attained bv means of instruments the
construction of which will presently be described.*
By these instruments, an unerring and almost uninterrupted
record of all magnetic changes is now maintained at the Koydl
Observatory, Greenwich. These results could not have been
obtained by personal observation ; for even if every telescope were
oonstantly watched by the eye of an assistant ^which would require
a very numerous staff), the results would still be liable to errors
of observation ; and occadonallv the magnetic variations are too
rapid and transient to be contmuously recorded by an observer.
It may further be remarked, that since the employment of this
apparatus at Qreenwich, the number of assistants in the magnetic
department has been reduced, and the fatigue of night duty has
been dispensed with entirel;^.
624. Magnetic registration is undoubtedly the most useful
application hitherto made of the beautiful art of photography.
The method suitably applied to each of the ma^etic instruments
may be thus described : — A concave metallic mirror, three inches
in diameter, is attached to each magnet by a frame possessing all
requisite adjustments : the rays of hght from a lamp or ^as-bumer,
placed at a distance of about two feet from the mirror, pass
through a small aperture in a metallic plate and fall on the mirror,
whence they are reflected to a focus at a distance of about nine
feet. The reflected pencil should pass as near as convenient to
the source of light, m order to reduce, as much as possible, the
* The merit of these instramenta wm acknowledged by the aw»rd of •
council medal bj the jurors of the Qroat Exhibition of 1861, and of a pre-
mium previously offered by the OoTernment, to their inrentor, the present
editor of this treatise.
It is desirable that self*re){isteriog magnetic instruments should be pro-
Tided with the means of making eye obsenrations, in order that the photo-
graphic results should be from time to time compared with direct obaerra-
tions on the same instruments. In the set of instruments designed by the
writer^ and erected under his superintendence at the Imperial Obserratory
at Pans in 1856, the three bar-magnets are hollow cyhnders about eight
inches long, and one inch in diameter, having a collimator scale flnelr nued
on glass at one end, and ao achromatio lens at the other, and the ooUimator
•cafe is viewed by the lens through the tube. This tubular fbrm is probaUlj
the best adapted fur the purpose.
ADTOMATIC BSQOTBATIOH BT PHOTOOBAPHT. 361
distortion of the image fiom oblique reflexion. The source of
light bein^ fixed, it is clear that the moyements of the focal point
of li^ht will correspond with those of the magnet : but the angular
deTiation of the luminous image is evidentlj double that oT the
mirror ; for each of the angles of incidence and reflexion is in*
creased or diminished bj the same quantitj. A cylinder covered
with photographic iiaper is so placed that the point of light m^y fall
OD it, the axis of tne cylinder beins^ parallel to the motion of the
local point. The cylinder is camea round on its axis by clock-
work, and, by the combined movements of the point of light, and
of the cyunder, the magnetic curve is self-traced upon the sensi-
tive paper. The photographic process has also been applied to the
barometer, and to the wet and dry bulb thermometers ; but the
mode of application is di£brent from the preceding, the light not
hcing reflected from a mirror. The description of the figure will
render further explanation unnecessary.
Fig. 886 represents the bifilar self-registering apparatus, which
is supported by a framework of brass tubes, springing firom the
four comers of a black marble slab (which, when in actual opera-
tion, would be cemented on the top of a stone pillar firmly fixed
in the ground, and insulated from the floor of the observatoy) :
these tabes, about four feet long, converge alternately to four
points of the torsion plate ; thus they compose a framework
possessing ffreat stiffiiess.* To the suspension-frame of the
magnet, a plane glass mirror, and a concave metallic speculum are
attached. The plane mirror is for the purpose of making eye-
observations with the telescope in the usual manner. A gas-light
or lamp is so placed, at a distance of about two feet in front of
ecu:h speculum, that an image of a small slit in the copper chimney
surrounding the burner may fall on the sensitive paper attached
to the registering apparatus. This consists of a stand supporting
horizontaUy on fnction rollers two concentric glass cylinders, round
the inner of which is wrapped a sheet of prepared photographio
Saper: the outer or covermg cylinder keeps the paper moist
arine the twenty-four hours it remains in action. A bent arm,
attached to the axis of these cylinders, is carried round by a fork
at the end of a hour-hand of a timepiece specially constructed
for the purpose (230, note). The horizontal motion of the tracing
point of light, combined with the vertical motion of the paper,
traces out the magnetic curve. A light is attached to the
reg^teriog apparatus, for the purpose of drawing a standard- or
hue-line on tne paper ; by the varying distance of any point of
the magnetic curve from this line, the magnetic variation is de-
termined. At the distance at which these instniments have
been placed, an angle of 1^ is Represented by two inches on the
• In iiutmmenta more recently construoted, a triple has been preferred
to a qaadmple lopporCh
I«per;but tb« MBlevtlm maj be enlarged at pleMtiTe,bf placing
tliem liirther apart.
B la a conoave (pMulam attacbed to the magnet.
c, ft plane glass mirrar also attached to the maenet, for maJdng
oleerratioiiB b; a telescdpe. id the old method, when reqaired.
p, the toreion plate, readini; to minates bj two Temiera.
B, a frame atanding upon the torsion plate. A poliey, capabla
of being raised or lowered bj a screw, is attached to this frame ;
the mafpiet is anspended bj a skein of ontwisted silk fibres paeaing
over this pnlle;.
I, a gas-burner eDcloaed in a copper chimnc;, from which no
light can escape, except a small pencil which pasaea tbrongh a
uannw slit k, capable of being a^jnated by a screw; on the
breadth of Ibis alit, the breadth of the register line depends.
L L, a frame supporting a combination of two planoHjMTaz
cylindrical lenaes. The pencil of light paaaine throngh K, fidli
on the mirror a, and ia refiected to the cjlindrical leaaes; by
Ibeae, the image of the slit is condenaed to a point
of light on the surf"ce of "f- ***-
MH, the regislcrine appaiata ' '" *
two concentric cjlindera, betwt
photographic papvr ia plaoed.
e, the maccelic curve traoed
light
o, a gas-bnmer, fixed to the i
the cjlindera mat.
r, a plaao-conTBZ prismatio h
the lop of
qtt, an opaque boi (ben repi
line), which prut«cta the pholo«
from extraneoua light. A pencil
paaaea thrtingh r, and ia brought to ■ tbcus on the nirboe of the
paper.
b, the base line, deacribed by this point of light.
M u, the bifihir, or hurizontal force magnetometer.
T T, the apparalUB for producing an automatic temperature oom-
pensalion, cuUBJating nf two zinc lubes, which are clamped t<> a
glass rod by two ad.jiitible clamps, w; the auspenslon-sliein
pMMS over ■ pulley attached to e, and the enda are attached to
MAOirSTlC flTOBMS AMD D18TUBB1.1ICS8. 953
two hooks, w w ; as ihe temperatare rises, these hooks are ap-
proximated to each other bj a quantity equal to the difference of
the expansion of the ghaa rod and the zinc tubes, between the
damps, T y, and thus we torsion force is diminished ; the position
of the clamps is so adjusted, that the diminution of the torsion
force shall be equivalent to the loss of power in the magnet, as the
temperature rises (588), and vice versd, when the temperature
decreases.
The bifilar and its appendages are enclosed in a plate-glass box
as a protection from currents of air, as well as from sudden
changes of temperature ; and the suspension'skein is enclosed in
a glass tube which passes through a stuffijig-box in the Ud of the
former; these are omitted in the figure, in order to avoid con-
idsion.
The declination magnet, with its suspension-skein, &c., similarly
sapported and enclosed, is placed at an equal distance on the op-
poeiie side of the registermg apparatus, m m,^ and its reflected
pencil traces the register-line, a. The register-lines a, e, are traced
pa opposite sides of the base-Hne, &, in ofder to avoid confusion.
A blackened zinc case is placed over the registering apparatus,
when in actual operation, to prevent any light from falling on the
paper, except the two pencils which describe the magnetic curves,
and another which passes through a prism on the top of the
inner case, q q, and draws the base line : in order to avoid con-
fofiion, this also is omitted in the drawing.
625. The magnetic observations that nave been for some years
past assiduously conducted in various parts of the globe, have re-
pealed the occasional occurrence of what have been termed moff-
netic itorma; during these, the magnetic elements are subjected
to great and violent changes, and a comparison of observations
has frequently shown that these disturbances are experienced
simultaneously over large tracts of the earth^s surface. A small,
but well-marked disturbance occurriog in a photographic register.
kept by Lt.-Gen. Lefroy, RA., at Toronto, m Canada, was found
to agree precisely in time, and very nearly in amount, with a
similar disturbance indicated by the Greenwich register.
The disturbances of the dechnation, and of the horizontal force,
are usually found to ag^e very nearly both in time and ioi amount,
bat the same amount of agreement between the disturbances of
^e vertical force, and either of the former, has not hitherto been
observed to exist.
The occurrence of Aurora Borealis has invariably been found,
both at Greenwich and elsewhere, to be accompanied by consider-
able magnetic disturbance ; and especially when brilliant corus-
cations are observed to shoot up towards the zenith, large deflec-
tions of the declination and bifilar magnets occur simultaueouidy,
but discharges of atmospheric electricity do not seem to have any
jnmilar e£Eect on the magnetic instruments, for the precise time of
A ▲
854 VAORKTIBM.
near and WyTd flashes has been freqnentl/ noticed at Greenwiob^
and no corresponding disturbance is indicated by the register.
Flashes of lightning have, howeTcr, frequently been found to
destroy, and sometimes to reverse the polari^ of the electric tele-
^ph-needles ; this will be subsequently shown to be due to an
intense current nassing through the ceils of wire by which they
are surrounded, but it has no reference to terrestrial magnetism.
626. Earth-currenti of electricity have recently been the sub-
ject of observation and photographic record at the Royal Obser-
vatory, Qreenwich. For this purpose, two insulated telegraph
wires of some miles in length are employed, one being nearly in
the direction of the magnetic meridian, and the other nearly
perpendicular to it One of the terminals of a galvanometer is
connected with each of these wires, and the other two terminals
with the earth ; the line-wires are also connected with the earth
at their further extremities. Whenever derived earth-currents past
through either of these wires, the needle is correspondingly
affected^ and its movements are traced by a pencil of light reflect^
from an attached mirror, as in the magnetometers. It is fre-
quently the case that any considprable earth-currents are found to
beaccoinpaniedbycorresjpondingdistuibancesoftheinagnetometers.
627. The intensity of^the action exerted on a magnetic needle
by the earth varies remarkabl v in different parts of its surface. As
a general rule, that action is less at the equator than at the poles,
and is weaker in the warmer than the colder parts of the earth.
Professor Hansteen has applied tbe term uchdynamie lines to
lines connecting the different parts of the worid which act upon
the magnetic needle with equal force. In their position tliey
approach the isoclinal lines ^569) already described, although they
still more closely correspcna to the lines of equal temperature or
i$o4hemud lines. The ma^etic mtensity is greater in the
western and northern, than m the eastern and southern hemi-
spheres. Hansteen has given the foUowmg values of the terrea-
trial magnetic foroe in several localkies.
St. Petersburg 1-403
Berlin . . . 1*364
Stockholm . 1342
Paris . » 1*838
London . 1*330
Yienna . 1*325
Madrid . 1*294
Florence . 1*278
Borne . . 1*264
628. The action exercised upon the earth by a magnetic needle
is not strictly an attractive, but rather a directive force ; for if a
magnetic bar be placed on a cork, and allowed to float on the
Bunace of water, it will not traverse the surface so as to reach its
northern side, but will remain where it was placed, a line joining
its poles becoming parallel to the magnetic meridian. The bar
will thus point towards both poles of the earth without evincing
any tendency to move towards either. Hence the influence of the
earth's polarity on a needle is directivef not aUractive, and may
MAOVSnC mTALB.
355
^.887.
,.--^
be represented bj two eqiud and opposite pAnJlel forces. If a
magnetic needle at rest in the
magnetic meridian, ab, Fig.
387, be made toaasome the
direction oh, the resultant of
the forces parallel to ab,
wbich act on o, to move it
towards ▲, maj be represented
b7^ lo, parallel to 1.0. Bat
^18 force maj be resolved
f 69) into two others, one, i p, parallel, and another, p o, perpen-
dicular to o c : the line p g will therefore represent that part of
the force, ig, which is effective in moving o towards a. At the
other end, h, of the needle a similar resolution of force will also
apply ; the forces acting on the opposite ends of the needle in
opposed directions will constitute a eoupU ^81), which tends only
to direct the needle o h into the line of tne magnetic meridiai^
▲ B. This directive action on the needle is alwavs proportional to
o p, tAe fine ^ the angle g i p, which the needle makes with the
magnetic mendian.
629. If a bar be irregularly magnetised, maffnetio proper-
ties may be developed, not onl^ at its poles, but in certain
intermediate positions; this anses from an irregular distri-
bution of its magnetism, and is generally conuectea with some
peculiarity in the structure of the bar, if not in the mode
m which it has been magnetised. In such a bar, if placed be-
neath a sheet of pastebMurd, and iron filings sifted over it, the
existence of its several JPig,SBi,
K>les will be demonstrated
St the manner in which
toe iron filings become
ananged. Instead of a
single series of curves as
in 554, as many additional
series are developed as
there are intermediate p<^s in the bar ; pointing out the position
of what are called consecutive poles. Thus, in Mg. 388, c, (^ are
tlie terminal, and a, b^ the consecutive poles.
630. In the foregoing remarks, the only substance mentioned
as capable of assuming and presenting maguetic phenomena, is
iron. It has, however, been long known that two other metals
at least nickel and cobalt, possess a similar property, although in
a much less degree than iron. From some researches of
Coulomb, however, it appeared probable that some organic sub-
stances were obedient to the influence of magnetism. The
whole subject has received a vast and unexpected development
in the hands of Faraday. This distinguished philosopher
foond that when a powerful electro-magnet was employed, the
A A 2
350 . KAaSBTISlC.
following bodies were acted upon with yrpng Iniennij, and
hence they moBt be added to the category of the magnetic metak,
iron, nickel, and cobalt.
Manganese. | Cerinm.
Chromlnm. ! Titaninm.
Osmium.
Palladinm.
Platinum.
It was, moreoTor, discovered that the salts of these bodies were^
as well as those of iron, nickel, and cobalt, obedient to the power-
ful electro-magnet, when made into bars b^ filling thin glass tubes
with them: and even their solutions were similiarT^ acted on. Green
bottle-glass, crown-glass, and even a roll of writing-pajper, are at-
tracted by the magnet in consequence of their containing iron. ^
631. tron has oeen shown to be always susceptible of in-
duced magnetism, which may be readilv excited by various
processes. A bar of soft iron placed in tne magnetic meridian
(609), almost instantly, under the inductive influence of the earth
acting like an artificial magnet (595), lacquires a low degree of
polarity ; if the iron be too close and compact to allow this ready
disturbance of the magnetic equilibrium of the bar, a few blows
applied at one extremity, to cause it 'to vibrate will, generally,
very considerably aid the inductive influence of the earth. A bar
of iron heated red hot, and allowed to cool in the direction of
the magnetic dip (611), will generally be found to be magnetic,
and bare of iron left for some time in this position, or one
approaching to it, will acquire a low degree of magnetism:
hence pokers, tongs, iron hooks, or other ferruginous bodies,
long kept in the magnetic meridian, and especially if at an in-
clination of about 70 degrees with the horizon, are always found
to be more or less magnetic. A thin rod of iron, as a piece of wire^
may be rendered magnetic, by forcibly twisting it until it breaks.
A strong electric discharge will produce a similar effect on a
needle, and even, according to some observers, exposure to the
violet rays of the prismatic spectrum.
632. A steel bar ma^ be more readily rendered magnetic by
various processes, technically termed touche$j all depenaing upon
inductive action (595). The simplest mode is to pass one pole of
a magnet several times over the whole length of a bar of iron or
steel, of course always in the same direction ; the end of the bar
last touched by either pole of the magnet becoming the opposite
pole. This is usually termed the process of the nngle touch.
According to Dr. Scoresby, a largo
Fig, 888* amount of magnetism is thus commu-
nicated by placing a piece of thin
sheet-iron or hoop-iron on the end of
the magnet ; he states that a maxi-
mum effect may be produced by one:
pass alon^ each side of the bar.
Another and convenient mode is to jom the opposite poles of two
^
Jf
I.I 1
1
I
TABIOUB MODSB OF MAOXXTIBJLTIOS, 357
magnets, a, b. Fig. 889| to place them oyer the centre of the bar
of Bteel, c, and then to separate ▲ and b from each other, drawine
them in contrary directions over o. They are then remoyec^
again placed together, reapplied to c, and once more separated ;
and by repeated application on both sides the bar c nltimately
acqnires a consideraole degree of magnetic energy.
The process of the aeparalU tatieh is somewhat similar to the
last, except that the ends of the bar, c, rut upon the opposite
poles of two sets of magnetic bars made by fastening three or four
together, with their pdes in the same airection. a aod b are,
instead of simple bars, similar compound magnets, not lying on
the bar c, but eleyated at an angle of about twenty-fiye or thirty
d^irees ; they are united, and then separated by drawing them to
the opposite ends of the bar o, as in the last described nrocess.
633. In the process of (Epinus, or the double Umekj the bars
are similarly placed, as in the $epar(xU touch last described, but
the magnetising bars are inclined at an angle of fifteen or twenty
degrees, and not separated ; but moved from the middle to the
ends of the bar of steel backwards and forwards, commencing
and ending the friction in the middle. In Fig. 390, a b is the bar
to be magnetised, n « « qaa
and fC$\ the fixed ^'^^
magnets on which it
rests, and bs, h^s^
the moveable magnets , ^
kept asunder at s h* . ■ "^T — . ■
by a smaU piece of ' *' --^^ 1 I ^
wood: by this process,^' '* ». *
rraeaied on both sides, very thick bars may be readily magnetised.
The magnets employed in these processes do not give up any
portion of their magnetism to the oars, they are used merely to
eaoeUe by induction, m the manner already explained (557).
634. An excellent mode of exciting magnetism in bent steel bars
is the following, the merit of which is due to Jacobi of Ctesden.
Let A B, Fig. 391, be the bar to be magnetised, place it on the
table in contact with the poles
of a horse-shoe magnet, bs,
then place a bar of soft iron on
the poles of b s, and elide it over
AB, m tne direction ot ine
arrow ; then lift it ofij replace it,
again ^de it, and so on. After
repeating this half adossen times,
the friction should be applied to the opposite sides, and the bar
will be found powerfully magnetised. Feschel succeeded, by
stroking a horse-shoe of steel of one pound weight six times in
this manner, in rendering it so powerfully magnetic, that it lifted
with eaje twenty-siz pounds and a half.
358 HAOKETX8M.
635. Several ma^ets are not anfrequentlj fastened tpgether,
lia 901 ^^^^ ^^^^'^ similar polea in the same direction,
'^' * constitutinff a compound magnet, or bcMery of
magnet$. In this case they are pecnliarlj fitted
for lifting heavy weights, as, hy applying a bar
of soft iron, ▲, Fig. 392, commonly called a
keeper, to their poles, it becomes, by indoctrye
action (595), a magnet, and will adnere to the
poles with a veiy considerable force. In con-
structing magnets, it is usual to draw, with a
file, a line on that end of the bar which it is in-
tended to convert into a north pole, or that
which, if freely suspended, would point towards
the north pole of the earth.
636. The capacity of steel bars, to receive by induction
and to retain magnetism, appears to depend on several cir-
cumstances. First, the quality of the Aeel has a considerable
influence; this depends either on the pxecise quantity of carbon
combined with the iron, or on the presence oif some other ele-
ments not well ascertained, in very minute quantities ; but it is
known that the most powerful magnets may be made from the
best Swedish iron' converted into steel. The power of retaining
magnetism with undiminiBhed intensity appears to depend prin-
cipally upon the uniform hardness- of a bar : and as a thin bar
can be more perfectly hardened than a thick one, a thin magnet
may, ccBtens paribus^ be made more powerful for its weight tnan
a thick one. When a bar approaches to saturation, any concus-
sion or even fiiction against a hard substance will diminish its
power ; and any elevation of temperature, above the temperature
to whieh it hat previoudy been raieed eubeemieiUly to ite mag-
netieationj will have the same effect. If the temperature be
raised to that of a red heat, the magnetism will be entirely dft*
stroyed, and at a white heat, iron and steel become wholly insus-
ceptible of magnetic induction.
637. Whenever a steel bar magnet is raised to any tempera-
ture not exceeding 100° F. and a little less than that to which it
had previously been raised, it is found to suffer a temporary dimi-
nution of its power, which it regains when its temperature is
again diminished ; and this temporary loss of power is nearly, and
has generally been assumed exactly, proportional to the elevation
of temperature. It difien considerably in different magnets, but
generally bears some inverse proportion to the pennanent intensitj
of the magnet, if originally magnetised to saturation.
If / be the force of a magnet at 32'' F., then, at any tempera-
ture, 32* +0 the force will be
/(l-JL.«-B.<«-0.«'-&C.)
where the ooef&cienta a, b, &c., must be determined in each par-
DSmtlllirATIOV OF MAOHETIC FOBCZ. 859
ticokr case, a has a c<m8iderable, and b, a yerj flensible Talne,
bat c, &c., are]^beTODd the limits of errors of observation, and may
be altogether neglected.*
SimiJuMrljr, horse-shoe magnets if left to themselves, gradnallj,
and in a space of time ▼aryin^ with their hardness, lose their
magnetic properties ; this may be prevented by connecting their
Soles by a keeper of soft iron, which, becoming magnetic by m-
action, reacts on the free magnetism of the magnet, and tendls
to augment rather than to dimiuiflli its intensity. In the weakly
magnetic metals, their power becomes remarkably increased by
exposing them to a temperature below zero, and nickel at a tem-
peratoie of 630° F. entirely loses its magnetic properties.
638. The coercing force of the other ma^etic metids, by which
is meant their power of retaining magnetism once developed in
them, especially nickel, is not so energetic as that of iron, accord-
ing to the experiments of Biot. The mrs used for these researohefl
were prepared b;^ Baron Thenard, and were as free from iron as
the cnemical snll of that philosopher could render them. M.
Biot found that the magnetic intensity of bars of steel and nickel,
of the same size, were to each other nearly as 68 : 21 ; the inten-
sity of the steel magnet being more than three tunes as great as
that of nickel. The magnetic intensity of cobalt has not been
so carefullv examined as that of nickel.
639. A oeautiful illustration of the mode of determining the in-
tensity of forces acting on a needle, by the number of oscillations
it performs in a given time, is found m the demonstration of the
law of intensity of magnetic action, for which among a host of
other invaluable investigations, science is indebted to M. Coulomb.
A small needle, suspenoed by a single fibre of silk, and protected
from the influence of aerial currents, performed fifteen oscillations
in one minute ; let the directive force (628) of the earth producing
these be called m. A long steel magnet placed in the ntapnetic
meridian^ had one pole brought within the distance of four mches
from the needle, which then made forty-one oscillations in one
minute ; the force thus exerted may be called m'. On removing
the pole to a distance of eight inches, the needle made twenty-
four oscillations in the same time ; this force may be represented
by m''. The action of the magnet on the needle m the nrst expe-
nment is m'— in, snd in the second m^—m, because its effects
resulted from its own magnetic force plus that of the earth| then,
m^~m_(4iy-(15)«_1456_...^
m"-f»""(24)«-(16)«~ 351 -*^*°-
Here, in the second experiment, when the distance of the needle
from the pole was twice that of the first experiment, the magnetic
* For a new method of determiniDg these temperature coefficiente. and,
u the author believes, a more exact method than any preTiously emplojedi
the reader is feforred to a papm in the Fhilosophioal Transaotio&s for 1860.
860
MAOXSTIBX.
intenflity was foand to be diminished by an amoant as nearly iir
accordance with the law of inverse squares of the distances, as
this experimental investigation oould be expected to exhibit.
640. Artificial magnets have been constructed by reducing to
powder the native magnetic oxide of iron, and forming it into mufb
with wax and oil. They may also be constructed by forming the
artificially prepared black oxide of iron, into bars with wax, and
magnetismg them by one of the processes already described.
A great number of mineral and even organic matters appear,
from the researches of Coulomb and Fanulay, to be capable of
assuming a faint and transient degree of polarity. And when
studjdng the science of electro-dynamics, we shall learn that all
metals during the passage of an electno current are capable of
exhibiting magnetic properties.
641. Hitherto, when a body, not itself possessing magnetic pro-
perties, has been alluded to as obedient to the action of the magnet,
we have found that it is equally attra/cied by both poles. Thus
when a bar of soft iron is suspended over the poles of a horse-shoe
msgnet, so as to be free to move, it attains a state of rest in a
{KMition parallel to a line connecting both poles, or in the direc-
tion of wnat has been aptly called a line of magnetic force (692).
Faraday, however, discovered the remarkable fact that a large
number of bodies are mutually repelled by both poles. And thus,
when formed into bars and suspended by means of a long ana
slender thread between the poles of a powerful magnet, they
vibrate, and ultimately come to rest in a line equi-distant front
both poles, and perpendicular to the lines of magnetic force ; in a
direction, consequently, at right angles to that taken by a bar of
iron. As this power, although obvious, is still far weaker than
the attractive power exercised on iron, some little management is
necessary to render it evident.
642. The best apparatus for this purpose consists of an electro*
Fig. 803.
magnet, on account both of the im-
mense power which may be commu-
nicated to it, and of our being able to
reverse or destroy its polarity at wiU,
An electro-magnet^ ns, Fig. 393,
capable of holding a bar of iron oa
eitner pole with a force of at least a
hundred pounds, should be firmly
fixed to a wooden support. From a
point above the middle of a line join-
ing the centres of n and s, a fibre of
silk, A B, should be suspended, having
attached to its lower end a double
hook or cradle of thin copper or pla-
tinum wire. The substance to be
examined should be made into a bV|
DIA-MAGVanC B0DIZ8. 861
01^ if in powder or aolntion, placed in a tbin elass tnbe, and carefnilj
allowed to rest in the little cradle. A cylinder or case of glass
oogfaty in many cases, to be placed round tne apparatus, to prevent
ihe interference of currents of air. The wires c, z, of tbe coil, snr-
Tocmding tbe iron bars sbonld then be connected with tbe battery.
The iron bars instantly become powerfolly magnetic, and the body
placed in the cradles will either be attracted by both noles ana
assmne a corresponding position, or be repelled by botn, taking
np a position at right angles to a line connecting them ; or, lastly,
be quite unaffected. A bar of iron will illustrate the first conditioni
one of bismuth the second, and a tube full of nitrogen the third.
Those bodies which are attracted, howeyer feebly, by the mag-
netic poles, Faraday proj^osed to call magnetic, according to gene-
ral custom, and those which are repelled, he termed dia^moffnetie;
whilst those which obey neither force are regarded as indifferent^
643. The first substance in which these dui^magnetic pro-
perties were detected was a heavy^ glass, composed of silicated
oorate of lead. A bar of this two inches long and half an inch
thick was suspended between the poles and allowed to come to
rest. As soon as the bar was quite quiet, the wires c, z, of the
apparatus were connected with a battery of ten elements of
Grove's construction (Ch.XIIL] : the bars instantly became power-
ihlly magnetic, and the piece of glass was quickly movea away
from^ both |x>le8 round its point of suspension, and took up ita
position at right angles to a nne connecting the poles. The position
of the piece of ^lass was uninfluenced by revenung the magnetism
of the bars, being always repelled by both poles imder all cir-
oumstances, so that it might be regarded as a magnet pointing
east and west, in relation to the north and south poles of the electro-
magnet. If but one pole of the magnet be employed, the sama
repulsive action is exerted, although of course with less energy. >
644. To produce the effect of pointing across the Hues of mag-
netic force, the form of any homogeneous dia-magnetie body must
be elongated. A cube or sphere will not thus point, but two or
three {Maced side b^ side in a paper tray win act as a single elon-
gated mass. Portions of any shape are, however, repelled ; thus,
if two firagments be suspended between .the poles parallel to each
other, they appear to attract eadi otheri in consequence of being
aimultaneonsiy repelled by both poles.
645. Flint-dass .is similarly acted upon by the magnet, but
not so powerfully as the heavy glass. Cylinders of phosphorus,
sulphur, and caoutchouc are readily affected by the magnet. A
large number of cirstalline bodies as well as ether, alcohol, oils,
water, and blood, when enclosed in tubes, were all found to be dia-
magnetic, and to be repelled. Animal flesh is thus acted upon ;
hence, as Faraday has observed, if a man were suspended nori-
zontaUy over the poles of a sufficiently laree magnet,^ ne would be
lepeUed, and point across the magnetic field.
862
VA0NBTI8M.
646. Among the metals, the following were found to be most
energetically diamagnetie in the order in which they are placed.
Bismnth.
Antimony.
Zino.
Tin.
Gadmimn.
Mercury.
Silver.
Copper.
Bismuth is very readily thus acted upon, and a small har of it^
two inches long, and half an inch wide, is peculiarly fitted for th«
exhibition of the phenomena now described.
, 647. The fact of salts of the magnetic metals obeying the
littractive action of a powerful electro- magnet has been already
alluded to (581). In the case of iron, some curious and highly
interesting anomalies were observed, evidently connected with
the constitution of the salt. Thus the chlondes, iodides, sul-
phates, phosphates, chromates of iron, and even Prussian blue, all
obeyed the attraction of the maenet, whilst the yellow and red
ferro-prussiates of potash, in which the iron does not play the part
of a Vase, were repeUed and appeared dia4nagneUe : a fine illus-
tration of the rektion between the foroe of magnetism, and the
molecular constitution of a salt.
Faraday placed solutions of protosulphate of iron in thin glass
tubes, and so suspended them that they oould be immersed
in glass vessels placed between the poles of a venr powerful electro-
magnet. He thus discovered, that when a tube was filled with
the solution of iron, and immersed in a solution of iron of the same
strength, it was utterly indifferent to the action of the magnet.
When immersed in a much stronger solution, it was repelled like a
dUtrmafj^neiief and when in a much weaker solution attracted like a
magneiU body. Water being dia-magnetic, and sulphate of iron
magnetic, a solution can be preparea of such strength as wheo
suspended in the air to be abeblutely indi£ferent to the action of a
magnet
648. The following list has been given by Faraday as showing a
Magnetic .
Iron.
Nickel.
Cobalt
Manganese.
Chromium.
Cerium.
Titanium.
Palladium.
Crown glass.
Platinum.
Osmium.
Oxygen.
Azote.
Arsenic.
Ether.
Alcohol.
Gold.
Water.
Mercury.
Flint fU
Cadmium.
Heavy glass.
Tin.
Zinc.
Antimony.
Phosphorus.
Bismuth.
dia^magnetie.
SFFBOTB or MAOVETIC POLABITT. 868
gradation of the intensities with which different bodies exhibited
magnetic and dia-magnetic phenomena; the bodies at the ex-
tremes of the list exhibiting their respectiye properties with
greatest intensity.
ThaUinm has oeen stated to be strongly dia-magnetic, bnt its
Sreciae position in the scale of dia-magnetism has not yet been
etermined.
It may here be remarked that Prof. Tyndall has obserred that
dia-magnetic bodies exhibit dia-magnetism by induction, just ae
magnetic bodies exhibit magnetism, when surrounded by a ooil of
insulated wire, through which a galyanic current is jpassmg.
649. If any strongly magnetic or dia-ma^etic miid be placed
in a shallow yessel, as in a watch-glass, m the magnetic field
(554) of a powerful magnet, its properties will be -. ^^
immediately manifested oy the form of the surface. "^
If magnetic, the fluid will be concaye in the centre,
and more or less heaped up towards the poles, as
A, Fig. 394 ; if dia-magnetic, the fluid will be re-
peUed from the maigins next the poles, and raised
m the middle point between them, as b.
650. It is really difficult to guess at the limit which may exist
to the power of magnetism in controlling or influencing molecular
forces. The elaborate inyestigations of Faraday haye opened out
a field of rich promise. A force which a few years ago was sup-
posed to influence masses of iron only, has been by these researches
shown to act upon almost eyery form of ponderable matter. It is
perfectly true that Le Baillif some years ago ascertained that
pieces of bismuth and antimony acted on the magnetic needle,
aiKl that Coulomb made a similar statement regarding many
organic substances; but neither of these philosophers followea
up their obeeryations, and to our own distinguished countryman
is due all the merit of the important discoyeries of which an
outline has here been giyen.
651. There are some few remarks on record respecting the in-
fluence of magnetic polarity on the reduction of metals, and on
crystallization, hitherto regarded as but of little importance, but
the discoyeries of Faraday make all such statements now mattera
of interest, and render a further inyestigation of them necessary.
Berzelius states, on the authority of Hansteen and Maschmann,
that when the centre of a U-shaped tube is filled with meroury,
and a solution of nitrate of silyer poured into either leg, the re-
duction of the silyer and growth ofthe Arbor Diana takes place
equally in either leg when they are placed respectiyely east and
west. When, howeyer, they are placed parallel to the magnetic
meridian, the silyer is reduced in greater abundance in the
northern leg. Murray has stated that when iron wires are placed
in weak solutions of nitrate of silyer, no change takes place, but the
silyer is reduced immediately the wires are rendered magnetic by
toringing'near them the poles of a powerful magnet.
36i VAGHXTIBX,
Another cnrioQB statement hui been made by Lndecke, tbmt
when a glaaa Tessel containing a concentrated solution of salt is
allowed to rest on the poles of a powerful horse-shoe magnet, the
crystals which form will be collected at the bottom of the glass
in every part, except a space corresponding to tbe two poles and
the magnetic field between them. Ludecke mentions solutions
of acetate of lead and sal ammoniac as readily exhibiting thitf
Shenomenon. This might be explained by regarding them as
ia-magnetic, had he not also stated that sulphate of iron pr&>
sented the same effect, which salt^ being magnetic, would not
permit of the same explanation.
A somewhat contrary phenomenon has been observed by the
writer. A large bar-magnet was suspended for three weeks in a
water-bath, and a long cylindrical bulb of a thermometer was
placed in the same bath, and very near to one extremity of the
bar ; a ring of oxide of iron was so firmly deposited on that part
of the bulb which was opposite the extremity of the bar, as not to
be capable of being wiped off by a cloth.
652. Several phenomena have been observed in cr^tallized
bodies, afpareiuLy contradictory to the general magnetic, or di»-
magnetic character of their elements ; these were ascribed hj
Pllicker to a repulsive power inherent in the optic axes of the
crystals ; and by Faraday, to a modifying influence of crystalliza-
tion on the direction of tne mimetic axis, to which he has applied
the term magne-crystallic axis. These observations have, now-
ever, been brought within the scope of a eeneral law deduced
from the investigations of Prof. Tyndall and M. Knoblauch on this
subject.
653. The general law is this : — That if ike parijdea of which a
moM contUU are more doeeL^ aggregated in one direction than in
any other ^ that line of directum vnu, when free to do so^ tcAe an
aanal or trantverte position in the magnetic jidd^ according as the
tody ie magnetic or dia-magnetic, whether the line of greatest
density corresponds with the longest dimension of the mass, or
otherwise. Tnis very important law may be elucidlated by experi-
ment. The method pursued by Prof. Tvndall has b^n to reduce
various substances to powder, to make them into a stiff paste with
gum-water, or other adhesive matter, and having compressed the
paste into the form of a thin flat calce, to diy it under continued
pressure. In a mass thus artificially prepared it is manifest that
the lines of greatest density must be all perpendicular to the plane
of the cake. Brick-shaped masses should new be cut from these
cakes, of the relative dimensions 3, 4, and 10, for example, the
greatest density being in the direction of the least dimension. If
these be successively suspended in the centre of the magnetic
field, with the direction of maximum density vertical, and there-
fore perpendicular to the lines of force alike in all positions, then
the long dimension of the mass will take an axial position, if it be
DlBBCnOH OP 9B&iTBST XBEBOY. 865
magnetic, as, for instance, if made of carbonate of iron, and a
transverse position, if it be dia>magnetic, as one of bismuth. But
if the masses be suspended with their greatest and least dimensions
both in the horisontal plane, then the superior inductive action in
the line of greatest density will overcome the leverage of the
lazger dimension of the solid, and the position taken up will be the
leverse^of what it was in the former experiment, and (mparently
in opposition to its magnetic or diarmagnetic property.
Ii the len^h of the mass be considerable compared with its
other dimensions, as, 3, 4, and 30 or 40 for example, the length of
Veterage will then sensibly oppose the influence ot greatest density
and the mass will take an intermediate position, wmch is, in fact, a
jesultant of two conflicting forces.
If a rhombohedron (24, V) be cut from the carbonate of iron
mass in such direction that tne axis of the rhombohedron coincide
with the line of maximum density, the artificial solid will comport
itself in the ma^etic field exactly as the similar natural crystal^
from the pulverisation of which it may have been derived. The
above results afibrd a beautiful illustration of the manner in which,
the simplification of first principles invariably follows the advance*
meat of scientific knowledge.
36$
CHAPTER Xn.
Fig. 996.
FRAHKLnrio* ELBcrnucmr.
654. If a large glass tube, previoasly made dir and wann, ho
briskly nibbed, tor a few seconds, with a piece of silk or woollen
cloth, also dry and warm, and then held near small pieces of jMiper,
pith, or cork, placed on the table, these light substances will be
attracted by the excited tube, and leap towards it. After adher-
ing to its surface for a short time, they will be repelled towards
the table, after touching {which, they will be again attracted
by the tube ; and these phenomena will be repeated, until the
properties excited by the previous friction on the surface of the
glass disappear. A piece of amber, sulphur, or sealing-wax, after
exdtation oy a woollen cloth, will exhibit the phenomena of
attracting light bodies, like the ^lass tube.
655. Suspend a light ball of pith of elder by a long silken thread
from the ceiling, or any con-
venient support. Fig. 395v
and anproacb towards it an
excitea glass tube; the ball
will be attracted, and, after
adhering for a short time to
the tube, will be repelled to a
considerable distance, nor will
it be again attractea until it
has touched some substance connected with the earth, and has
thus lost the peculiar properties it had acquired by contact with
the tube.
Then bring towards the pith ball thus repelled by the tube, a
piece of sealine-wax excited by briskly rubbing with a piece of diy
flannel, the ball will instantly be attracted, soon however becom-
ing repelled, when it wiU rush ^ward the glass tube, if held suf-
ficiently near. It will thus vibrate like a pendulum between the
excitea glass and sealing-wax, being alternately attracted and re-
pelled by each.
656. From these simple experiments it appears that oertaio
bodies acquire by friction properties which they did not previously
exhibit, but which properties are readily manifested by the attrao-
• Thit term wsa first applied by FundAj, in contradistinction to Toltaio,
and in commemoration or the illostrions Franklin, who was one of tiie
earliest experimenters in electrical science.
ooin>vonov ahd ihbulatiov.
^67
tlm kad. repolniDii of light bodies. As these phenomena werer
fint obBenred hy Thales, b.c. 600, in pieces of amber (^X<jcr(K>v),
the term eUetrieUy has been applied to the properties thus excited^
It appears also, from the observations jnst made, that the elec-
tricity excited bj friction on the sorface of glass is communicated
to pieoee of paper, or pith balls, placed in contact with the glass,
ana that the hodies thus acquiring electricifj are repell^ by
the tube until they have changed their electrical condition by
contact with some other body ; and as, when thus repelled by ex-
cited gUss, the ball is attracted by excited resins, there is valid
reason for concluding that the electricity developed on these sub-
stances by friction exists in opposite states, or conditions. That
which is acquired by excited glass is termed the vitreous, or posi-
Hve electricity, and that excited on amber, and resins, the
retinofUf or negative electricity. It appears, moreover, that'
hodies in one electriecd Hate are attracted hy thoee in the oppo-
site, and repelled hy those in the same sUOe, A substance ex-
Ubiting electricity in an active state, or as it is now commonly
called^ j^otential electricity, is said to heeUetrified positivdy, if its
electricity be positive ; and negatively , if it be negative.
657. In consequence of the sealing-wax, glass, &c.. in the pre-
ceding experiments, acquiring electricity bv friction, tney are said
to be idiodeebriCj whilst those not ordinanhr manifesting this pro-
perty, as metals, are termed anelectrics, from the general law of
JFV.896.
TT
I
Dodios similarly electrified repelling each
other, arises a very convenient mode of de-
tecting the presence of free electricity. For
this purpose, instead of a single pith ball
(665), two are attached to the ends of a
piece of thread, and suspended by the
middle across a fit support, cut off from all
ekc^cal communication with the earth, by
means of a pillar of glass or resin. Fie. 396.
On touching this little apparatus with the excited tube or sealing-
wax, electricity will be communicated to it, and the balls, being
similariy electrified, will repel each other, and separate to a con-
siderabfe Stance, forming^ the simplest kind of electroscope, or
indicator of potential electricity.
658. Insert into either end of a hollow metallic cylinder c, Fig.
397, supported by a glass pillar, a
wire or rod of some metal, as brass,
B, and one of glass, shell-lac, or sealing-
wax, A ; and suspend from each a pair
of pith balls, attached to thread or
cotton. Then touch the middle of the
cylinder, o, with an excited glass tube ;
immediately the pith balls suspended
fit>m the brass rod b, will separate from
f-
Fig.ZVr.
n
368 FHAlfflfLTlfflO KLBCTBICITT.
each other, whilst those stispended from the glass, jil, will retbam un-
affected. This arises from the fact of certain bodies, as metals,
cotton, thread, &c., possessing the property of conehuting elec-
tricity^; whilst others, as sealing-wax, glass, silk, &c., are incapable
of being freely traversed by it. On this account, bodies nave
been divided into two great groups ; conductors and nonrconduo-
tors of electricity f the former Ming in general identical with
aneileetrieSf and the latter with idioelectrics. The line of de-
marcation between these two great classes is h^ no means strictly
defined, as a large nomber of substances exist which conduct
electricity when presenting greskt energy, and insulate it when
feeble ; or the conducting powers of which vary with their tempe-
rature. Electricity is not recognised on the surfeoe of metaUio
bodies submitted to friction unless carefully insulated, in conse*
quence of their so readily transmitting electric force, that the
BurfisMse-ezcitement is diffused through the substance of the metal
when continuous with any conductor, as fast as it is generated.
659. Among conducting bodies may be ranked all metab, char-
^al, water, steam, all animal and vegetable substances containing
water, and manj other substances : whilst glass, and all vitrifica-
tions, gems, resins, sulphur, organic substances perfectly free from
water, and ice, are all more or less perfect non-conouctors and
idio-electiics. A substance supported by a non-conductor, as when
placed u^on a stool with glass legs, is said to be insulated^ fit>m
its electric communication with the earth being intercepted.
660. The capability of electrical excitation appears to be an
universal attribute of matter. Electricity may be aeveloped either
by friction, or by many other mechanical and chemical means :
but whenever one body, in consequence of either of these means
being applied, manifests the presence of electricity, some oUier
body simultaneously shows the presence of the opposite kind of
electricity ; if the first becomes positive, the secona becomes nega-
tive, or vice versd : thus in the experiments previously mentioned,
when the glass tube is positive, the silk muber is equally nega-
tive ; and when the stick of sealing-wax is negative, the flannel
rubber is equally positive. It will be found better to abandon the
terms vitreous and resinous, inasmuch as the electricity of glass
depends on the nature of its surface : if rough, as by grinSng,
the electricity developed is of the kind called resirumSf and there-
fore, that teim is not distinctive.
The two forms of electricity, although possessing some disUne-
tive characters, produce generally the same physical effects, the
amount of effect produced depending on the relative difference of
electrical condition, or potential energy^ as it has been termed, in
the agent and the body acted on.
661. Electroscopes, — Instruments designed to exhibit the
presence and kina of electricity, without reference to its potential
energy, are called electroscopes; these are constantly called
i|i requisition, in prosecuting the study of electrical science.
ELEOTBO0OOPE8.
369
Hie pidr of pith balls already described ^» 398.
(657) is finqoentlj called bj this name,
and employed to detect the presence of
electric potential. As the currents of air
always m motion render the indications of
the pith balls obscure, they are frequently
eitspended by linen threads, or fine wires,
from a metallic rod fixed by a resinous
cement in the neck of a glass bottle or
cylinder, a, Fig. 398, and surmounted by
a metalfic plate, b. ^ On touching the top, b,
of the apparatus with an excited piece of
glass or resin, the electricity is difliued along
be metallic rod, c, in consequence of its
being a good condactor, and reaching the pith balls, they, becom-
ing similarly electrified, repel each other (656], and by their
mntual repulsion the presence of electricity is mdicated. The
electricity does not escape from the rod, c, to the earth, in conse-
qaenoe of the glass jar supporting it being a non-conauctor, and
If the outside of the glass vessel, in which the pith balls are sus-
pended, be moist, they will still more rapidly lose their electric
state, in consequence of their potential energy being conducted by
the film of moisture to the earth. For this reason it is absolutely
necessary to carefully dry the exterior of the glass vessel, in order
to ensure the success of an experiment. To prevent the deposition
of moisture on this as well as on all other electric apparatus, it is
usual to cover the upper ^art of the glass externally witiii a solution
of shell-lac in alcohol ; this, on drying, leaves a nearly transparent
covering of an excellent insulating substance, which is much less
liable to attract moisture from the air than the nncoated glass.
662. The best electroscopes are generally furnished with a con-
trivance for rendering the insulation more per-
ibct. In this arrangement the metallic rod to
which the pith balls are attached, passes
through a glass tube, a, Fig. 399, covered
both externally and internally with lac var-
nish ; this rod is retained in ite place at b by
a plug of silk, lac, or other non-conducting
sabstance ; the advantages of this contrivance
are sufiiciently obvious, for it is evident that
any electricity communicated to the plate o,
cannot be dissipated excepting through the
unless the whole of the interior of the
Jl^. 300.
C
air,
tube. A, and the outeide of the apparatus, be-
come covered with moisture. The late author
c
J
B B
370 FRANKLXVIO ELSCTRIOITT.
repeatedly found such an instrament perfectly sensitiTe to mera
traces of electricitVi after having remained nnnsed, and even
coyered with dust^ during six montns.
When it is required to detect minute qoantities of electricity,
the weight of the pith balls in the first described electroscope in-
terferes with the delicacy of the instrument ; on this account two
slender slips of leaf-gold, hangine parallel to each other, are with
great advantage substituted for the pith balls.
Two slips of tin-foil, d, d, are usually fixed alon^ the inside of
the glass case of the instrument, so as to touch its base, which
must be of metal, or some good conductor. On communicating
electricity to such an electroscope, the gold leaves separate, and,
if the electric charge be too powerful for tne tenacity of the slender
laminae, by approaching or touching the slips of tin-foil, they
become readily unelectrified.
663. An ingenious mode of determining the quality of very
feeble charges of electricity has been devised by Dr. Radcliffe.
For this purpose two similar gold-leaf electroscopes are mounted
on glass stems 10 or 12 inches long, one of which is thoroughly
coated with a solution of shell-lac. If these stems be slightly
excited by friction, and the caps be placed in connexion witn
the earth, the two pairs of ^old-leaves will become charged by
induction (676), with opposite kinds of electricity: if now a
feebly electrified body be placed in contact with each cap in
succession, the divergence of the ^old-leaves will be sli^tly
increased in one instrument, and dtminished in the other, and
thus the quality of the charge is determined. By these instru-
ments manifestations of electricity have been detected in the
living body, sometimes positive and sometimes negative: and
likewise in fresh blood, as from an animal just slaughtered.
664. JSlectromet&rs. — The instruments above described, merely
indicate the presence^ and not the precise qwaWtUy^ of electricity
present in any substance in an active state. Tor a mode of gaining
an approximation to the knowledge of the Quantity of electricity
we are indebted to the torsion balance of M. Coulomb. It con-
sists of a slender bar, b. Fig. 400, formed of melted shell-lac, fur-
nished with a gilt pilh b^ill at one end, and a little vane of gilt
paper at the other. This is suspended by a fine metaUic wire, c,
or still better, by a filament ot spun glass, in the middle of a
cylindrical cage of glass. The upper end of this wire, or glass
thread, terminates in a key, d, furnished with an index, and capa-
ble of moving in the centre of a circle, o, graduated into 360^.
Through a hole, e, at the top of the glass cage, a rod of lac, f,
terminating in a gilt ball, is inserted ; being prevented falling in
by a stop at e. This ball is generally termed the carrieritdl^
on account of its being used to convey the electricity of an ex-
cited body, to the electrometer, so that its potential may be deter-
mined. To use this instrument for measuring the amount of
olectricity, the rod r » nmiorad, and iM
b«H bronght in contact with tlMiubgUucs
to be exniniDed ; the ball srajuires >oma
actiie electricitj, aod on being placed in
the glaai cage, it shares its electricit;
with the ball terminating the horisont^
needle, b : the two being similarij electri-
fied, then repel each other, and as p i«
fixed, ■ necesBsrilT mores, and describes a
certain angle, wbich it ret^u antil it
low* iU eleetridt;. To measnre the
amoant of energj thus acquired bv tha
b*lla, tbe ke; i>, to which the glass thread
c is factaned, is tamed ronnd, until, by
tbe tonioD, or twisting of the thread, tbe
ball of B is compelled to come fn contact
with that of r : then tbe nomber of de-
grees described by tha index fixed to the
leTolnng ke<r, d, gives an approximation
to tbe amooDt of eleotridtj acquired b;
the ball of r, during tU contact with the
electrified bodj ; EiiBcai
rionstr bean staled (390), tbe t
"ly.b* . . ..,. . .. -
thread in proportional to the deflecting force,
~"': FiUitr'i Eleetrometer.—'n^H " *
or deflexion of the glusa
J^.«>1.
9 on tbe
tame principle as the preceding, namely, that the amount of
lension is delennined by the amount of repnlBion of two bo^es
similarly electriSed : but the constnictioa in somewhat difierenl.
A central metallic stem terminated abore by the ball P, Hg. 401,
ia eSectnally inaulnted by the vul-
canite haae to which it is attached.
Ttii* stem is perftirated at a, to
allow the paewge of a bent wire
c D, to the centra of which a small
maenetised Deedle is attached, and
bou are poised on a fine »lael point
beneAth e: thus od acquires a
■mall direclive force. Two rods
with balls, 1, B, at their extremities, '
are fixed parallel to saoh other into
tbe lower part of tha stenL so that
tbe ends □, d, may be both nearly
in oontacl with a and b respectively.
The instrument is so placed for use,
that OD may be in the magnetio
meridian, and also in contact with
ab. It is sometimes found de-
sirable to place cj> in cloee prox'
BB 2
imity to A and b, bnt nnl in actual cootact. id aiiti U> prercnt
adhedon. If anj charged bod; b« now brought into contact with
P, a cbai^ will be aimultaneonBl; cominanicated to A ■ and o d,
and tb« antonntof rapnlsion will be a meamreof ita teoiion. Thia
instrnment it highl; Mnritint, and ver; conTcniant.
666. Tlotnaon'f Eledrom^er. — Thii inatnulient ia cotutmcted
on the principle that if two indefinite panllel pianos be un-
eanaliv electrified, the rapuliinn of n email portion of one fran
Ibe otber will be ioTerael; as the diatance betwOen them, and not
iiiTeraelj ae the aqnare ol' that diitanoe, which ia atrictl; the caaa
odIt when the extent of the electrified inrfacei ia amall ccanpared
with their dUtattce : thia may be readiij prored bj analfiU.
JVd tnt '*' "' ^^' ^^' "^ ^' **° eepantel? inn-
Uted plane*, of which a maj be reached
throogti the coTcied apertnra c, bj removing
ila cap, and A ii in metallio conneiion with a
central pillar, on which the op, d, alide*.
When the cap ia cIom down, it ia in contact
with the ciQler braas caae of the inatrnnwnt,
■nd thna the plate is placed in conneiion with
the earth. When the cap J* raiaed, it i« in-
■nlated from all except tfae cap, a. In the
middle of a ia an aperture neariy filled b; a
plate E, which ia in one piece with a amall
■creen r ; and the pieco aria balanced im a
atrettbed wire placed beneath b, the airection of which hum*
throogh (he centre of gravity of the piece ; and it is so balaDoed
a finewire Bird in front of it._ In order that the plale_ .__,
keep its charge, the interior air ia kept dr; b; some pieoea of
pnmice moiataned with atrong aulpbnric acid, contained in a leaden
box in the npper part of the inatmment: Ihia is not ahown in
the figure, which must be considered not aa an exact rcpreaent*-
tion of the inilniinent, bnt mere!; aa a diagnun to explain ita
conEtructton. The plate a ia raiasd or lowered b; a micromoter
screw bj mrana of a dinded plate o, placed above the cover of
■he inatmment. When both plates are charged, and the index r
ia brongbt into c<nncidenc« by ralaing or lowering a, it ia evi-
dent from what has preceded, that [he ifistaace between ihe plate*
will be a measure of the difiference of their electric potentials.
The object of this inatmment ia to gire a comparative Talne
of the free electricity, electric tenaion, or (aa it ia now freqiMDlIr
and mora appropriatelj called} the "potetUii^' of any two givm
bodies; and it ia convenient, for the aake of oompariaon, that the
earth abould be one of these. To make an experiment, therefan,
the plate b ia charged hy a amatl sleclropboma (3S6), ihroo^
the aperture, o, and a ia pUosd in conneiioa wiih the eaith
BLBGTBICAL QUAUTT OF YABIOUS BOBSTANCBS.
373
by depressing the cap d : a it then raised or lowered until the
index comes to zero, and the micrometer reading is recorded.
The plate a is now insulated by raising the cap, d, and the body
to be compared, a telegraph cable for example, is put in connexion
with A. The index is again brought to eero, and a second reading
of the micrometer is taken : it is evident that the difference of
these two readings will be a m-'asurn of the difference of the
potentials of the earth and the cable. The accuracy of this result
18 seen to depend on the constancy of the potential of b during the
'time occupied by the experiment : this is found practically to be
Buch that no sensible error is likely to arise from this caose.
667. It has been already stated, that in no instance can one
kind of electricity be excited without a corresponding portion of
the other being also excited ; in the present state oi our know-
ledge, no general nile can be given as to the kind of electricity
developed by the friction of different substances, farther than
the data which the results of experiments on this subject have
fiimished. Many substances, excited or robbed by one rubber,
evolve negative, and when submitted to the friction of another
composed of a different material, evolve positive electricity ; thus,
smooth glass becomes positively electrified, when rubbed by
flannel or silk, and negatively, when excited by the back of a
living cat. Sealing-wax, on the other hand, becomes nositive
when rubbed by metallic substanct^s, and negative, wnen by
almost everything else. A very useful table, exhibiting the re-
Bulta of numerous experiments, has been giveu by Cavallo : —
Snbctsnoes
excited.
Back of a cat
Smooth glass
Bough glass .
Tourmaline. •
Hareskin . . .
White silk . .
Black dlk . .
Seafing wax .
Baked wood .
Kind of
Eleotrioity.
Material forming the Bahber.
Pontive
Positive
Positive
Negative
Positive
Negative
Positive
Negative
j Positive
j Negative
i Positive
Ne^live
, Positive
Ne^tive
Positive
Negative
Every substance hitherto tried.
Do., except the back of a cat.
Dry oiled silk, sulphur, metals.
Woollen-cloth, paper, wax, human
hand.
Amber, a current of lur.
Diamond, the human hand.
Metals, silk, leather, hand.
Other finer fiirs.
Black silk, metals, &c.
Paper, hand, hair, &c.
Sealing-wax.
Furs, metals, hand.
Metals.
Furs, hand, leather, cloth, paper.
Silk.
Flannel
374 F&ANKLXNIO BLECTBICITT.
668. Faradaj sabmitted the following bodies to friction, and
found that any one of them became negative with the Bubetances
above, and positive with those beneath, in the list.
Catskin, or bearskin. Flint-glass. Wood.
Flannel Cotton. Ghim lac.
Ivory. Linen, canvas. Mutals.
Qaill. White silk. Solphor.
Rock-crystal. | The hand.
The mode of rubbing often makes a remarkable difference ; thns,
a feather merely brushed against a piece of canvas will be nega-
tive, whilst if drawn forcibly between its folds, it will be positive.
Two pieces of flannel drawn across each other will possess diffe-
rent electric states, according to the direction of the friction.
Another curious example is presented in the coarse long hair
growing from the node and forepart of the back of an old
chamois ; this if drawn between the finger and thumb from the
root to the point, becomes strongly positive, but if drawn in the
contrary direction is equally negative. It forms a convenient
test for determining thj electrical condition of small pieces of
mineral substances.
669. Electricity is excited not only by friction, but by almost
every form of mechanical action to which any substance can be
submitted; mere pressure is quite sufficient for this purpose.
Take two pieces of common window-glass, each presenting a sur-
face of about four square inches, to the centre of each fix a piece
of sealing-waz, to serve as a handle ; press the discs firmly to-
gether, and, whilst in this state, bring them near to a gold-leaf
electroscope (662), no diver^nce of the slips of pold wid ensue ;
but suddenly separate the pieces of glass, and bring one of them
near the^ electi-oscope, and the immediate neparation of the gold
leaves will demonstrate the presence of electricity in the discs,
one of which will be found positively, the other negatively, elec-
trified. Sulphur poured, whilst melted, into a conical glass, and
fiirnished with an insulating handle, as a piecie of glass or silk,
will, when cold, indicate no charge of electricity, until the cone pf
sulphur be lifted from the glass, when the former will be found
negatively, and the latter positively electric. On tearing asunder
pieces of cloth, suddenly separating a pair of drv. and warm siUc
stockings which have been rolled up together for some time, or
rapidly uiifolding a roll of flannel, there is abundant evidence of
the evolution of potential electricity as shown by the action of these
bodies on electroscopes, and even by their evolving flashes of light
and sparks.
^70. Certain minerals, especially tourmaline, and many of the
family of zeolites, have potential electricity developed by heat,
one extremity of the crystal becoming negative, and tho other
positive ; the signs of electricity gradually increasing from the
BUPSIFICIAL DDTRIBCTIOV OP BLXCTBICITT. 875
middle, where' they are absent, towards either extremity of the
prism. Minerals, poiweBsiDg this property, are ctA\edpyr(h€lectric,
and their crystals are found to possess peculiar characteristics
i27). It may be stated that, in general, no idio-electric substance
608) can be pressed, bruised, rubbed, or submitted to a change
of temperature without suffering some change of its electrical
state; one or the other kind bemg deyeloped in the body, in
greater or less proportions, according to circumstances.
671. If the excitation of the glass tnbe (654) be performed in a
darkened room, a pale lambent flame will be observed on its sur-
face, each time the tube is drawn through the piece of silk, accom*
panied by an odour resembling that produced b^ phosphorus,
arising from the development of ozone. On brinemg the glass
near any conducting body, as the hand, a small but vivid spark
will be obserred to pass between them, attended with a faint, but
sharp crackling noise. The evolution of this electric light was
fint distinctly noticed by Otto de Gnericke, at the latter end of
the 17tb century, whilst submitting a ^lobe of sulphur to the
friction of the hand ; about the same time, Bovle observed the
light emitted by an excited diamond ; and Dr. Wall, that given
offfirom a piece of excited amber, on the approach of the finger.
This electric light can be easily observed in a dark room by
drawing a piece of dry and warm brown paper, about eighteen
inches long and four broad, through a piece of warm flannel, on
bringing the hand near the paper, as it is rapidly withdrawn from
the foldb of the flannel, bluish flashes of light, two and three
inches in length, will dart off in various directions, accompanied
by a loud crsckling noise.
672. Electricity thus excited in, or communicated to, any sub-
stance, does not appear to penetrate into the interior of the mass
to any extent, but to resiae almost exclusively upon its surface.
Coulomb found, that on suspending, by silken threads, a conduct-
ing body, in which various pits and uepressions had been made,
and communicating to it some electricity from an excited tube,
the carrier-ball of nis electric balance (664) being applied to the
bottoms of these cavities, gave no indication of electricity on being
placed in the electrometer ; although, when brought in contact
with the surface of the conductor, it became strongly electrified ;
proving that electricity is almost entirely limited to the surfaces
of insulated bodies. This circumstance is, as Faraday has shown,
easily explained by the inductive influence of the electricity pre-
sent in surrounding objects, and even in the comparatively distant
walls of the room. This most talented philosopher, among other
experiments made with a view of obtaining some light on this
matter, constructed a room of a light framework covered with
canvas. This was carefully insulated, and Faraday entered
it. On bein^ connected with the conductor of a powerful
electric machine, it appeared so highly electrified, that flashes
376 FBAKKLXiriG BLBOTBIOITT.
of light darted off from the outside of this insulated room
towETCls the walls of the apartment containing it, and yet no signs
of potential electricity could, during this time, be detected in its
interior.
Faraday has further illustrated hv an ingenious experiment the
fact of the external distribution of electricity. A cone of gauae
or muslin is attached by its base to a ring, insulated by a
glass stem; a piece of silk thread passes through, and is
attached to, the apex of the cone, so that it may be inverted at
any time, by pulling that end of the silk thread that happens to
be inside the cone. If the cone be now electrified by contact
with an electrified body, it will be found that the charge is all on
the outside, and that none can be detected inside. If tne cone be
then inverted, the inside (previously outside) will again be found
free from electricity ; and this experiment may be repeated until
the charge is dissipated by the atmosphere.
673. Primary £leetrottatic Laws. — Having considered some
of the principafand simplest phenomena of electricity in a general
sense, it becomes necessary to be acquainted with the nature of
the exact laws governing them ; for a knowledge of these, we
are almost entirely indebted to the researches of M. Coulomb,
who brought to bear, on this subject, the most accurate experi-
ments, combined with the most refined resources of mathe-
matical investigation. The following have been fully verified by
experiment :-^
A. Two bodies, similarly electrified, repel each other (665)
with a force varying inversely as the square of their mutual
distance.
B. Two bodies, differently electrified, attract each other with a
force inversely as the square of their dintance.
It must here be observed that the8e laws are strictly true only
when the electrified bodies are considered as pointa, i.e., possessing
iio sensible magnitude, and approximately true only when the
electrified surfiices are small compared with the distance between
them.
C. When either of the bodies presents an indefinite plane sur-
face, the attraction or repulsion oetween them is simply in the
inverse ratio of' the distance.
D. Whenever electricitv has been excited by (notion, or by
any other means, on any idioelectric substance, it appears to be
always confined to the smface of the substance, on which it haa
been excited, in the form of an exceedingly thin layer, not pene-
trating sensibly into the substance of the mass (672).
£. JBodies^ carefully insulated on resinous supports, lose, bv expo-
sure to the air, a certain proportion of their free electricity, depend-
ing to a great extent upon tne moisture present in the atmosphere ;
the loss, per minute, appearing to bear a ratio to the cube of the
weight 01 hygrometric moisture in the air.
POTBimAL DBFEVDEHT OH BUBFACB.
t
877
F. Bodies electrified and inmilaied imperfectly, as on silk, or
elass uncovered with resin, lose a portion of their electricity, bj
Its escaping along the surface of the imperfectly insulating sup-
port, prorided the electricity he of considerable energy, but if weak,
It is completely insulated ; hence the loss of eleotnoity is at first
rapid, but quickly decreases.
674. As a necessanr consequence of this law of superficial dis-
tribution, it follows that, the quantity of electricity in any given
body remaining the same, its potential will be diminished, by in-
creasinff the surface over which the electricity is distributed.
A hcMiow tin cylinder, A, fig. 403, about eight inches in length,
is insulated by a glass support, b ; an inner tin cylinder, c, pro-
vided with a ^ass nandle, moves readily in -ai aha
the outer one, and from this proceeds a bent
wire, D, from which a coric-ball electroscope
is mispended. Let a be touched with an
excited glass tube ; the electricitpr diffus-
ing itself over the apparatus, will cause
the pith balls to become electrified, and con-
sequently to repel each other. When these
balls are about one third of an inch apart,
raise the inner cylinder, c, by its ^lass
handle, as high as possible, without entirely
moving it from a ; the electricity will ble
expanded over twice its previous superficial
extent) and a smaller quantity will be left
in the nith balls, which will consequently
approacn each other. Then depress the
inner cylinder, o, the electricity will again be spread over a lesser
surface, and the pith balls will separate as at first
675. A simple and more effective mode of demonstrating the
same fact, is to insulate a small cup of
tin or other metal, a, Fig. 404, having
a wire fixed to its exterior, carnring a
pair of ^th balls. A piece of thick
orass chain having a silxen string tied
to one end is placed in the cup. On
giving the latter a spark from an excited
glass tube, the pith balls will divei^, and
on then raising the chain by means of the
silken string, so that ten or twelve
inches of it are out of the cup, the pith
balls will immediately collapse; return
the chain and they will again diverge, and
•o on.
Another mode of showing the same result is by means of a
sheet of tin-foil rolled on a roller with an insulated handle, and
■npported on glass legs. If this be unwound, and eledsrified, its
378 nuirsLivio blbctbioitt*
potential will be foand to be increased as it is wound up, and
diminished again as it is nnwonnd from the roUer.
676. Let GAD, Fi^. 405, be a conducting body, as a cylinder
of sheet zinc, placed on an insulating support ; a cork-ball elec-
troscope, F, being suspended from one ena of the cylinder. If a
body subject to that particular kind of molecular action, which
has been designated as "positive electricity," as for example,
a charged metallic globe, b, be brought within about six inches
from c, the pith balls p will instantly separate, indicating the
presence of potential electricity. This cannot arise from any elec-
tricity baring passed from b to c, as on removing e to a consi-
derable distance, the balls p
^.405. will fall together, and ap-
pear nnelectrified ; on again
approximating b to c, the
balls will again diverge,
and so on. This pheno-
menon arises from the
positive electricity in e,
5 reducing some kind of
isturbance in the but-
3 rounding particles of air,
this disturbance is trans-
mitted from the molecules of air to those of the conducting body,
c D, through which it is instantaneously diffused (for conducting
power, or as it has been called ** conductivity,'* has probably no
other signi&cation than the power of freely receiving and trans-
mitting from molecule to molecule that particular form or phase
of dynamical action, that has been termed '* electricity"), and on
the surface of which it is manifested, but chiefly at those parts of
the surface which are nearest to, and farthest from, the inductive
body E. The action exercised b^ b, is called induetionf from ita
active electricity or "potential" tnducina the electric state of o d.
It is convenient to follow Faraday in calling the tube b, whence
the induction is exerted, the inductivej and the c^linderj c a d, oa
which the result of induction is manifested, the tnduelnc body.
677. The inductive action of an electrified body appears to be
a polar action; each successive molecule in anv given line of
action, polarising that contiguous to it, that is, inducing an
opposite state on the proximate side, and a similar state on the
remote side ; and, as iias been already remarked in magnetism.
(598), the energpr of each particle is more and more manifested, in
proportion as it is less and less counteracted b^ the contrary ener-
gies of particles on either side of it. Ihus, if the ^linder cad
be carefully examined whilst within the inductive inflnence of the
positively electrified ball, b, the end c will be found to be nega-
tively electric, and the end d, positively ; whilst the middle por-
tion at A will be found to be neutral and unelectrifiedL So that
THKOBT or IXDUCnOH, 379
the difiiribntion of electricity on the surface of the cyKnder may
be compared to that in an excited tourmaline (670), which sab-
stance evinces a natural tendency of its molecnles to become
electrically polarised in a determinate direction ; for the terminal
faces of the crystal (87) are differently arranged at the two ends,
and the same arrangements of those faces always correspond with
the electro-positive and negative ends respectively: hence it
appears that the electro-polar properties of the molecules are in
some manner connected with their crystalline polarity. The dis-
tribution of electricity on the cylinder may likewise be compared
to that of magnetism on a soft iron bar magnetised by induction
(595) :^ and the analogy of the cylinder to a map;net may be car-
ried still farther, for if the cylinder, instead of bemg a single body
as supposed, consist of two placed in apposition at a, so long aa
they are in contact, no potential is manifested at A, but the
instant they are separated a short distance from each other, the
now exposed end of c becomes positive, and that of d, negative,
and both these polarities disappear on again placing them in
contact, thus showing a striking analogy to the fractured
ma^et (597) : and if an excited tourmaline were fractured, a
nmilar electro-polar condition would probably be manifested in
each portion : tne experiment cannot be made without a consider-
able sacrifice, as suitable tourmalines are now of rare occurrence.
It may here be remarked that the development of electricity in
the toarmaline by heat, which entirely disappears again on cool-
ing, is one of the innumerable points of evidence of Uie correlation
of physical forces, that are oontiuually presented to the student
of science. It seems hardly to admit of a doubt that the electric
potential of the heated tourmaline is nothing more than a mani-
festation in another form (in conseqaence of the physical consti-
tution of the crystal) of a portion of that energy which has been
communicated to it in the form of heat, or thermic potential.
678. The apparatus remaining as above described, and the
pith balls being separated by mutual repulsion, let the cylinder
oe touched at d by the finger, or by any other conducting body
connected with the earth ; the pith balls will collapse, from the
positive electricity being transmitted bpr the conductor, to the
earth, bat the negative electricity is mamtained in the end, c, of
the cylinder, by the inductive influence of the opposite electricity
of the ball, n. Now remove the finger, leaving the cylinder ih-
Bolated, and separate b to a considerable distance nam c ; the
negative electricity in which, being released from the influence
of E, becomes diffused over c d, and the balls, f, will instantly
separate with negative electricity. If this experiment be repeated
with an excited piece of sealing-wax, amber, or sulphur, instead
of the globe, b, tne same phenomena will occur, with this differ-
ence, that the induced electricity will always be of the opposite
kind, as would, of course, be expected h priori.
380 FSAHKLIHIO BLECTB1C1T7.
679. The application of this inductive inflaence present b tbe
readiest mode of ascei'taining the kind of electricity present in any
excited substance. For this purpose, excite a glass tube by fric-
tion, and hold it about a foot distant from the cap of the gold-leaf
electroscope (662) ; the leaves will diverge with uositive electricity,
the cap of the instrument Ixrcoming negative uy induction. On
touching the cap with the finger, the unconstrained positive elec-
tricity escapes to the earth, and the leaves collapse — negtUive
energy being retained in the cap by the induction of the positively
electrified tube. Now, remove first the finger, then the tube, and the
gold leaves will diverge negative electricity. £xcite, b^ friction or
otherwise, the substance of which the electric state is to be ex-
amined, and hold it near, but not in contact with, the cap of the
electroscope ; if the substance be positively electrified, the cap of
the instrument will become negative by induction, and the trans-
mitted polar action will render the gold leaves either less nega-
tive, or neutral, according to its intensity ; they will become leas
divergent in the first case, and collapse entirely in the second.
But if the inductive body be negative, the cap will become posi-
tive, and consequently the gold-leaves more stronglv negative, and
their divergence will consequently be increased. By tlus process,
it becomes exceedingly easv to discover which kinj of electricity-
is present in any excited suostance.
680. In the preceding experiments, the induction takes place
through the column of air separating the excited tube from tbo
conductor or electroscope. A similar action is capable of taking
place when other non-conductors are interposed ; tnese substances,
m consequence of their enabling induction to take place through
them, have been termed dielectrics. Dielectrics differ ccaisi-
derably in the degree of facility with which induction takes place
through them, indicating the existence of a spec^ inductive
capacity. Thus, sulphur, lac, and glass, have much higher in-
ductive capacities than air.* The following table contains the
results of 8ir W. Snow Harris's experiments on the comparatiye
inductive powers of several diclectncs : —
Air . • 100 i Wax . . 186 \ Sulphur . 1-93
Besin . . 1-77 { Glass . . 1 90 { Shell-lac . 1*95
681. Induction has been demonstrated by Faraday, to be essen-
tially a physical action, occurring bettoeen contipvoue particlee,
never Uudng place at a distance without polarizing the molecules
of the intervening dielectric; causing them to assume a peeuUear
constrained action^ which they fnaintain so dongas they are under
the coercing influence of the inductive body. Thus, in the expe-
riment already detailed (676), a space of six inches existed be-
tween the inductive excited globe and the inductric cylinder, the
* On €his sntiject the elaN>rate pap«n of Faraday. In the PhikMophioal
TnoMotiou for 18S8, should be oonaoltcd, espeeial]|y % Utt-Ttl.
IHDUCnON TNFLTJSNCEfl ALL IKTBRTEHIRO MATTEB. 381
electricitj of which was affected by its action ; bat it mast not be
aasamed from this, that the disturbance of the natural electric
state of the conductor arose from an action at a distance ; for
most satisfactoiy evidence has been adduced by Faraday that the
intervening dielectric air has its particles acted on in a manner
analogous to those of the conductor c d, Fig. 405, by the inductive
influence of the charged globe. The theory of induction depending
upon an action between contiguous molecules is supported by the
fiict, which would be otherwise totally inexplicable, that a slender
rod of glass or resin, when excited by friction and placed in con-
tact with an insulated sphere of metal, is capable of influencing
the latter b^ induction most completely, even at that point of the
ball which is most distant from the roa, and, conRequently, inca-
pable of being connected with it by a right line not passing
thmugh the ball. Faraday excited negatively
a cylinder of shelMac an inch in diameter, by ^' ^^*
rubbing it with a piece of warm flannel ; and @)
placed on its top, which was cut concave for the •"'^^^s. •
pmrpofle, a lai^ge brass ball, a, Fi^. 406. It is <^f \
ohvioas that a molecular perturbation of this ball ^^1 ]
most be created by the inductive influence of the ^-^ V J
excited lac, as its lower half becomes positive, ^y p^- — ^
and the upper half, negative. If, then, a be
tOQched with the finger, the negative electricity /^
is discharged, and the ball remains positive, like V>
the cover of an electropborus (686). If the
canier-baU (664^ of Coulomb's torsion electro-
meter be placed m any of the various positions
shown by the figured circles in Fig. 406, and
then returned to the balance, the force of torsion required to re-
store the horizontal beam of the instrument to its proper position,
will give the inductive force exerted by the lac-cylinaer. Wher-
ever the carrier^ball is placed, both it and a must be first uninsu-
lated, and then insulated, before removing^ it to the electrometer.
The numbers in the cut snow the comparative amount of inductive
influence exerted by the cylinder in different positions. Thus, at
the top of the ball, a, the carrier-ball received a charge of positive
eleotricity of 130** by induction from the cylinder. So tnat we
must either consider that induction is exerted in curved lines, or
propagated through the intervention of contiguous particles. New,
as DO sinffle force can act in curved lines, excepting under the
coercing influence of a second force, we are compelled to adopt the
view of induction actingthrough the medium of contiguousparticles.
682. This inductive action appears to be manifested in every
electric phenomenon : thus, in the simple experiment of attracting
fight bodies by an excited tube (654), the positive electricity on
the tube acts hy induction on the jsieces of oaper, rendering their
surfaces opposed to its action negative, and tnus they are attracted
382
FBAMXLINIC BliBCTRXOITT.
J^. 407.
b^ the tube, in obedience to the law of motual attraction between
differently electrified bodies. The following experiment illnstratea
in an interesting manner the development of electricity by indno-
tion. Let a pane of dry. and warm window-glass be supported
about an inch from the tame by
means of two books or blocks of
wood, B, B, Fig. 407 ; and let several
pieces of piper or pith-balls be placed
oeneath it. On exciting the upper
surface by friction with a silk hsiid-
kerchief, this acts by induction on
the lower surface of the glass, the
intervening dielectric becoming pola-
rized in the manner alreadjr expiauied.
The lower surface of the glass, thus becoming electrified by in-
duction through its substance, attracts and repela alternately the
light bodies placed beneath it in a sunilar manner as the excited
tube (664--656).
683. All cases of electrical repulsion are in reality referable to
attraction under inductive influence. Thus apparently the two
slips of gold leaf, similarly electrified, repel each other; thia
repulsion, however, is really the effect of the attraction of sor-
roundiuK bodies of which the electric equilibrium is disturbed by
their inductive influence : their inner and opposed surfaces do not
manifest any free electricity.
684. Induction takes place through a thin plate of a perfect con-
ductor, as readily as through a non-conducting dielectric. A thin
piece of gold leaf may, by the inductive power of an excited
electric, become intensely positive on one side, and as powerfully
negative on the other, as long as it is within the influence of the
inouctive body.
685. Tfie lUlectrophonu, — ^Into a circular tray of tinned iron
or zinc, a, Fig. 408, about eight or ten inches m diameter and
one inch deep, pour a mixture of two parts of shell-lac and one of
Venice turpentine, until it is rather more than half filled, and let
it cool graaually. A circular plate of stout tinned iron or brass,
c, about two inches less in diameter than a, is furnished with a
glass handle, b, fixed into its centre. Remove the metallic plate
from the resinous cake, ▲, and excite the
latter by friction with a warm and dry
piece of fiannel; then place on it the
plate c: under these circumatancee the
negatively electrified cake rendere the
under side of the plate, c, positive, and
the upper side ne^tive by induction. If
then c be lifted off by its glass handle, it
will be found destitute of tree electricity.
Keplace c on a, touch the former with
Fig.4M.
THB BLE0TBOFH0BU8. 383
the finger, aod its negative electricity, excited by the indoctiye
influence of ▲, will ^ transmitted to the earth ; then let o be
raised, by the handle, b, and it will be fonnd to exhibit a charge
of positive electricity, which will be discharged, on the ap-
proach of any conductor, in the form of a yivid spark, the
plate resuming its naturally neutral state. Again, jplace o on a,
touch it with the fin|B;er, negative electricity a^n escapes to
the earth ; lift off c, bring any conductor towards it, and another
spark of poeitiye electricity occurs. This process may be repeated
an almost indefinite number of times, the cake ▲ losing none of ita
electricity by the operation, as it acts solely bv its inductive in-
fluence on the metallic plate d. Indeed, alter being once excited,
a spark may be obtained from this inHtrument, during many
weeks, without any fresh excitation, and on this account it haa
been used as an electrifying machine, and was by its inventor,
the celebrated Volta, termed eUdbrofcro perpetwy. This deetro-
phoTus is a most valuable instrument, not only from its affording
a beautiful illustration of inductive action, but from its yielding a
targe supply of electricity.
686. A small electrophoms, about 2} or 3 inches in diameter, ia
a veiy convenient instrument for charging various experimental ap-
paratus, such, for example, as Profl Thomson's Electrometer (666).
A disc of vulcanite, or ebonite, is an exceedingly good material
for the induction plate, as being both durable and highly excitable.
If the surface of the vulcanite plate be connected by a pin with
the metallic cell into which it is inserted, the trouble of touching
the insulated plate at each induction is saved, as the negative
electricity passes through the pin to the earth.
687. A veij useful modification of the electrophoms is made by
coating a thin pane of glass on one side with tin-foil to within
about two inches of the edge. Placing it with the coated side on
the table, excite the other surface by friction with a piece of silk
covered with amalgam (693), then carefully lifting the glass br
one comer, place it on a badly-conducting surface, as a smooth
table or the cover of a book, with the unooated aide dotontoarda.
Touch the tin-foil with the finger, then carefully elevate the plate
by one comer, and a vivid spark will fiy from the^ coating to any
conducting body near it ; replace the plate, touch it, again elevate
it, and a second spark will be produced. An electric jar may be
charged, in a few minutes, with an apparatus of this kind only
four inches square. This modification of the electrophoms is a
most convenient instmment in the laboratory where electricity is
required for eudiometric purposes, and where the introduction of
an electric machine is inconvenient.
688. If a given amount of electric potential be communicated
to a surface exposing sixteen square inches, and a similar amount
be communicated to another of but four square inches of surface,
it is obvions that each square inch of the former will contain but
^Si psAHKiJinc BLBOTRicrrr.
one«fonrth of that present in eyery square inch of the latter;
hence, although the amounts of potential are alike in both, yet
BSf in the former, that energy is distributed over four times
as much surface as in the latter, there will he found so much
less power of producing the phenomena of attraction and repulsion,,
induction, or light. The potential of the smaller sur&ce, is con-*
sequently said to be gp^ater than that of the larger.
689. A rounded surface, as a brass knob, on being held near to
an electrified bodf , allows induction to take place with much lesv
facility than a pomted wire similarly situated, on account of the
inductive action being concentrated on a smaller surface, causing
thereby a greater electric energy on the surface pf the point, than
on that of the knob ; for this reason, whilst a rounded surface may
be brought within an inch of an excited tube without abstracting
much of its free electricity, the point of a sharp needle, held at
four times that distance, will almost immediately effect the
transmission of all the electricity present on the tube. For this
reason, all kinds of apparatus destined to retain electricity,
are terminated by knobs or rounded surfaces ; and those intended
to facilitate its free and rapid transmission are furnished with
points. Similarly it is eyident that an electrified sphere has its
electricity equally diffused oyer its surface, whilst, in the case
of a prolate spheroid, the greater quantity is found at the ter-
mination of its long diameter, and of a cube, at the apices of its
solid angles.
690. With the exception of the electrophorus (685), no in-
strument for furnishing large (Quantities of electricity has yet
been described. The first machine constructed for this purpose
was contrived by Otto de Guericke, of Magdeburg ; it consisted
of a globe of sulphur, turned by a winch, and submitted to the
friction of the hand. Improvements were yery gradually intro-
duced into its construction : first, a globe or cylinder of glass was
substituted for the sulphur, and then the silk rubber was used, in
lieu of the hand ; the last great addition consisted in the adapti^
tion of a metallic conductor, so as to expose a large surface to the
inductive influence of the excited glass. The revolving glass
electric was used by Hawksbee in 1708, the rubber and conductor
being introduced in 1741 ; Boze, of Wirtemberg, contriving the
latter, and Winkler the former; thus rendering the electrical
machine nearly complete.
691. Two forms of the electrical machine are used in this
country, differing firom each other in the shape of the revolving
electric, which in one is a cylinder, and in the other a circular plate
of glass ; each varying in diameter from eight or ten inches to
two feet, beyond which size it is inconvenient to use a cylinder,
but plate machines are made of three feet, or more, in diameter.
The best form of the cylinder machine consists of a cylinder of
glass, levolving by means of a winch, between two upright pieoes
385
«f itoatuid well-dried wood, a, i, rig.tca.
Vig. 409 : thU ii aubmiCted to tha
fiictiDD of B rubber, fanned of u
olilaiig piece of wood, p, about tbrei
or four iaches (barter thari the
Cjlioder, covered witb leather,
fbrnuhed wilh a (Up of gilt, b,
tendiiig over oesrlj' half the circ
ference of the glue. The rubber
caa be placed at any distance from
the cylinder, supported by a Btroug
S'ua pillar, and connected with a
diog foot of wood, filed bj means
of B screw. On the side opposite to
the rubber is a cylinder, t, of hollow
tiiiDed iron, or, what is
MTsred with tin-foit, and about three or four in .
this is termed thepriTM condtictor; it is, like tbe rubber, insulated
on BgtasBleg. Tbe aide of the conductor neit to theglssec;liDder
b fumiahed with a row of pointed pieceH of wire, to allow of its
mora ripidlj aojuiring an electric state from the reToliing indue-
tire glass. This piece of the apparatus baa a number of holes, of
variooB diameters, bored in it, to permit the insertion of wirea of
various aizea ; the moutha of these holea, as well as eTer; other
part of the condnctar, except the poiots already mentioned, must
be carefnlly freed from all sharp edges or prominences, which cause
a rapid diuipatiou of electricity (669}.
69!. Tbe plate machine consists of a circular pbte of thick ^lass,
revolving vertically, by means of a winch, between two upnghit,
A, 4, Fig.410; two pairs of rub- J^.iio.
ben, formed of slips of elastic
wood eorered with leather, and
fhrnisbed wilh silk flaps, are placed
at two equidiatanC portions, b, b, of
the plate, their preuiiTe upon which
may be increased or diminished by
means of brass screws. Tbe prime
conductor consists of two curved
arms of hollow brass, supported
horizontally by a glass pillar from
one of the uprights t. ; its anna, |
where they appnHch the plate at
c, c, are funuBhed with pomfs, for
the same reason as in the cylinder machine.
Great advantage is gained by causing a row of metallic points,
ootinected with the prime condnclor, to be presented to both sur-
fitces of the revolving plate, instead of to one only, as in Ute
■ "■- if theii
« machines.
886 FRANKLIMIO BLBCTBICITT.
It 18 very difficult to give an opinion of the oomparative merits
of these two machines, — for an equal surface of glass, however,
the plate appears to be the most powerful ; but it has one ^reat
inconvenience, viz., the difficulty ot obtaining ne^tive electricity
from it, in consequence of the uninsulated state of Us rubbers: some
plate-machines are, however, specially constructed for thiH purpose.
693. When an electrical machine is required for use, it should
be placed within the influence of a good nre, so that its several
parts may become dry and warm. The rubber and conductor are
to be removed, and the plate or cylinder rubbed with a piece of
flannel, dipped in oil, until it becomes quite clean and bright ; the
layer of oil thus left, bein^ removed with a linen cloth. The rubbers
are then to be made quite dry, and their silk flaps wiped clean ; a
little amalgam made into a soft paste with lard, to be spread over
the surface of the cushions of the rubbers, unless there happens
to be plentv left on from a previous experiment, in which caae
the suiiace is to be cleaned bj[ rubbing it with a piece of rough
brown paper, or by scraping it with a knife. The rubber, or
rubbers, are to be then applied, and by means of the a<^u8tinK
screws, made to press with moderate force against the surface of
the cylinder or ntate. On then turning the winch, and holding
the hand towaras the revolving glass near the lower surface of the
silk flap, the electric discharges will be felt between the hand and
glass, like a brisk wind, attended by a crackling sound, and in
tne dark, by a lambent blue flame, 'rhe prime conductor is next
placed in such a manner that its points stand about one-eighth of
an inch from the glass : on holding the hand towards it, whilst
the winch is being turned, vivid sparks, often some inches in
length, oppear ; these are attended by a loud snapping noise, and
on striking the hand, produce a pungent pricking sensation, some-
times causing a papular eruption on the skin. These sparks, and
all similar visible manifestations of the existence of electric
potential, are produced by the induction of the electrified on the
approaching body, through the medium of the intervening atmo-
sphere. Whenever the transmitting medium is, like the air, a bad
conductor, that is, incapable of freely transmitting the electric
form of molecular action, a portion of that action will be manifested
in other forms, namely, those of light and heat. Whenever the
amount of potential in the excited body is larse, induction on the
atmosphere alone will take place, accompanied by a more diffixsed
luminosity, in the shape of a brush.
694. During the excitation of electricity by the machine, and
indeed in other cases in which luminous discharge (619) takes
place, a peculiar odour like that of phosphorus is evolved. This
odour has been traced by Professor Schonbein, of Bale, to the
formation of a substance termed by him (wone, and which is now
known to be an allotrapie form of oxygen, possessing some moet
peculiar and characteristic properties.
ACTIOS OF AMALGAM. 887
695. The deTelopment of potential electricity apon the prime
(K>nductor is so intimatelj connected with the theory of induction
already explained f677), that the remarks there made will he suf-
ficient to remove all ohscnrity as to the mode in which it is effected.
On rotating the glass plate or cylinder, a molecular disturhanoe
takes place at the points of friction between the rubber and the
flass, m consequence of which the surface of the glass is rendered
ighly posit iye, and the rubber equally negative. The excited
suHaoe of the glass is covered with silk (which is frequently var-
nished) to prevent its potential energy from being dissipated by
induction on the surrounding atmosphere, until it reaches the row
of points already mentioned, when the chief part of that energy is
expended in polarising, by induction on the points, the molecules
of the prime conductor, which becomes strongly positive, especially
at those parts farthest from the glass, whilst the metallic comb is
negative, so long as it is under the influence of induction. The
prime conductor being insulated, it soon acquires so high a degree
of potential energy, that it is incapable of acquiring more, until a
portion of that energy is expendea in induction on neighbouring
Dodies, or in transmission through a conductor in contact with it.
Similarly, the rubber, if insulated, soon acquires such a degree
of negative energy, that its action on the glass ceases, but if con-
nected by a conductor with the earth, or with any large mass of
conducting matter, the acquired energy is constantly expended on
the conductor, and the action of the machine continues. Just so.
if a body be exposed to a continual source of heat, as a ball of metal
anspended over a gas-burner, it will become more and more heated
until a point is reached, at which the heat is dissipated by radiation
as fast as it is acquired, when no further elevation of temperature
can take place. But if a good conductor of heat be now Drought
into sufficient contact with the heated ball, its temperature will
immediately be lowered, and cannot again be raised to the same
point, by the same accession of heat from the supposed source.
From what has been stated respecting the nibber, it is evidently
necessary that it be connected by a good conductor with the
earth ; for this purpose it is best connected by a wire or chain
with the water- or gas main of the house.
696. Much discrepancy of opinion has existed concerning the
modus cigendi of the amalgam applied to the rubber ; it certainly
acts yeiy powerfully in increasing the excitation of electricity.
The best combination for this purpose consists of two parts of zinc
and one of tin, melted together, and added to six parts of mercury,
previously heated in a crucible : the mixture bemg stirred until
cold, is readily reduced to a fine powder, which requires merely to
he formed into a paste with lard, to be ready for use. It has been,
with Kood reason, supposed that the oxydation of the amalgam, by
the friction employed, aids at least the increased excitation; for
#in>RJgaTnii of gold, and other difficultly oxydisable metals, do not
cc2
388 FKAHKUSflC BLECTBICITT.
appear to increase the development of eleotricitr. In accordance
with this view, Dr. Wollaston found that an electrical machine,
when worked in an atmosphere of carhonic acid, gave no evidence
of electricitv : the accuracy of this statement has however been
questioned by later observers. One mode in which the amalgam
acts is certainlv by affording a soft cushion of good conducting
matter, which thus presents an excellent surface for inducing elec-
tricity on the revolving glass.
Instead of an amalgam, the bisulphide of tin, or aurum musivum,
as it is often called, may be rubbed upon the cushions of the
machine, and with similar results. This latter substance acts
probably like the amalgam, by undergoing oxydation, as bv fric-
tion it absorbs oxygen, and is partiaUy converted into bisulphate
of tin. In a similar manner also iron pyrites, by friction, is partly
converted into sulphate of iron. The chemical influence of fric-
tion, indeed, is more energetic than is usually supposed; even
siliceous minerals, as mesotype, basalt, and feldspar, oecome, ac-
cording to Becquerel, partly decomposed, giving up, when long
triturated in a mortar, a portion of their alkali in a free state.
When the prime conductor is connected with the earth, and the
rubber of the machine insulated and connected with a second
prime conductor, sparks are seen on approaching the hand, or
other conductor, towards it ; these are termed sparks of negative
electricity, and as in the case of sparks from the former prime
conductor, they arise from the opposite polar induction of the
rubber on the neighbouring conducting body.
697. Ebonite or vulcanite, which is a mixture of sulphur and
caoutchouc submitted for some hours to a much higher tem|>era-
ture than that required to form the ordinary elastic vulcanised
caoutchouc, is an excellent material for the disc of a plate-machine ;
being, however, soil, compared with glass, it requires a much
softer form of amalgam, than that recommended for the glass
plate.
A verjr efficient form of prime^onductor is a large ring aboat
two feet in diameter, placed in a vertical plane above the machine,
in which situation it is much more out of the way of losing energy
by induction on neighbouring bodies. This form was proposed
and adopted by Dr. Winter of Vienna. In one particular case,
in which sparks onlv seven inches long could be obtained from an
ordinary prime-conductor, sparks of twenty inches might be ob-
tained from this.
698. Some years ago, a workman on the Newcastle and Carlisle
railway observed an electric spark to issue from the boiler of a
steam-engine on the approach of his hand. This curious pheno-
menon induced Sir W. Armstrong to investigate the subject, and
his researches, with the later ones of Faraday, have developed a
mode of exciting electricity to an almost indefinite extent. It
appears that whenever a current of steam escapes from a boiler
THE HTDBO-BLECTRIC MACHINE.
389
Fig, 411.
with sufficient Tiolence to cany off mecbanically particles of water,
it will, in its course tbrough a proper escape-pipOi excite by the
eviction of the water agaiust the ndes of tne pipe an enormous
amoant of electricity.
699. Upon this principle is founded the constraction of the
hydro-electric macoine of Sir W. Arm-
strong, Fig. 411 : this consists of a sphe-
rical or cyundrical boiler of wrought iron,
at least eighteen or twenty inches in dia-
meter, of sufficient strength to bear a
nressure of sixt^ or seventy pounds on the
Square inch. This boiler, ▲, rests on a
small furnace of sheet iron, b, and furnished
with a bent chimney, c. The whole is care-
fully supported on four stout legs of glass.
The boiler is provided with a proper safety-
valve, D. From its upper part, a tube an
inch or more in diameter rises, furnished
with a stop-cock at e. To the end of this
tube is fixed a spherical vessel of brass, f,
about six inches in diameter; into the
upper part of this a tube, furnished with a
stop-cock and a peculiar jet, is fastened.
The construction of the jet is shown in
Fig. 412, in which the whole is seen in
section, ¥ being the spherical vessel, a the
stop-cock furnished with a stout brass cap,
H, into which is firmly screwed the jet, 8.
This represents the section of a conical
plug of box-wood, terminated by a brass
month-piece. The shaded parts represent
the metallic portion.
Having filled the boiler about half full
of water, and placed burning charcoal in
the furnace, in a short time the water will boil, and after the air
has been first expelled, the stop-cock e should be closed, and the
globe p and its escape-pipe screwed on. When the quantity of
steam generated is equal to a pressure of fifty or sixty pounds to
the inch, open the stop-cocks e and g, some of the effluent steam
will be condensed in f, and the particles of water violently driven
forward with the vapour through the wooden mouth-piece of the
mg. 412.
and it is necessary to obtain an efficient discharge of its electricity,
for which purpose a coil of thick copper wire connected with the
earth may be so placed, a few inches from the escape-pipe, that
the current of steam may traverse it.
390
With Buch a bydro-elsctrio machine, sn larKS a qnnntitj of ele-
tricity maj be obUiaed as to enable it (o replooe with adTantags
the ordiDarj electric machinea. Tba onlr Dejection to its geneml
adoption is, that unloaa the boiler be anfficielitlj large, the iteam
quickly aesumea too hieh B atata of teneion, and an explosioD may
be not impowible. Sucb an Bccidoot baa in fact actnally occurred.
TOO. It ie remarkable, that, 80 long aa the globe p containa
merely a little pure water coadensed Irom the ateam, the eioita-
tion of electricity ia abuudaat; but if a little autpliuric acid or
common aalt be placed in it, all generation of electricity ceaaea,
apparently in conaeiiuence of the water being rendered too good a
condnctor, and thus allowing of the difiuaioii of electricity, aa aooij*
aa it ia develripcd by fiiction. If ■ little oil be dropped in, the
eicitatioD of electricity continuea, but ia changed in coancter, the
boiler being poeitive, and tho aieom negative.
There can acaroely be a quaatioQ of the accui«cy of the DpioioQ
of Faraday," (hat fnction ia really the exciting cauae of electrici^
in this mBchine ; for if the Btaam be allowed to escape eTen in
torrents, and under high pressure trom the opem'ng of the aaietj-
valve, no electric excitation oocura. Hence the neceadly of wi ar-
ranging the opening of the escape-pipe, aato preaent aome impedi-
ment to the paaaage of the ateam, in order to augment the friclioD.
701. A now form of electrical machine has been produced by
H. Holtz, of Berlin, the general arrangement of which ia shown in
Fig. 413. Into a wooden stand are inserted four npright glaaa
_ ,. bars, i, which are con-
'*■ «*• ii«cted above and belc™ by
four boriionlal and paraJlel
bant, B ; and the front and
' back uprights are con-
, nect«d by two paralJel
I wooden ban, c, which anp-
/ port at their middle paints
I the aiia of a rotating glass
plate. A small pulley
placed near the end of the
same axis ia coDnected bj
a band with a much lai^r
puliev placed beneath it,
its axis; ibia lattersmalt pulley
_, cond laree pulley on a separate
axis at the side, to which a winch ia attached. By turning the winch
a very rapid rotation may be communicated to the plate, by means
of the multiplying pnlliea. A second glass plate, about two inches
larger in diameter than the former, ia placed behind it, having a
hole in the centre, LhruuKli which tbe aiia passes freely. The
brger pUte is supported by foor wooden damps, h, sliding on the
* PUL Tiana. IMS, p. 17.
M. BOLTZ' MACHINE* 391
glasB bars, b, and ib thus completely insnlated. This plate has
two lar^ apertures opposite each other, each bounded by two radii
containing an angle of 43° or dO*", and by arcs of two concentric
circles. At the correspoDding radial sides of these apertures two
strips of paper are attached, and perpendicularly to the middle of
these a second strip of paper, cut to an obtuse point and projecting
over to near the middle of the aperture. The fixed plate may be
adjusted to any required dintance from the rotating plate, by means
of the sliding clamps ; and both plates are coated with a solution
of sheil-lac, to prevent the dissipation of electricity by moisture
adhering to their surfaces. A vertical glass rod passes through
the middle of the front wooden bar, c, terminated by knobs through
which pass two horizontal brass rods, terminated by " combs" or
rows otpoints, placed radially in front of the rotating disc. Across
the front ends of these brass rods are two stout wires terminated
by knobs, and furnished with glass handles by which the knobs
may be placed at any required distance from each other. The
fixed plate is placed about half an inch from the rotating one, and
it is so placed that the radial paper slips may be near^ but not
quite behind the combs. If the disc be now rotated by means of
ine winch, no signs of electricity appear, but if one of the slips of
paper be charged by an excited glass tube or by a stick of sealing-
wax (or still letter, by a glass tube, or a rod of baked wood thickly
▼amished with shell-lac), and the movable plate bo put in rapid
rotation, a copious stream of sparks, sometimes four or five inches
in length, will pass between tne knobs ; and this extraordinarily
large evolution of electricity continues until the charge of the
paper is dissipated by the moisture in the air. It is a singular
tact, that so long as the paper remains unelectrified, the plate
rotates freely with very little force, provided the various bearing
points be sufficiently lubricated, but the instant the paper is eleC'
trified, a considerable resistance is experienced to the rotation of
the plate ; and this resistance increases with increiu»ed rapidity of
rotation, and the consequently increased potential evolved.
702. The action of this curious machine has been to many quite
a scientific riddle, but it appears to be capable of a simple expuna-
tion. When the molecular disturbance due to electrical induction
has been excited in any body, iome time, although probably an
exceedingly small interval, is required for the subsidence of the
disturbance, after the excited is removed from the inductive bod v ;
but the removal must be effected very rapidly, in order that the
disturbance may survive it. Now a portion of the rotating plate in
front of the excited paper undergoes a disturbance by induction,
but in consequence of the rapid rotation, this portion of the surface
reaches the points before the induced disturbance has subsided,
and acts on tnem inductively, and this process bein^ repeated by
each succeeding portion of the plate, the comb and its conducting
rod acquire a high degree of potential by the continuous accumu^
392
FRAVKLIKIO BLEOTBICITT.
Mlg.41i.
lation of Bmall inductionfl. It is probable that the continual
attraction existing between the inductive portion of the plate and
the opposed comb constitutes a perpetual drag on the rotation, like
a friction-break, and produces the resistance ezperienoed.
To this presumed explanation of the phenomena there is in
some degree a parallel case in Optics. As it will subseqnentlj
appear, phosphorescent bodies are those which are capable of re-
taining for a short period the light impressed upon them, and
again emitting a portion of it : that is to saj, a portion of the
luminiferous molecular action is imparted to the phosphorescent
body, which again transmits that action to surrounding matter.
But in some substances this influence of light is so transient, that
if they be enclosed in a glass tube capable of rotating oo it^ axis,
and placed behind a slit in an opaque screen, and then strongly
illuminated from behind, no light will be given ofi* through the
slit, unless the tube be put in rapid rotation ; otherwise the phos-
phorescence subsides before the tube has made half a revolution.
And in this case, also, the amount of light radiated is up to a
certain point increased hj greater rapidity of rotation, because a
greater amount of ene}]gy is accumulated at the points of radiation.
703. If a pointed wire be held towards the insulated rubber of
an electric machine in action, it will by induction
become highly positive ; the electric tension at
the point soon becomes so hi^h as to produce dis-
charge through the dielectric air, in the form of
a brush or pencil of rays, as at the point of ▲,
Fig. 414. When, on the other hand, a similar
Soint is held towards the positive prime con-
uctor, it acquires a high state oi negative
tension, and luminous discharge occurs, not in
the form of a brush or pencil ; but the end of
the wire becomes illuminated with a minute but
brilliant star of light, as at the point of b. By
using similar wires, the electric state of a con-
ductor may be ascertained by the character of the
luminous discharge occurring at the point of a
wire held towards it.
704. If the conductor and rubber of the electric machine be
connected with each other, or with the earth, by means of a con-
tinuous conductor, as a piece of wire, the electric discharge will
take place alon^ it invisibly, unless the machine be extremely
energetic, in which case the wire will appi^ar surrounded with a
lambent flame. But if the conductor be interrupted, then vivid
sparks will appear at each rupture of continuity, arising from in-
ductive action taking place at every one of these points.
£xp. A. Connect the prime conductor and rubber with each
other, by means of a brass chain ; on working the machinoi vivid
sparks will appear at every link.
ISTEBRUPTEO DIBCHARGE.
893
2.
Fig. 415.
B. On a plate of glass, Fi^, 415, paste some stripft of foil,
haTing portions cat out, so that
the spaces represent letters. On
connecting the first piece of foil
wiih. the condnctor, and the last
with the ground, the letters will
appear in characters of fire, in con-
sequence of luminous discharges
in the form of sparks occurring at
each division of the foil.
C. Draw, on a pane of glass, a
serpentine line witn varnish, and
place on it, before it dries, metallic^
spangles, about one-tenth of an inch apart; on connecting the
first of the series with the machine, and the last with the ground,
a serpentine line of fire will be represented^
D. I ( in a similar manner, the Ro 4ia
spangles are placed on a glass ^'
tube, Yig. 416, in a spiral direc-
tion, a spiral line of sparks will
be produced.
705. In all these experiments, it is better to allow the electri-
city, before passing through the tin-foil, chain, or luminous
condnctor, to acquire some degree of tension ; this is conve-
niently effected by means of an instrument called Lane's eUetro-
QK/sAA.^^jQ
^.417.
meter J or more properly, discharger.
This apparatus consists of a curved
arm of varnished glass, b, fixed by
a brass stem into the prime con-
ductor, A. Fig. 417, and terminating
in a ball, c, through which passes
a rod furnished with two brass
knobs, D, B, capable of being placed
at any required distance from the conductor. If any of the above-
descnbed pieces of apparatus be connected with the ball d, elec-
tric discharges will take place through them, as soon as the
electricity has acquired a sufficient potential to efifect an inductive
discharge through the air between ▲ and k.
706. Induction takes place through a greater space in an air-
pump vacuum than under ordinary atmospheric pressures, a cir-
cumstance arising from the resisting dielectric medium being
diminished in density. This led to the error of oonsidering a
partial vacuum as a conductor of electricity, which is not the
case, polarization of the particles of rarefied air being transmitted
through it readily, providing the two conducting surfaces be suf-
ficiently near to permit induction to take place : this will occuf
at a distance of five feet or more, in a very good vacuum.
If a glass tube, a, Fig. 418, two or three feet in length, be fiir-
394
FIUNKLIKIC ELECTBICITT.
nisbed at either end with a brass hall projecting into its interior,
and carefollj exhausted of its air bj means of a good air-pump,
on connecting its upper end, n, with the prime con-
ductor of a machine in action, and its lower end, c,
with the earth, b becomes positive, and induces a
contrary state on the ball at c ; the induction taking
place with facility, in consequence of the atmospheric
pressure being removed, is accompanied by a beauti-
ful blue light, filling the whole tube, and closelj re-
sembling the aurora borealis. This luminous dis-
charge undergoes some verv interesting changes:
when the rarefaction of the air is considerable, the
tube is filled with a purplish lambent flame ; if a little
air be then admittea, the continuous column of light
is replaced by distinct flames repeated several times
in a second, and darting from one ball to the other ;
and if more air be allowed to enter, the discharge takes
place in beantiful zig-zag lines of brilliaut light, like
flashes of lightning, occurring, however, at consider-
able intervals.
707. Since electric induction takes place with very great faci-
Jltf 410. ^^^y through highly-rarefiea air, it is easy to under-
^ ' stand the rationale of the Leyden vacuum. This con-
sists merely of an electric jar coated as usual exter-
nally, its interior being[ exhausted of air, by means of
the air-pump, and having a point projecting into its
■interior, ana connected externally with a knob. This
apparatus. Fig. 419, may be used like the common
electric jar, induction readily taking place from the
point over ito whole internal surface. On charging
and discharging it in a dark room, the point of the
wire in its inside becomes beautifully illuminated with
a star or pencil of rays (647), according as the electricitv in the
interior of the jar happens to be of the positive or negative cha-
racter. To this the term reciprocating dieeharge has been applied
in contradistinction to the discharge taking place in a vacuum
between two electric terminals.
708. Every conducting substance, insulated and connected with
the prime conductor, or rubber, may be considered as part of
either, so far as their electric state is concerned : thus if a man
standing on a btool, furnished with iubulating glass le^, touch
the prime conductor, he virtually becomes part of it, being simi-
larly electrified, and all the phenomena proper to the prime con-
ductor may be observed at any part of his surface.
709. The electric spark does not impart to the finger a sensa-
tion of sensible heat, on account of the impression being too
transient, although it is capable of exciting sufficient caloric to
produce the combustion of iufiommable substances.
S95
Exp. a. Coaaect > Hhallow matallio cap with tbe prime con-
dactiir, and pour ether into il ; on bolding the finger, or a knob of
bra« over it, the electric indactioD taking place thnmgh it will
evolve BuXdent heat to inflame the ether.
B. Pat iuto a bottle giaDolated sac, and some dilate ml-
phoric acid ; Si !□ ile neck a, cork famished with a tube, ti
natioriii " '■— ■ '---' .-.i ....... . ..
<J. A braag tube, moonted on a Block like a pistol barrel, is
fomiBhed with a plug of baked wood, or of loma other good in-
■tUator, A, Eig. 420, which la Bcrewed into it. Through thia a
bnaa wire pnwei into the inte-
rior of tbe barrel, bat without Hs-iW.
toncbine it; the brass tabe is ^
then filled with an eiploeive
mixture, b/ hohliDg it for a few
eeconda over the moatb- of a
battle Mutninin^ tbe ingredients
for the prodoctiou of hjdrogen
gas. On clorii^ the mouth
quicklv, with a cork, the charge
ia retained, and on approaching
(he Imob B to the pnme conductor, a spark ii produced in the
interior of the barrel, the gases are exploded, and the cork driveu
out with considerable violence, attended with a loud report ; this
apparatus is termed Volta's electric pistol, from the name of its
iDventor.
710. Tbe amountofrepukion is made use of aaan approximate
indication of the potential elecCricit; accumn-
lated in tbe prime conductor of an electrical Jv-**'-
machine, hi- means of an instrument, called
Henley's electrometer, consisting oF a gn-
doalea semicircle of ivorj', a, lig. 4!I, at-
tached to a rod of wood, d. Laving a projecting .
pin at the bottom, which is inserted in a hole /
in the upper surface of the prime conductor, t
From the centre of t. depeads a light index,
terminating in a pitb-Mli, o, and readily
moving on a jinnt. On working the machine,
tbe electrometer becomes, like the conductor,
pDsitivel/ electrified; the pith bail o is con-
aoentl; repelled b; the stem d, and re-
es from it ; raising the index even to 30*,
if the action of the macbins be suScientiy
strong.
711. Various electrical toja have been deiiaed, aa illuftrstiona
396
FBANKLINIC ELECTBICITT.
Fig. 422.
of attraction and repulsion ; of these the following will serre as
examples : —
Exp. a. Place in one of the holes in the primA conductor, a
figure-head, covered with a plentiful supplj of long hair ; on ro-
tating the cylinder, the haijrs becoming smiilarlj electrified, repel
each other, and " stand on end."
B. Suspend from a brass rod, inserted into the
conductor of the machine, by a piece of chain, a
plate of brass. Fig. 422, about four inches in dia-
meter, and about two inches beneath it, place a
second of rather larger size ; on electrifying the
conductor, the pobitive electricity of the upper
renders the lower plate negative by induction,
and discharge would ensue, if they were not too
far apart. On the lower plate place some figures
of the pith of elder, or paper, and on turning Uie
machine, they will begin to dance between the
plates, beine alternately attracted and repelled
by each of them. .
C. Suspend from a rod on the conductor, the
apparatus well known as the electric bells. The
two outer bells, a, b, Fig. 423, are suspended by
brass chains, whilst the central, with the two
clanpers, hang from silken threads; the middle bell is connected
with the earth by a wire, or chain : on turning the cylinder, the
bells A and b become positively
electrified, and by induction, the
central one becomes negative; lu-
minous discharge taking place be-
tween them, if the electricity be in
too high a state of tension. But if
the cylinder be slowly revolved, the
little brass clappers will be alter-
nately attracted and repelled by the
/y^ outermost and inner bells, producing
- a constant rinsing so long as the
machine is worxed.
D. Fix to the conductor a bundle
of threads, — each about eight inches long, tied at both ends; on
turning the machine, the threads being similarly electrified will
repel each other, and as they are connected at top and bottom,
their centres will recede from each other, and separating, the
threads will represent a skeleton spheroid so long as the winch of
the machine is turned.
712. If a pointed wire be fixed to the prime oonduotor, a discharge
takes place silently from it, in the form of a luminous pencil of
rays, on working the machine ; this is accompanied by a brisk cur-
rent of air, veiy sensible to the finger, when held near the point.
J^.428.
OF
t
Eip. A. Rp. 43*. Ri fonr nnet of
laateboard obliqnely, like the Mils of a
(indmill, in a circular pieca of cork A,
urtiished with a ateel needle Tor an axle ;
uipend tbia from b, one of (ha polea of a
lar-magnet, and on bolding it towarda tbe
K^int of a wire fixed in the conductor fi^
u that tbe cDireot of air excited bj the
li>«^haT^ {mm it ma* strike the ranee,
he little apparatus ml] begin to revoWe
fith great rapiditj.
Tbecarrentof airthii.i set in motion bj discharges frompoiutod
virea, is sufficient to react npon them, and caose them to move in
in oppoHte direction to the cnrrent, pru- jn^, ks.
-ided the; be moveable on an axis. -^ — k-
B. PUca the cap of the electrical flj,
ijrniflhed with four pointed wires b*nt
lear their terminatjons at right angles,
in a pivot fixed in one of the boles of
he prime conilnctor, Fig. 425. On tarn-
ue the wincb, tbe wire will rapidlr tv-
roT™ in a direction oppoaed to the points,
13 nhown by the arrows, eibibiting in the
lark a complete circle of light.
713. The mechanical force of an electric discharge is very c<
•iderable. pronded its effects be concentrated in a Ma ^1
reiT small space.
Fill a phi«l, i. Fig. 42S, with oil, or other ni
:finilucting tluid, pass thrangh tlie cork a copper
cire bent near its lowor end at right angles, so
that its point may press Hgainat tbe inside of the
glass, snd suspend it by the apper end of the
wire from ths primo conductor, 'ibe point of the
wire in the phial will assume a high state of posi-
tive electric potential (689). On bringing towards
it a brass knob, or a knuckle of the hand, induo-
Liiin and consequeal discharge will take place
ihrough the glass, which will become perforated
with a
h media diSeriiu; from
.... Thas, in ranibd air,
;s light is blue and less vivid, than when under ordinary atmo-
spheric pressure. Faraday found that, in nitrogen, ic was veiy
brilliant, bluish, and aonorons ; in oxygen, less brilliant, and
white ; in hjdrc^n, crimson, and accompanied by little or no
siiuod ; in carbonio acid ita tint was rather more green than in
air; in coal-gas it waagreon or red,«oiqetimo8 both, with frequent
inlermptions by black spots ; and In hydrochloric acid gas, white
398 FBAMK.LIN1C ELECTSICTTY.
without any of the dark spots so frequently present in the case of
the other gases. Occasionally the spark appears interrapted in
its centre oy a non-luminous spot, owing to induction taking place
at that point in a more diffiised manner than nearer the inducting
surfaces. In common air, the luminous electric dischaige or
spark, heoomes modified in tint according to the surface at which
it takes place ; thus, from a lai^ brass oall, it is white and bril-
liantly luminous, whilst on diminishing the size of the ball, it
becomes bluer and more scattered, assuming the form of a brush,
which itself depends upon a series of intermitting discharges
taking place with considerable rapidity. From the surface of
ivory, the discharge is crimson-coloured ; from silvered leather it
is ^en ; from powdered charcoal, yellow ; and purplish, when
taking place on the surface of most imperfect conauctors of elec-
tricity. The li^ht of the electric discharge is capable of under-
going decomposition by a glass prism, ana polarization by reflec-
tion or absorption, like ordinary light.
715. Several varieties of electric discharges have been pointed
out, and are readily distinguished by their attendant phenomena.
A. Conductive IHseharge. — ^This takes place when bodies dif-
ferently electrified are connected by means of a good conductor.
It is unaccompanied necessarily by any mechanical e£G9ct| or dis-
placement of portions of the conductor.
B. DUrvptive Ducharge. — Under this term is indoded all the
varieties of electric discharge, accompanied by lights from the
faint lambent gleam at the extremity of a wire, to the vivid flames
and sparks accompanying the transmission of potential electricity
between good conductors. In all cases of this discharge, an actual
displacement of particles through which it occurs, takes place.
We have a go63 example of it in the frequent rupture of electric
jars by spontaneous discharge taking place through them ; the
perforation of a glass bottle iiill of oil (660), is also a case of this
C. Convective DUeharge. — A form of discharge in which,
under the influence of electric currents, ponderable matter is set
in motion. Thus, the aerial currents from points (712), are
examples of the convective discharge. Another series of cases
in which ponderable matter is transferred by the electric current,
is found in almost all instances of discharge between metallic sur-
faces, or charcoal points; minute portions of the material of
which the conductor is composed being conveyed from one surface
to the other, so as to cover it with a superficial coating of vola-
tilised matter. The transfer of solid matter, in these cases of
convective discharge, always takes place in the direction of the
positive current.
716. When two insulated conducting bodies are differently
electrified, and approximated towards each other, so as to lie
within the influence of their mutual induction, but not saffi-
TBB C0XDEH8ER.
899
1.^
Kpir
n
cf
2i-
]
cientl J near to pepuit of lominotifl dischar^, no signB of eleo-
tricity wiU be conunonicated by either to pith-ball electroscones
connected with them, until the bodies are separated to a consider-
able distance from each other. The opposite electricities are then
said to be diagttised or paralysed^ by their matoal inductive action.
Let two plates of metal, ^ ^^
A, B, Fig. 427, Mven or '*
eight inches in diameter,
be insulated on varnished
glass legs, c, d, fixed into
pieces of wood moving in
a groove in the boara b.
To the back of each of
these plates is attached a
brass wire, furnished with
a binding screw; these
hold wires, o, h, irom each
of which is suspended a pith-ball electroscope.
Separate a and b from each other, and touch one with an
excited piece of glass, the other with excited resin, the pith balls
connected with each plate will diverge, one with negative, the
other with ^itive, electricity. Gradually approximate the plates,
and as their mutual distance diminishes, the pith balls will by
degrees collapse, until ▲ and b are very near to each other, when
they will appear totally unelectrified. The apparatus being in
this state, gradually separate a and b, and, m proportion as
this is done, the pith baits will diverge as before, proving that the
potential electricity of the plates hi^ not been destroyed during
the course of the experiment : that of either plate having been
held in abeyance by the inductive influence of thitpposed plate.
Next, remove by the contact of any conductor all potential
electricity from both a and b. brin^ them vdthin one-sixth of an
inch of each other, and toucn a with an excited glass tube ; it
thus becoming positively electrified, acts by induction on b, the
opposed surface^ of which ^ becoming negative, its pith balls
diverge with positive electricity. If b be now touched by the finger,
or any other conductor, all the potential electricity not held induc-
tively on the surface of the plate, will escape, and the pith balls
will collapse. The plates are now in the same condition, as in
the former part of the experiment, when they were oppositelv
electrified, and on again separating them, both pairs of balls will
diverge as before.
If Tefl to themselves the plates will retain their potential for a
period of time varying with the dryness of the atmosphere : if,
as in Prof. W. Thomson's electrometer (666), a special provision
is made for the removal of aqueous vapour by absorption, the
potential will be retained for a considerable period of time,
amountiiig perhaps to several days.
400 FKAHKLIKIC ELECTBICITT.
717. Any other dielectric may be substituted for air in these
experiments : and if a plate of gutta-percba or sheU-Iac be used,
the electricities accumulated in the two surfaces may be increased
to a very considerable extent (687).
£xp. A. Place a large pane of glass, about fourteen inches
square, between the two plates of the apparatus Fig. 427, and bring
A and B so near to each other as to be in contact with the pane.
Connect a with the prime conductor of the electric machine, and
work the latter so as to render the plate powerfully positive : this
will act by induction through the pane of glass on b, as before (664),
which, on approaching the hand to the back of b, will produce a
series of sparks, or discharges. After a certain time these will
cease ; then remove the wire connecting a with the prime con-
ductor, and leave it insulated ; the plate a. will then be chained
with positive, and b with negative electricity, both in a state of
high tension. Connect the two plates by means of a curved wire,
and ditruptive discharge results, attended with a vivid flash of
light, and a loud snap. If, instead of using a curved wire, the
plates be connected by the fingers of both hands, the same di»-
charge ensues, accompanied by an exceedingly disagreeable and
painful sensation, extending across the arms and chest of the
experimenter, well known as the electric shock.
' B. Instead of placing a pane of glass between the two metallic
plates, coat it on each side with a piece of tin-foil, leaving about
one inch and a half all around uncovered. On connecting one
piece of tin-foil with the conductor of the machine, and the other
with the earth, the glass dielectric will become chai^ged as before,
the side connected with the conductor acquiring a powerfully
positive, and the other an equally energetic negative chai^ge.
718. The ehargCf thus communicated to the pUte of glass,
penetrates its substance to a certain small distance, as was first
pointed out by Mr. Henley.
Exp. a. Coat two thin pieces of window-glass on one side
only with a piece of tin-foil, considerably smaller than the glasaea ;
place them toeether, with their uncoated sides in contact. Charge
this double pmte as before, and then attempt to separate them,
they will be found to adhere veiy firmly together; on pulling
them asunder, the naked side of that plate which had been con-
nected with the conductor will be found to be positively, and that of
the other plate negatively electrified.
£xp. B. This may be still more readily shown, in the manner
proposed by Faraday, by charging in the same manner two
plates of spermaceti covered on one side with tin-toil. The im-
perfectly insulating character of this substance enables us to detect
this penetration of the charge more readily than when glass plates
are used.
At the instant the discharge takes place, the two electricitieB,
accumulated in a stete of mgh tension on the coated surfiMMs
LEYDEH JIB. 401
of the glftM, pass from a state of constraint into one of rapid motion,
constatnting the eUetrie current. In all cases of disruptive dis-
chaige, the current is bat of momentary duration, and ceases the
instant the electric equilibrium of the dielectric is restored.
719* Induction, and subsequent charge, do not appear to be
materially modified by the figure of the glass, its thickness only
influencing these actions ; easteris paribus^ the thinner the glass
tile more powerftil charge will it hold. As the plate is a very
inconvenient form of apparatus, on account of its being readily
injured, ^lass jars or bottles coated with some conductor, are
almost universally substituted for it This, indeed, was the first
arrangement used, forming the celebrated electric or Leyden phial,
so called from the place of its discoyery, by Guneus, or Mnschen-
broek, in 1770. White and green glass answer almost equally
well for the construction of electric jars ; wide-mouthed glass jars
are very convenient, but on account of their expense, common
wide-mouthed green bottles may be substituted, provided they
are free from air bubbles, and specks of unvitrified matter. Cylin-
drical glass vessels are frequently employed in the construction of
electric batteries.
720. The ordinary Leyden phial, or jar, consists of a glass jar
of any convenient size, coated internally f^id externally with tin-
foil to about three inches from its mouth. The
jar is closed by a dry and varnished cork, or ^' *^*
by a wooden disc, a. Fig. 428. A stout brass
wire, furnished with a ball of the same metal,
passes through the cover, a, and has several
thin pieces of wire, or a chain fixed to its end,
B, so as to touch the inside coating in several
places. The knob thns corresponds with the
internal coating. When narrow-mouthed jars
or bottles, as the common sixteen-ounce phials
of white glass (which, from their thinness,
form excellent electric jars) are used, it is
better to coat them internally with brass
filings, instead of tin-foil, on account of the
difficult of its application to their inte-
rior. For this purpose some thin glue should
be poured into them, and the botlle turned slowly round, until its
inner surface is covered to about three inches from the mouth.
Brass filings are then put in, and the bottle well shaken, so that
they may be diffused equally over its surface ; on inverting it,
those which are in excess will fall out, and the bottle will be left
coated internally sufficiently well for its intended purposes. Some
jars should always be provided with hooks, instead of knobs, as it
IS requisite frequently to suspend them from the prime conductor.
To preyent the too rapid deposition of moisture on the uocoated
part of the glass, and the consequent escape of the charge, it is a
D D
402 rRAKKLnno slsctbicitt.
good plan to TarDiBh the jar aboTe the external coating, with m
solution of shell-lac in alcohol, or with the common spirit-TarniBh
of the shops : taking care to warm the jars before, and after its
application.
721. If the knob of a Lejden jar be held about half an inch
from the prime conductor, whilst its outside communicatea with
the earth, a rapid snccession of sparks will pass between the knob
and conductor, which will continue for some time, and tiben cease.
The jar will then be charged^ its inside becoming positively, and
its outside coating negatively, electrified; neutnuization being
prevented by the interposed glass, unless the tension of the elec-
tricity be considerable, in which case, discharge often ensaea
either by passing through the glass, which is then perforated, and
the jar rendered useless, or else by passing over the surface of the
uncoated shoulder of the bottle m the ibrm of a bluish lambent
brush of flame, constituting the spontaneous dischai^. If the
potential be not sufBcient to produce these phenomena, and the
cMttle be set aside, its electricity becomes gradually dissipated by
the conducting action of the surrounding atmosphere.
722. When an electric jar is^ charged, its discharge may be
effected by connecting its outside coating with the knob, by
means of a thick curved wire, which is generally furnished with
a brass ball at each end. This instrument, the discharging-rod^
Pig^ 489. is usually attached to a
glass handle, and has a
p cradle-joint, like a pair ci
T ^^""^Vs^ compasses, so as to allow
' ^Ni,. the metallic arms to be
r~r^ ^< placed at different dis-
tances from each other,
Fig. 429. The jar may be
also dischaiged by graap>
ing the external coating
with one hand, and touch-
ing the knob with the
other, in which case the person who performs the experiment
experiences the peculiar and painful sensation, termed tne shock.
in nis arms, and, if the jars be large, through his shoulders ana
chest. A charged jar, the outside of which is negative, and the
inside positive, is said to be positively electrified ; and to be
negatively electrified when the electricity of its internal coating
is of that kind.
723. In accordance with the conditions of the induction and
disguise of electricity (716), it is obvious that an insulated jar
cannot be charged.
£xp. A. Place a jar on any insulating support, as a stool with
glass legs,- with its knob in connexion with the prime conductor ;
on workipg the machine for some time, and examining the j^,
XUOTUOAL BATTBBT.
403
it wQl be found to be almost destitute of any electric charge.
For on connecting its outside and inside coating, by means of
the discharging rod (722), a minate discham takes place, a faint
spark onljr appearinjg between the knob of the discharging rod,
and the jar.
£rp. B. Place the jar in the same position, and while the
machine is in action, bring the finger near to the outside coating,
yivid sparks will pass towards it, arising from the discharge ot
indnoed electricitj from the outside of the jar. After a certain
time these sparks will ceane, and on applying the discharging rod
to the jar, the flash of light and loud snap mat ensue, prove that
die jar has received a considerable charge.
If the knob of a second iar be substituted for the finger, it will
become charged b^ ^e electricity proceeding from the outside
of the first jar; this mode of charing is termed, by the French,
'* charger en cascade.'' And in this manner a series of jars, Fig.
430| can be readily charged, representing a polar arrangement, in
FSg. 4A1.
which the knobs of the jars are all positive and the outside coat-
ings all negative.
724. The charge of an electric jar varies, coBterU paribus^ with
the extent of coated surfietce ; and on this account, veiy large jars
have been constructed. These, however, have several inconve-
niences, and amone them may bo mentioned, the necessary thick-
neee of the glass when the jars are
Tery large, preventing induction to
any great mtensity taking place
throQgh them. On this account,
several small iars coated in the
usual manner (720), are placed in
a box lined with tin-foil, or other
good conductor, so as to connect
their ontsides, whilst their knobs,
and consequently their insides, are
connected by brass rods : the whole
oonstitnting the eSfctric battery^
Fig. 431. As the interiors of all the jars communicate, they may
be charaed as a single jar, their exteriors being connected with
the earth. A hook, a, is fixed in the side of the box in contact
with the metallic lining, so as to allow of readily connecting a
chain or wire with the outside of the jars.
dd2
404 fhakklinic electricity.
725. In charging a battery, its interior is connected bj means
of a wire or chain with the prime condnctori and its exterior con-
nected with the earth ; and for the purpose of tracing the progress
of the charge, the quadrant electrometer (710) is fixed in one of
the holes of the pnme conductor. On taming the machine, the
potential of the prime conductor, being now shared bj a largely
extended conducting surface, is augmented much more slowly;
but by degrees it is accumulated on the conductor, and acting on
the electrometer raises its index, which, when the battenr has at-
tained its utmost charge, seldom rises above 40** or 50 : as the
tension of a batteir charge never equals that of a single jar, pro-
bably on account of the larger surfi<u^ exposed to inductive action.
The battery may be discharged like a single jar, by connecting its
outside and inside, by means of a jointed discharger, or a chain.
Great care should be taken in this operation to avoid passing any
of the charge through the body, as the shock from a powerfdl
battery may be attended with serious consequences.
726. Residual charge, — After a large jar or battery has been
discharged, its two surfaces should be left connected for some
time, as a residual charge^ arising from the return to the coatings
of the electricity which had penetrated the substance of the die-
lectric (718\ often takes place, and may give a severe shock to a
person toucning the battery without this precaution. Acoor£ng
to Reiss, the amount of potential exhausted by the first discharge
amounts to ^ only of^the entire charge, ^ being left for the
residua] charge.
727. When the two surfaces of a charged jar are connected by
means of the jointed discharger (722), or by a lon^ metallic wire,
the current of electricity traverses the conductor with an enormous
velocity. In fact, even with the largest circuit yetfemployed, the
time occupied appears to be almost inappreciable : from a series of
very beautiful experiments performed oy Prof. Wheatstone,* it
appears probable that the electric current, in passing through a
conducting wire from one side of a charged jar to tne other, is
transmitted through the conductor with a velocity equal to about
576,000 miles in a second of time.
728. From the above remarks it is obvious that the coatings
are by no means essential to an electric jar ; they act only as sur-
faces limiting the inductive action, the charge itself residing, as
has been already shown, in the glass. This mav be further
proved, by providing a wide-mouthed glass jar with moveable
coatings ; cuarging it (721), and removing the coatings, these will
be found un electrified, and on replacing them by anower pair, the
jar may be discharged, the flash accompanying which act, will be
found scarcely less than that of a jar of which the original coat-
ings have been retained.
A jar may also be charged without metallic coatings ; to show
• Phil. TraasMtionv, 1834, p. 6»1.
URITSB8AL DI8CHASOBB.
405
Ftg.4aS.
♦♦♦♦♦
this, let a glass tambler be grasped by tbe hand, j^. 431.
and its moath held over a pointed wire, fixed on
the prime conductor of a machine in action ; it
will become charged, and on fitting a pair of
coatings to it, it may be discharged like a com-
mon jar. If, instead of being thus discharged, it
be inverted on a table over some light pith balls,
fig. 432, these will be attracted and tnen repelled
by its internal surface in a very curious manner,
and the discharge will thus be gradually effected.
The coating, as might be from these facts ex-
pected, need not to be continuous ; it may consist
of a number of separate pieces of tin-foil fixed at a
small distance from each other. Jars thus qoated
are termed diamond jars, fig. 433, from the bril-
liant scintillations appearing on their surfaces when
they are charged and discharged.
729. When the transmission of electricity, neces-
sary for the discharge of a jar, is effected by various
conductors connecting the two surfaces, the charge
18 said to pass through them, and Tery important and interesting
mechanical and chemical effects are thus pn)duced. For the pur-
pose of passing the charge through different bodies, a very conve-
nient apparatus, called the universal disehargerf is employed:
this consists of two brass wires, a, b, Fig. 434, terminating in
points, and on which balls are
screwedr; thev are furnished JV«^84,
with a ball and socket or cradle
joint, so that they are moveable
in any direction on the tops of
the glass snpports, c, d. A
hollow wooden support, b, is x-
fixed midway between them; [
into this is screwed a small ^
wooden table, having a slip of
ivory inlaid on its surface, on which any substance to be subjected
to the action of the current is placed. A small press is sometimes
placed in e, instead of the table f, for the purpose of submitting
Dodies to the action of the charge whilst under pressure.
730. The following experiments, requiring for their performance
s charged jar, exjxxdng at least a square foot of coated surface, will
iOustntte exceedingly well the general properties of accumulated
electricity.
A. Fix to the outside coating of a jar a curved wire, a. Fig.
435, terminated by a metallic ball, b, and rising to the same
height as the knob of the jar, c. Charge the latter, and hang
midway between b and c, a cork ball, suspended by a piece of silken
thread. The ball will become immediately attracted by c, then
406
FBAKKLISIC ELECTSIOITT.
Tig, 436. repelled, and attracted bj b, again repelled,
and so on, continuing this vibratoiy motion
until the jar is discharged.
B. Insulate a charged electric jar on a
support with a glass leg, and connect the
electric bells (711, G), with its knob. They
will remain at rest, until the outside of
the jar is placed in connexion either with
the earth, or with the chain attached to
the middle bell, when the clappers will be
set in active motion, and wul oontinne
striking the bells until the jar u diacharged.
C. Place some gunpowder on the table of the uniTersal dis-
charger, unscrew the luiobs from the wires ▲, b, and immerse their
points in the powder, at about half an inch from each other. Ccm*
nect the outsiae of the charged jar with the rod A, by means of a
chain, and touch b with its knob, the charge will pass through the
powder, and scatter it in all directions without inflaming it : an
effect probably arising from the disruptive violence with which the
electric discharge occurs, the powder oeing scattered before it has
had time to be ignited.
D. Place some mor^ gunpowder on the table of the discharger,
and arrange the apparatus as before : connect the outside of a
charged jar with a, by means of a piece of thick string toaked m
wateTf instead of a chain ; touch b with the knob of the jar, and
the gunpowder will be instantly inflamed. The action of the wet
string appears to favour the combustion by impeding the velocity
with which the electricity traverses the powder, and thus allowing
time for the production of its calorific effects.
E. Tie some tow looselv over one of the balls of the jointed
discharger (722), and dip it in powdered resin ; place the naked
ball in contact with the outside of a charged jar, and bring the
other in contact with the knob. Discharge will take place, and
the resin will burst into flame ; the combustion being favoured by
the badly-conducting nature of the tow and resin.
F. Place between the knobs of the universal discharger (729),
a thick and dry card, and discharge a jar through it. A perfora-
tion wiU be produced, the card at that point being Imrred outward
in both directions, as though the force producing the perforation
had emanated from the centre of the thickness oi the card in two
opposite directions.
U-. Colour a card with vermilion, unscrew the balls from the
universal discharger, and place the points on opposite sides of the
card, one about half an inch above the other; and dischaiige ajar
through them. The card will be always perforated at the point
opposite to the vrire connected with the negative side of the jar.
A black line of reduced mercury will be found extending from the
point where the positive wire touches the card, to the place of per-
KZAMPLES OF OIMUPTITB DI8CBJLB6B. 407
fbrfttion. This curious effect ia attributed to the greater facility
with which the reduction of the metal takes place during the trans-
ference of electricity, when the Temiilion is exposed to the atmo-
3 there ; for if this experiment he repeated in vtieuo, the perforation
ways takes place at a point intermedicUe between the two wireit,
781. When electricity is accumulated in large jars, or, still
better, in a series of jars constitutine the battery, it is capable of
producing results which simulate the effects of lightmng ; and
mar be considered as bearing the same relation to the area in
which they are exhibited, as the former does to the great theatre
of nature, in which its no less grand, than awful, phenomena are
displayed. The mechanical effects accompanying the discharge
of an electric battery are extremely interesting, but the calorific
phenomena it produces are still more so. In these experiments,
the universal discharger should always be used to apply, and the
qnadrant electrometer to afford a comparative measure ofj the charge
employed.
H. Place a sheet of white paper on the table, and let a fine iron
chain about two feet long, connected with the wires a, b, of the
discharger (677) lie upon it Transmit the charge of six jars,
each presenting about a foot of coated surface, through the chain :
on removing this from the pajper, the outline of the chain will
be observed marked upon it, with a deep stain at each link. The
paper is often burnt through in places, if the charge be sufficiently
powerful.
K. Tie on one end of each rod of the discharger the end of a
piece of fine steel wire,* about four inches long, and allow the
chsj^ of the battery to pass through it. The wire will undergo
combustion, accompanied with a vivid flash of light, being oon-
verted into oxide, which is disnersed in all directions.
L. Place a slip of gold-leaf oetween two pieces of paper, allow-
ing ite ends to project, and press the whole firmly together by
means of the little press of the universal discharger ; let ito rods
A, B, (Fig. 4S4) toucn the projecting portions of the gold-leaf, and
transmit the charge of a battery through the apparatus. On re-
moving the paper firom the press, it will be founa stained of a deep
purple hue from the oxidized gold, the metal being entirely con-
verted into an oxide by the dischai^e.
M. It, instead of using paper, the gold leaf be pressed between
two plates of glass, they will be generally broken to pieces, and
the gold forced into their substance by the explosion. This
arises from the substence of the conductor being too small to sus-
tain the amount of molecular action that constitutes the charge of
the battery.
732. The identity of electricity, from whatever source it may
* The waiek-baIcMoeȤpring wire h bett railed for tlui pnrpoM, that lold
ss No. 82 readily undergoing combattion by s very low cbarge.
408 PRANKUKIG ELEOTBICXTT.
have been derived, maj he demonstrated by a Bomewbat more
complex experiment, by whicb tbe resultB of either of the experi-
ments C, D, or M, as well as tbe chemical action of an ordinaiy
▼oltaic current (765), maj be produced by tbe discharge of the
same battery.
Fig. 436 represents a square wooden stand, to which are at-
j^. 439. tached two binding screws, a, l ;
a screw tipi>ed with platinum,
passes Terticallv through a
raised piece of brass, b, under-
neath tne point of which is a
brass stud, c. Two wires pass
through upright studs, d, b, so
that their points, f, o, may
stand opposite each other on
fhe surface of a raised block, b, k, are two brass clamps, by
which a piece of gold-leaf may be clamped between two pieces oif
glass, ana a metallio connexion maintained, a has a metallic con-
nexion underneath with b, c with b, d with n, and k with l, and
L with one binding<screw of a galvanometer (Ch. XIV.}, the other
binding-screw of which is connected with the outside of the
battery. The galvanometer screws should be furnished with two
wires, the ends of which may be brought sufficiently near to e/tch
other, that any current of such intensity as to injure the instru-
ment may overleap the intervening space. The ends of the wirra.
F, o, are placed at a small distance from each other, and covered
with a little gunpowder. A small bit of card,^ or thick paper,
moistened with a solution of iodide of potassium, is laid on a piece
of platinum foil, which, bein^ placed on the stud o, is lightly
compressed by the platinum point of the screw b.
It the binding-screw a be now connected with a discharger (722)
by a piece of silk thread five or six feet long, moistened with distilled
water, and the charged battery be discharged through this, the cur-
rent will be BO much impeded by the resistance of the wet silk, that
in passing from b tocit will decompose aminute quantity of the salt,
and leave a slight stain of iodine at the point of contact : the cuirent
will then be conducted by the particles of gunpowder between p
and o, and by the gold-leaf through the coil of the galvanometer (of
which the needle will be deflected oy its passage), to the outer coat-
ing of the battery. If the discharger be next connected with a by
a piece of string moistened with salt and water, and the battery
equally charged, be again dischai*eed, the current will, being less
retarded, pass from b to c too rapidly to produce any decomposition ;
it will then pass from p to o with sufficient intensity to inflame the
powder, and after having been transmitted by the gold-leaf from
H to K, it will pass directly from one binding-screw of the galva-
nometer to the other, beinff now sufficiently intense to destroy or
even to invert the magnetism of the galvanometer needles : this
HEAT ASD UOHT EYOLTED BT DI8CHAB0E.
409
tDBtrament may therefore now be remoyed from the circuit..
Lastly, let a be connected with the discharger by a thick copper
wire, and the battery charged and dischar^d as before, the ends
of the wires p, o, having been coyered with a fresh quantity of
gunpowder ; the dischargCi being unimpeded, will now take place
with violence, tiie card oetween b and c will be perforated, the
gunpowder scattered without ignition, and the gold-leaf will be
burnt or oxidized, as in the experiments L and M, in consequence
of the heat excited by the passage of the electric current tbroueh
it. From these experiments it appears, that the nature of the
e£fect8 that a current of electricity is capable of producing depends
alone and entirely on the tntenmty of the current, or, in other
words, on the amount of resistance which it encounters in its pas-
sage, I'rom the inability of some portion of the intervening matter
to transmit freely and rapidly the molecular action of electricity.
733. The facility with which metals are heated by the electric
tfnrrent appears to bear a certain relation to their conducting
powers. As a general rule, the greater the resistance offered by
a metal to the passage of the current, the greater the eyolution of
heat. The following table exhibits the results of the experiments
of Sir W. Snow Harris on this subject.
Metal0.
Heat
OTolved.
B«Birt-
•ooe.
Metals.
Heat
eTolved.
Resist-
ance.
Lead . .
Tin . . .
Iron . . .
Platinum .
72
36
30
30
12
6
5
6
Zinc . . .
Gold. . .
Silver . .
Copper . .
18
9
6
6
3
1-5
1
1
734. The electric discharge is capable of communicating tran-
sient phosphorescent properties to various bodies over which it
passes ; thus su^r, fluor spar, aud carbonate of lime oontinue to
emit a green light for some seconds after the charge has passed
over their surface. This is best seen by placing the bodies between
the ends of the wires of the universal discharger, and passing the
charge of a large jar through them in a dark room. In this
instance some portion of the electrie energy is absorbed by the
medium and converted into optic energy^ or light.
If a glass full of water be allowed to rest on the ends of the
wires, the passage of the discharge under the glass will render
the whole beautifully luminous. The most curious experiment of
this Idnd is made by placing the ends of the wires of the dis-
charger about a quarter of an inch apart, and pressing the end of
the thumb over them. On then discharging a jar through the
wires, the thumb will for an instant appear illuminated with a
real light, as if suddenly rendered transparent. Eggs, fruit, &c.,
may thus be rendered luminous ; but these latter phenomena are
410 PKAHKLIHIC BLECTBIOITT.
not precisely of the same oliaracter aa the fonner ; these simply
show the diiFiised transmission of light similarly generated in the
intervening dielectric, air.
735. The opposite electric states of a charged jar may he beaa>
tifblly demonstrated hy means of the fignres of Leichtenberg. To
show these, make the resinoas cake of an electropbonis (685) dry
and warm : draw lines on it with the knob of a positively charged
jar, and siit over these places a mixture of sulphur and red-lead ;
on inclining the plate, to allow the excess of the powders to fall
off, every hne marked by the knob of the jar will b«s observed
covered with the sulphur, whilst the minium will he dispersed. On
wiping the plate, and drawing figures with the outside of the jar,
the stuphur will he dispersed, and the minium collected in a very
elegant manner on the lines described by the outside of the jar.
The rationale of this experiment is very obvious ; the sulphur
becomes negatively and the red-lead poeitivelv electrified by the
friction to which they are necessarilv exposed, and on allowing
the mixture to fall on surfaces to which one or the other kind of
electricity has been imparted, the sulphur will be collected on the
positive, and the minium on the negative portions of the plate,
according to the ordinary law of elecmc attraction.
736. The fact that the intensity of a chaige has no necessary
relation to the quantity accumulated must never be forgotten in
experiments witn charged surfaces. Thus, let an ounce phial be
coated like an electric jar, and charged in the usual manner, a
vivid although minute spark and distinct snap will accompany its
discharge. If the finger and thumb of one nand be usea to con-
nect the outside and inside coatings, an electric shock will be dis-
tinctly felt Then recharge this phial, and connect its interior
with the kaob of a coated jar holding a quart, and unite their out-
side coatings by means of a wire. Separate the two jars, and it
wiU be found that scarcely the trace of a spark, snap, or shock,
ma 4S7 ^^^^ accompany the discharge of either jar, al-
^' though the actual quantity of electricity must be
) the same as in the fonner experiment. The real
9 change undergone being a diminution of potential
«A^ energy in the charge from its diSusiou over a com-
1^ parativelv large surface.
CXD 737. The unit-jar, contrived by Sir W. S. Harris,
enables us to measure with considerable accuracy
, the comparative quantity of electricity accumulated
in a jar. This consists of a small coated phial, in-
sulated on a glass support, Fig. 437, the charge of
which is assumed as the unit of measufe. A smiJl
coated jar, a, (generally made of a piece of glass
tube,) is inverted on an insulating support ; a wire
furnished with a knob at each end, capable (^
moving through the ball, b, is connected with the
THB C01IDB1I8KB. 411
onteide coating of tlie jar : the wire and hall, c, are connected
with the inside of the jar. Let the outside of the iar, a, be con-
nected with the prime conductor of the machine m action, and
the end of the wire, c, with the knob of a larger jar. It is obvious
that the jar, a, will be positively charged externally, and that the
interior of the larger jar will be charged oulj bv induction through
the glass, until the potential of the prime conductor, and therefore
of D is sufficiently exalted to polarize the mass of air intervening
between j> and c, when a spark will pass between those points,
which will be repeated as soon as the potential of the prime con-
ductor is raised to the same degree, when another portion of
positire electricity enters the larger jar, and so on. Thus, as-
suming the electno potential required to charge and dischaige the
small jar as unity, toe number of luminous discharges occurring
between d and c will be an approximate measure of the elec-
tricity communicated to the larger jar.
738. The Condenser. — In consequence of the action of indue
tion causing the dUguited etate (716) of electricity, very minute
traces of potential electricitv may be detected with facility by
accumulation; instruments designed for this purpose are called
eond&isers. To illustrate their use, touch the prime conductor
of an electric machine in weak action, with a disc of metal fur-
nished with a glass handle, as the cover of the electrophorus (685),
and on bringing it towards the cap of an electroscope, the gold
leaves will be scarcely affected. Then touch the conductor once
more with the disc, holding beneath and parallel to it, at the
distance of about a quarter of an inch, a second disc of metal,
but unhuuUUed. Bemove them in position from the conductor,
and touch the cap of the electroscope with the insulated plate,
quickly remove the other plate, and immediately the gold leaves
will diverge to a considerable distance from each other. In this
experiment, the conductor being weakly charged, the plate of the
electrophorus employed can only remove a portion ot electricity
equal to its own surface, a quantity far too small to act upon the
electroscope. But on repeating the experiment, with a second
plate held parallel to the firs^ induction comes into play, the
electricity which first enters the insulated plate becomes Tot^nf or
dUguisedf a fresh portion enters, and so on, until the plate of air
confined between tne two discs of metal becomes charged (664).
On then separating them, the coercing force is removed, and
the released electricity readily acts on the electroscope. The most
ooQTenient form of tne condenser is furnished by the apparatus
previoariy made use of to illustrate the phenomena of induction
(716). ui order to apply this as a conoenser, remove the cork-
oall electroscopes, connect one of the plates, as a, with a gold-
leaf electroscope (662\ by means of a wire, and let the other
plate communicate witn the earth by means of a piece of chain or
wire ; then bring the two plates as near as possible to each other.
412
FBAHKLINIC ELEGTBICITT.
bnt witboQt allowing them to tonch. Bj means of a wire, or hj
absolttte contact, connect the body of which electricity is to he
examined, for a few seconds, with the plate a, then remove it, and
quickly separate b from ▲ : instantly tne electricity, left free in a,
will canse the gold leaves of the electroscope to diverge. In this
manner, minute traces of potential electricity can be readily
detected.
739. To explain the principle of the condenser, let the plates be
called p, N, the fonner oeing connected with the earth, the latter
being iuKuIated. Now, on placing an electrified body in contact
with N, a certain charge e enters it. This reacts by induction on p,
and the potentials of x and p mutually constraining each other, a
further charge enters n, which is again constrained by a further
induction on p *, and the fresh quantity of electricity in p then
reacts on n as before, ad infinitum.
To determine the measure of the influence of a condenser in in-
creasing a charge of electricity, let the potential imparted to x by
contact with the electrified body be taken as the umt : it will in-
duce on p a potential 0, which must always be less than the unit,
unless absolute contact took place between the plates. The eflfect
of e on X will be to develop oy induction a second charge of elec-
tricity, and hence the quantity in this plate will be equal to e x «,
or «*. The influence of this on p will necessarily set free a
quantity of electricity equal to e' x e, or e*, and so on. Hence the
result of this series of actions will be a converging geometrical
series, since e is less than unity ; and the accumulation of poten-
tial will be
in X,
and in p,
1— c*
c+«*+e"+«' + e' + &c.=
l-e«
740. In the condensers usually made in this country, the unin
sulated plate, p, Fig. 438, is made to move back on a hinge, ai
shown in the figure, when the electricity of the insulated plate, x
has to be examined.
JV-438.
^.489.
f
▲PPUOATIOn OF TRB OOHDBSSEB. 418
As it 18 difficult to place the plates of the condenser as close as
neoesaaiy, without their accidental contact often ensoing, it is usual
to cover their opposed surfaces with a thin layer of resinous varnish,
as a sohition of gum-lac in alcohol. When plates thus prepared are
used, the layer of resin hecomes the charged dielectric^ instead of
the thin plate of air. Thej are then most conveniently arranged
horizontally, as in Fig. 439, and this is the form in which they
are generally used on the Continent.
741. On a principle analogous to that of the condenser, in-
struments have heen devised for the pur]^ose of accumulating
potential electricity hv successive inauction. The earliest on
record is by the JEtev. A. Bennett,* a description of which would
be sunerfluous, as a more complete instrument on the same prin-
ciple has been constructed by Mr. C. F. Yarley, which might be
called a multiplying inductor. It consists of an axis on which
are fixed two opposite rows of equal and parallel segments of
brass a, b, c, &o., a', b', o', &c Of these it will be convenient to
consider the action of one row only. As the axis revolves, these
vanes simultaneously enter two rows of insulated brass shells,
which closely envelop, without any contact, their outer edges, and
their sides nearly up to the axis : let these be called a, 6, e, &c. ;
a', b\ €f, &c. These shells, or envelopes, are thus coupled : —
& to e, <i to e, &c. ; a' to 6', c' to (f , &c., in an alternate series.
The chafge to be multiplied (assumed to be positive) is now com-
municated to a, the axis is then turned by a winch, and at the
moment^ that a is entirely enclosed by a, an earth-connexion is
made with a, at a point near the axis not covered bv the en-
velope, whereby a negative charge, nearly equal to that of a,
is induced on a, and retained by breaking the earth-connexion
before it leaves a. The rotation being continued a enters a', and
oominji; into contact with it, when entirely covered, the whole
negative charge of a is distributed over the outside of a' and b'»
As A again enters a to receive a new inductive charge, b enters
b\ and by means of a similar earth-contact receives a positive
diarge by induction, which it again imparts by contact to % and c,
and this process being continually repeated, it is evident that a
positive coarge may be indefinitely accumulated on the last 6hell
of the series a,' to he limited only by defective insulation.
742. Aided by these condensing instruments we are enabled to
appreciate the disturbance of electric equilibrium in many cases,
in which, without their aid, we should quite fail to do so. The
following are some highly instructive instances of this kind.
A. Detection of EUetricity exeitedby Combustion. — Connect a
delicate gold-leaf electroscope, a. Fig. 440, with one plate, b, of
the condenser, placing the other, c, in communication with the
earth by a cham, d. Select a piece of well-burnt charcoal, b,
• Fhfl. Traaa. 1787.
414
FBAVKLinO BLECTBIOITY.
about four inchei long, and twisting a piece of copper wire firmly
round one end, connect it with the cap of the eleotroBoope. Ignite
the upper end of the charcoal, and keep it brilliantlj burning for
a few seconds by aid of a stream of air from a pair of bellows held
at a distance ; then opicklj remove the uninsulated plate, c, and
the gold leaves will oiverge with negative electricity. (Pouillet.)
B. Slectrieity evolved hy the Beaudion of Salte of Silver, —
Remove the charcoal in the last experiment and replace it by a
capsule of platinum containing a few grains of oxalate or citrate
of bilver. Apply the flame of a spirit-hunp until the capsule is
barely red hot, and then quickly remove it. The silver wOl be
reduced to its metallic state, and on liftinj^^ o£f the plate o, the gold
leaves will separate with negative electricity. (BUttger.)
C. EketricUy evolved by DecompoeUum of Nitrate of Copper,
— ^Place on the cap of the electrometer a few folds of wet bibmons
paper, and place on it a few cnrstals of nitiate of copper wrapped
m a piece of tin-foil pierced full of holes. As soon as the water
penetrates the foil from the paper the tin will be acted upon, the
nitrate of copper reduced, ana red flames will escape with a
copious evolution of heat. The gold leaves will diverge with
negative electricity on removing the uninsulated condensing plate.
(Bottger.)
D. Evohaion of EleetrieUy during the breaking up of Orystaii.
-^ Place in the platinum capsule used in Experiment B, a few
crystals of the double sulphate of potass and copper ; apply the heat
of a spirit-lamp until tney fuse, then remove the lamp. The
melted salt will soon solidify into a solid mass. In a few seconds
this will begin to break up with a loud crackling noise, and on re-
moving the upper condensing plate the electroscope will be found
charged with positive electricity. (Buttger.)
743. When a lai^e jar or battery is discharged by means of
a discharging rod without a glass handle, a slight shock is often
felt bv theperson holding it, although he forms no part of the direct
circuit. Inis arises from what has been termed the latertd ex-
ploeioTif or more appropriately the returning shock, and is owing
to the accumulated electricity occupying some time in passing
415
tbrongh the ccndoatiiig mediom, AlthonKti its rtpiditj ii ei
It therefore aots momsntuilj bj iudaction od th« nibataiice in
cODtAct with the conductor, m the hand, uid Ihni diMarba its
eqnilibrimn ; the restontion of which tak^s place the initaut
the diKhkTEB of the jar !■ completed, producing the ilight ibock
fliperiencedT The Uteral eiplonon i* «iliibit«d in the roUowing
•xperimeDti.
A. Chu-ge a, jar, and place on the t>ble, with one end in cootact
with the (mtmde coating, a pieoe of bru* chain. Diuharge the
jar by meanB of Ihedia^arginerod, and the instant the diichjuve
oGcnra, the chain, altboDgb uotforming any part of the dicoit, will
be illnminated by a ipuit appearilig before each link.
B. Let an ininlatM cofidDCter, q. Pig. 441, be placed aboat
three inchei from tbe end of the prime cwtdnctM', a, of an electric
macbiDe. A condnctor, d,
connected with the earth by ^- **i-
means of ■ cbun, is placed
■bont a qnartcT of an inch jj f
from o. Then i, being pon-
tively electrified, acts iiutno-
tivel}- on c ; and the indaced
chai^ of c acta on i>, utd
polansing tbe tbin stntlom of
•ir between c and d, a apark
D, on □, BO that c is left in a negative itale. On dis-
y Faraday,* admirably illnt-
kop of copper wire, abo, is insnlsted by nu-
penooD, haTing a Wl at One end, 0, the other
end, 1, being in coniieiion with the earth. Two
portions of the wire near the ends are brought
•ufGciently near to each other at e, that when
» charged Leyden jar is discharged
of the wire adjacent to B be now connected by
attached wires, f, a, with the inner and onler
CAatinnof a fnll'Sised Leydenjar. The poten-
tial of the charge will be so moified by tlw
Indnctive action of these coating that it will no longer be able to
orercome tbe reeiatacce of the inlervening air at b ; which epace,
when that potential is nndiminiahed, oSers less resistance to its
puBtge than the entire circait of the wire.
• Lntnm at Hay Instil. Ju. 1)
416 FBAXKIilinC BLVCTRICITT.
746. Hitherto, it has been presumed that positive and negative
electricity possess the same properties with regard to conductioD
and insulation ; differing only in the appearance of their luminooa
discharge ; the one being accompanied 07 a star, and the other bj
a pencil of light (703). A remarkable circumstance has been
observed, which tends to indicate the probability of the existence
of some more important difference between them, instanced in
certain bodies being capable of conducting one kind, and insulat-
ing the other, when they are in a state of ^stremely toeaib potential
energy. These bodies are termed «mjN>2ar; among them the
flames of alcohol, coal-gas, and sulphur appear to conduct podtiva
electricity, whilst the name of phosphorus, dry albumen, ivory, and
dry soap, conduct negative electricity. Of an approach to this
curious class of bodies we have an instance in atmospheric air,
which would appear to allow the discharge of positive, to take place
more readily than that of negative, electricity (730, G), although
Prof. Belli has stated the contrarv to be the fact*
746. Conductors, and nonconductors, pass into each other bj
insensible grades, and indeed rather di£fer from each other, in one
insulating better or worse than another, as they all offer more or
less opposition to induction and resulting dischanpe taking plaoe
through them ; and at length, such a point of inmfference to the
discharge of electricity is met with, that bodies are known which
allow discharge to take place through them in one direction, and
prevent it in another, as in the so-called unipolar bodies discovered
oy Ermann. Many non-oonductors insulate when cold, and con-
duct when heated red-hot, as glass. Others do not acquire their
conducting power until they are fused, as in the case of resinons
electrics, which allow discharge to take place through them when
they are fused, a circumstance first, it is believed, mentioned by
Cavallo,t and shown to hold good even with electric currents of
weak tension, by the elaborate researches of Faraday.
747. The atmospheric medium, by which we are surrounded,
contains, like eveiy other form of matter, a considerable quantity
of potential electricity ; sometimes of one kind, sometimes of the
other ; but as a general rule it is always of an opposite kind to that
of the earth. Different layers, or strata of the atmosphere, placed
only at small distances from each other, are frequently found to be
in opposite electric states.
various kinds of apparatus have been contrived to facilitate an
examination of the electric state of the atmosphere. These consist
in general of poles elevated about thirty feet into the air, provided
with a metallic point at their upper, and insulated at their lower
ends. The most perfect mode of insulation hitherto devised con-
sists in attaching the atmospheric wire to the apex of a hollow
cone of glass, under the cavity of which a small lamp is kept con-
* PofTgendorif, Annalen, t. xl. p. 73.
t Treatise oo Blectrioilj, p. 906. London, 1777.
BLECTUCITV. 4 1 7
■buitly burnintc; b; thii the temperature of the glaag tupport is
maintalDed Bufficiently ubare that of the snironading atmoephere,
to preveat the depoaitlon of moisture, and consequent escape of
electricity, orer the surface of the glass. The electric bells
(711, C), have been suspended from a coitdiictcr in contact with
such apparatus, bo that by their ringing, they may indicate the
presence of electricity iu the conductor.* A long Sshinc-rod,
raised aboTB ibs highest part of the housa, and proiided with au
insulated conducting wire, furnishes a very conTenient apparatus
for occamonal obseryations.f Tlie apparatus used by Saussure in
his researches whs merely a well.insulaled electroscope, provided
itith a poirited conducting wire about three feet in length, to take
up the electricity from the atmospbere.
For the purpose of aacertainiog the kind of "" ""
atmosphenc electricity, an instrument called
a dittinffaitking electroscope, fig. 443, ban
been employed : this consists of a small tubu
lar Lejnlen jar inserted in the neck of a gold-
leaf eleclroBcope, the inner coating of which
is connecled with the wire carrying the gold
leaves. This is doily charged with negative
electricity, and if carefully iosulaled with shcll-
lac varnish, will retain the charge with little
diminution for twenty-four hours. The leaves.
conae<|uenlly, are constantly divergent with
negative electricity, and their increased or
diminished- divergence will show (679) the
kind of electricity, if any, present in the atmosphere.
Tbe same result may on oblainoil by means of the podtivs and
negative eleclroiKopea of Dr. Eadclifle ((j(j3), and more conve-
niently, if the poteudal be very feeble.
746. By means of some of theBo apparatus, a knowledge of the
electric state of thosii portions uf the atmosphere nearest the earth «
may readily be obtained. Iu clear weather, indications of positive
electricity are always to bo met with in the atmotphere ; this is
weak before sunrise, becoming stronger as tho sun passes the
horizon, and soon afterwards gains its greatest stale of iutensit; ;
it then rapidly dimini-hes, and regains i[8 mininiiini state some
bouTB before sunset, alter wbich it once more iucrcasex. and gains
its second moxintum state ; and then decreases until the following
morning, t
M. Schnbler of Stnttgard, to whom tbe above observations are
due, has remarked that the atmospheric electricity increikses from
July to January, and then decreases. It is also much more in-
tense in winter than in summer, and appears to increase as the
t Canllo, p. Sao.
418 PKAVKLIHIG BLBCnUClTT.
cold increases. According to the obsenrations of M. Qaetelet,
atmospheric electricity exhibits a nuunmum intensity in Jannary,
and a minimum in June.
From a series of upwards of 10,000 observations made at the
Kew Obserratory in the years 1844-8,* it appears that negadve
electricity was observed only once in 81*4 times. In summer, the
hours of maximum intensity appeared to be 10 ▲.!!. and 10 p.m. ;
and of minimum intensity 2 a.m. and noon. In winter, the
mcucima were sit 10 a m. and 8 p.m., and the minima at 4 a.m. and
4 P.M. The greatest potential was observed in the most humid
months. The manifestations of negative electricity were usually
accompanied either by rain, frequently heavy, or by the occur-
rence of clouds of the cirrfhstreUuB, or eumuli-HrcUus variety in
the zenith.
749. Among the causes modifying the electric condition of the
atmosphere must be ranked its hymmetric state, as well as, pro-
bably, the nature of the effluvia which may be volatilized in anj
given locality. Thus, Sanssure has observed that its intensity is
much more considerable in elevated and isolated places, than in
narrow and confined situations; it is nearly absent in houses,
under lofty trees, in narrow courts and alleys, and in indoeed
places. In crowded cities it is most intense in the squares, and
upon the bridges. In some places the most intensely electric
state of the atmosphere appears to be that, in which large clouds,
or dense fogs, are suspended in the air at short distances above
the surface of the earth ; these appear to act as bodies charged in-
ductively by the earth.
750. Cavallo, from a set of experiments performed at Islington
in 1776, ascertained that the air always manifests po$iHve elec-
tricity, except when influenced by heavy clouds near the zenith.
This electricity he found to be strongest in fogs and during frosty
weather, being weakest in hot weather, and just previous to a
shower of rain ; and to increase in proportion as the instrument
used in its investigation is raised to a greater elevation. This
indeed necesoarily happens, for the earth's surface is always nega-
tively electrified, and acting inductively on the atmosphere, no
positive electricity can be detected withm four feet of the soifaoe
of the earth.
Mr. Crosse, of Bromfield, collected and examined the atmo-
spheric electricity by means of wires, insulated and supported by
poles and by the trees in his park. When these conductors were
about one-third of a mile in length, he frequently succeeded in
collecting sufficient electricity, to charge and discharge a battery
of fifty jars, containing seventy-thrae square feet of coated surface,
twenty times in a minute, accompanied by reports as loud as those
of a cannon.f
• Bep. Brit. Aaioe. 1850.
t Sturgeon's Jovirnal. Tol. i. p. 139.
ATlfOSPHBBIO BLIOTRiaTT. 419
751. The first satisCM^toiy attempt to oolleot the electricity of
the upper regions of the air, was made by Dr. Franklin in North
Amenca, in 1752, although it must be observed that a short time
previously, Dalibard, in France, had, by means of a lon^-pointed
conductor, raised in Mary-la-Ville, succeeded in obtaining vivid
sparks of atmoepheric electricity. Dr. Franklin raised into the
atmosphere a kite, formed by stretching a silk handkerchief across
two rods of light wood, and with thb, when the string had been
rendered sofficiently moist by the falling rain to conduct elec-
tricity, he obtained a copious succession of sparks, from a key,
fiutened to the end of the string.
Subsequently, M. Romas, in France, h^ increasing the length
of the string, obtained flashes of electric light from his apparatus,
ten feet in leneth, accompanied by a report as load as that of a*
pistol. Sbort^ afkerwaros Professor liichman, of St. Petersburg,'
was struck dead by a discharge from an apparatus, similar to that -
of M. Dalibard, with which he was experimenting.
Cayallo, in 1777, raised an electric kite repeatedly in the neigh-
bourhood of London, and obtained an enormous quantity of elec-
tricity; he found that the electricity frequently changed its
character, as the kite passed through different aerial layers, or
strata.
752. Perhaps the most in^nious mode of investigating the
electric state of the upper regions, is by means of the apparatus
used by MM. Becquerel and Breschet, on the Great St. Bernard.*
These physicists placed one end of a cord,
coverea with tinsel, about ninety yards in ^*
length, on the cap of an electroscope, and, /
tying the other to an arrow, fig. 444, they
projected it, with the aid of a bow, into the
air, and they found that the -gold leaves
divereed in proportion as the arrow ascended
into the atmosphere. In this experiment,
the effect would probably have been aug-
mented by surrounding the head of the
arrow with a small bit of cotton- wool mois-
tened with alcohol, and igniting it, previous to projection.
753. The existence of tree electricity in the air has been referred
to various sources; the phenomena of animal and vegetable life,
as well as chemical action, haye been called in to explain its
origin. Among others, the evaporation of water, and other fluids,
constantly taking place on the earth's surface, may certainly be
regarded as one of the sources of atmospheric electricity. The
eyolution of electricity by evaporation, may be readily proved by
placing on the cap of a ^old-leaf electroscope a small metallic cup
containing water, in which some common salt has been disBolvcJ.
• Traits de rSIeotricit^ et da Magn^tisme, t. iv. p. 110.
E K 2
420 FBAKKUiriO ELECTRICITT.
On dropping into it a piece of hot cinder, the vapour will ariae
copiously and carry off positive electricity, leaving the cup nega-
tively electrified, with which electricity the gold-leaves will di-
verge. If water, containing a minute portion of an acid, be
substituted for the weak brine, the reverse will occur, the gold-
leaves diverging with positive electricity, and the vapour being
negatively electrified.
The potential electricity evolved in these experiments has been
ascribea by Faraday to friction ; but it appears more probable that
the true cause is evaporation.
754. The clouds, consisting of immense masses of aqueona
vapour, are tolerably good conductors of electricity, and conae-
quently manifest a considerable potential energy. There can be
but little doubt that a cloud consists of an aggregation of minute
vesicles of aqueons vapour. These when similarly electrified do
not repel each other, and fly apart, unless they are quite beyond
the inauctive influence of the earth, or of any nearer body. Thus,
a glass feather fixed in one of the holes of the prime conductor of
an electric machine will appear animated, all the fibres mutually
repelling each other. But if the hand, or a large brass ball be
held near it, then the fibres will fall together, and losing their
appearance of repulsion, will bend towards the hand or ball under
the inductive, and consequently attractive influence, exerted by it.
765. Two clouds, being in different electric states, act upon each
other through the particles of the intervening dielectric, the air,
like the inductive surfaces, or metallic coating of a charged jar ;
and when sulficiently near to each other, dtaeharge occurs, pro-
ducing the vivid flash well-known as lightning, generally accom-
paniea by the loud reverberating sound of thunder. When, on the
other hand, induction takes place through the air, between an
electrified cloud and the earth, an explosion or disdtarge ensaes,
when the intervening particles of the dielectric are so arranged as
to admit of its occurring ; producing a second, and much dreaded
form of lightning. This mode of establishing an equilibrium be-
tween the oppositely electrified bodies, often ensues through the
medium of the nearest prominent conductor, which, if a tree, is
often riven in sunder ; if a building, is frequently dashed in pieces ;
and if an animal, severely injured, or even killed. These natural
events present examples of the commutability of physical forces ;
when the intensely exalted potential manifested in a flash of
lightning meets with a good conductor of sufficient magnitude,
that is a body havnug sufficient capacity to receive and trans-
mit the force, it is transmitted harmlessly to the earth : but when
that force is impressed on bad conductors, that is, on bodies not
sufficiently capable of receiving and transmitting it, as the atmo-
sphere, and non-metallic bodies on the earth's surtace, a portion of
tnat force is converted into, or is manifested in the form of, light,
heat, or dynamical force ; and thus the gazing rustic is killed or
LIGRTXIXO CONDUCTOBS.
421
JF^.MS.
blinded, the stack is fired, the oak is rent, or the towor is shivered
to pieces.
756. Several instances have occurred of the fatal efiects of a
tempest having been exerted on animals at a considerable dis-
tance from the spot where the most serioas results have taken
place, and where the violence of the storm appeared to have been
chiefly^ exerted. This will readily admit of explanation, on the
supposition of a lateral explosion or returning i^ock (743) having
occurred. Thus, if a b be a large
cloud, positivelv electrified, ap-
proaching at its end, a, Fig.
445, within striking distance of
the church-steeple, o, the ex-
tremity, B, will, oyits inductive
action, excite and retain a large
amount of potential energy in
any object at d, as a traveller,
for example, leaving him in a
negative state. When a has
approached sufficiently near to c, an explosion will occur, and
electric equilibrium will ensue, b being thus left unelectrified, no
longer exerts a coercing force on the electricity in d, which now
rushes back with violence to the earth, producing discharqe^ and
a restoration of electric equilibrium, witn such mechanical force,
however, as to even kill the unfortunate individual situated at d \*
an event which is stated to have actually occurred.
757. Science, and mankind generally, must ever remain debtors
to the ingenuity of Dr. Franklin, for proposing at least a partial
protection against these dreaded effects of the tempest, in the in-
vention of the j^aJtonnerrea, or lightning-conductors. These
consist of metallic conductors, of sufficient thickness, usually fixed
against the side of the building they are destined to protect ; their
upper extremities extending some feet above it, ana terminating
in a point, which is best constructed of some metal not liable
to oxidation : the lower end is buried in the earth, to the depth
of a few feet. For ships, ^flexible paratonnerres, composed of
copper chain, or slips of that metal, are fixed to the masts, and
extend from their highest points to the outside of the keel of the
vessel, so as to conduct the electricity harmlessly to the water in
which the vessel floats. Whatever form is used, one general pre-
caution is necessary, that aU and every portion of the paraxon-
nerre should be as perfectly continttous as possible^ for wherever a
break or interruption occurs, the potential energy in being trans-
ferred from one portion to another, is liable to produce the very
danger which these instruments are intended to avert.
* See Tnit4 El^mentaire de Fhysiqixe, par M. 1* Abb^ Hauy, p. 434. Paris,
1806. >
422
FR1.HKL1NIC ELECTRICrrr.
758. To illustrate some of these positions, the thunder-honse,
as it is termed, was inTented
^•^' by Dr. Franklin, ab, Fig.
446, is a piece of hard drf
wood, cut into the shape w
the gable end of a honse,
with a brass rod, terminat-
ing in a ball at c, fixed
against its side, and termi-
nating at D in a hook. At ■
this coqductor is intermpted
by a sauare block of wood,
fitting loosely into a cavity
_ made to receive it, having a
J wire fixed across it ; so that
when B is fitted in its place,
as in the figure, the conductor, o d, is perfect ; but when placed
in the opposite direction, as shown by the dotted line, the para-
tonnerre, c d, is interrupted in its centre.
£xp. A. Charge the jar f ; connect its outside with the hook
at the end of d, and its knob with the pointed wire supported on
its inndatinq stand, h, and bearing on its apex the brass rod k,
terminating in balls, and moving on it in any direction, as on a
pivot. Phice the window e in its place, so that the brass conductor
may be continuous, and cause k to revolve, so that one of the balls
terminating it may pass within half an inch of c. The jar will
be discharged, and the window b remain unmoved.
B. Repeat the last experiment, with the window b placed so
that its wire may be at nght angles to the direction of the wire
CD. On discharging the jar as before (A), the effects of the ex-
plosion will be exerted on b ; and it will be projected with violence
from the cavity into which it fits.
C. Let the apparatus be arranged as in (B), remove the knob
on c, and leave the paratonnerre pointed : on allowing k to re-
volve, the jar will be tUently dischnrged. The electric current
during this gradual discharge by the point, never acquiring suf-
ficient tension to act energetically on s, although it was displaced
with violence, when D terminated in a knob.
D. The protecting infiuence of pointed conductors is more
strikingly shown by the electrical toy, called the powder roaga-
xine, in which the interrupted portion of the conductor reposes in
a mass of gunpowder, placed in a wooden model of a house. If
the jar be dincharged whilst the paratonnerre terminates in a
Soint, the powder is unaffected ; but if a knob be screwed on, the
ischarge explodes the powder, and blows the model to pieces. In
repeating this experiment, a piece of wet string phould be used
to connect the jar with the baiBe of the paratonnerre, for reasons
already mentioned (780, D).
AURORA, AND M LIBORS. 423
759. When lightnme strikes a sandj soil with sofficient force,
it often penetrates to the depth of seyeral feet, fonoing a kind of
tube, known as afuk/uritey by the fusion of the a4J<^<^ii^ calcareoas
and sificeoQS particles ; and which, in almost every instance, has
been found to terminate in a subterranean reservoir of water.
The lambent lightning ho common in the sultry autumnal
evenings is unattended with the sound of explosion, and often
a,pjpe&rs in the most opposite regions of the sky. It has been in
many cases traced to the restoration of electric equilibrium dis-
turbed by storms actually below the horizon.
760. The well-known meteoric appearances so frequent on the
Sointed masts of shipping, known as Castor and Pollux, the feu
e St. Elm of the French, and Elmsfeuer of the Germans, appear
to depend on the gradual discharge of atmospheric electricity by
thepointed masts of the vessel.
The beautiful aurora borealis, so frequent in the north of
Europe, and of late years not of unfrequent occurrence in the
neighbourhood of the metropolis, depends, in all probability, on
the passage of electricity through a highly rarefied meaium.
From the calculations of Mr. Cavendish, it is probable that the
aurora usually appears at an elevation of about seventy-one
English miles above the earth's surface ; at which elevation the
density of the atmosphere must be but the ttt^^ P^^ ^^ ^^^^ ^^
the earth's surface, a degree of rarefaction far above that or-
dinarily afforded by good air-pumps. As electricity is difiiised
in a quantity nearly proportionate to the elevation above the
earth's surface, it appears very probable that, under favourable
circumstances, an inductive transmission of electricity would ap-
pear luminous to us, in the vast regions of rarefied air terminating
oar atmosphere, in a manner analogous to that in which it appears
on an infinitely smaller scale in an air-pump vacuum. When the
discharge of a large jar is e£fected through a tube filled with rarefied
air, it appears limiinous, not in flashes, like the artificial aurora
<706), but in a condensed form, like a beill of fire, falling through
the tube ; resembling in appearance another class of meteors known
Asftdling or shooting stars.
424
\
\ CHAPTER XIII.
VOLTAIC ELECTRICITY.
761. It has been already mentioned, tbat even two plates of
glass when pressed* together, and suddenly separated, assume op-
posite electnc states : but the same thine occurs more manifestly
when two discs of difi'ereut metals are similarly treated. To de-
monstrate this, tak^ a plate of copper and one of zinc, each about
four inches in diameter, and furnished with a glass handle in its
centre; connect a gold-leaf electroscope with the plate n of the
condenser (738), allowing p to be connected with the earth. Press
the copper and zinc plates together, holding them by their insu*
lating bandies ; suddenly separate them, and apply one to the
plate N of the condenser ; again press them togetlier, having pre-
viouslv touched them with the finger to restore their electric
equilibrium, and re-apply the same plate to the condonser. Re-
peat this about six times, then draw back the uninsulated plate
p, and the gold leaves of the electroscope will diverge with poti-
tive electricity if the zinc, and with negative^ if the copper plate
has been applied to the condenser.
762. The development of positive electricity in the zinc, and of
negative electricity in the copper plate, was attributed by the
illustrious discoverer of the fact, Prof. Volta of Pavia, to a pecu-
liar electromotive force, under which metals, by simple contact,
tend to assume opposite electric states. This theory has now but
few supporters, in consequence of the mass of evidence that has
been opposed to it by Fabroni, Do la Hive, and our illustiious
countryman, Faradav, to whom this branch of science is so largely
indebted. Tliese philosophers have very satisfactorily proved, that
whenever electricity is developed during metallic contact, it is
owing to some chemical action undergone by the more readily
oxidizable metaL So rigorously has this been demonstrated, that
it may be stated as a general law, thcU no cJtemteal action oceurSf
unaccompanied hy disturbance of electric equiUbriumt and eon-
eequewt development of. potential eUctridty^ although it is fully
possible for such to occur without our being able to detect it ; for
unless the electricity evolved is in sufficient quantity to circulate
as a current, or of sufficient tension to be collected by a condensing
plate, and to act on the leaves of an electroscope, it may escape
the evidence of our senses.
EI.£CTBO-P08inTB AND HEOATITB BUSHEKTS.
425
763. In every chemical combination, whether saline, haloid, or
of still more complex nature, the force by which the elements,
both proximate and ultimate, are held together, appears to bear a
dose relation to their electric state, and their separation is ^eno-
ntlly accompanied by the evolution of a weak current of electncit j.
So genera] is this fact, that the discoveries of Faraday have cer-
tainly very clearly pointed out the probability of chemical affinity be-
ing merefy a modification of electric attraction ; an opinion previ-
ouslv adopted, with some limitation, by Davy, Berzelius, and others,
not less deservedly celebrated in this branch of experimental science^
764. Among the ultimate elements with which chemistry has
made us acquainted, there are twenty-four which are characterized
by their electro-negative, and thirty-nine by their electro-positive
state in relation to each other : with these are given their chemical
symbols, some of which are derived from their Latin names.
I. ELBCTBO-NEOATrVB ElBMEHTS.
Oxygen 0
H^'drogen...H
Nitrogen ...N
Sulphur S
Phosphorus..?
Chlorine .,.Cl
Bromine ...Br
Iodine I
Fluorine ...F
Carbon C
Boron B
Silicon Si
Selenium ...Se
Arsenic ...A«
Chromium ...Cr
MoIybdenumM
Tuuj^sten ...W
Antimony ...S5
Tellurium..Te
Titanium ..Tt
Tantalium To
Vanadium. V
Niobium. ..Nt
Pelopium ..Pe
n. Electbo-positivb Elements.
Gold An
Platinum. .Pt
Iridium ...Ir
RutheniumBtt
Osmium ...0«
PalUdium..P(i
Rhodium .. JRA
Silver A^
Meroui7...H^
Copper ...Ctt
Uranium ...U
Bismuth ...Bi
Tin 8»
Lead P6
Thallium...TA
Cadmiom...CJ
Zinc Z
Nickel Nik
Cobalt Co
Iron F0
Manganese Mn
Lantanium La
Cerium Ce
Didymium D
Zirconium 2^
Yttrium ...Y
CsBsium ...Co;
Rubidium... Ri
Erbium E
Terbium ...Tr
Glucinium G
Aluminium Al
MagnesiumM^
Calcium ...Ca
Strontium.. Sr
Barium ...Ba
Lithium ...L
Sodium ...Na
Potassium K
These substances are, it must be remembered, negative or posi-
tive only in relation to each other, and their mutual chemical
afiinities appear to be in the ratio of the intensity of the diflference
of their comparative electric states. Thus potassium has a greater
affinity for oxygen than any other substance in nature, and ao-
cordinely we find that, whilst the former is in its combinations
powerrally positive, the latter is as energetically negative. In
426 VOLTAIC ELECTRICITY.
the list of uegative bodies every element is to be remrded as
negative to all below, and positiye to all above it in the list ; tbns
hydrogen is negative with regard to nitrogen, bot positive with
regara to oxygen : a similar observation applies to the list of
electro-positive elements. The electrical relations of many of
these bodies must not, however, be regarded as absolutely correct
in this arrangement ; many of them being arranged from their
chemical analogies only.
^ 765. Let a piece of zinc be amalgiamated by immersing it in a
little dilute Bulp>horic acid, and rubbing a few globules of mercury
over it with a piece of cork. Fill a ^ass with a mixture of one
part of hydrochloric acid and six of water, and place the amal-
gamated zinc in it. The brilliant surface of the zinc almost im-
mediately assumes a greyish tint from its becoming covered with
myriads of excessively minute bubbles of gas. These consist of
hydrogen, arising from the decomposition of the acid, its chlorine
uniting with the zinc, and the hydro^n, for which the metal has
no affinity, mechanically adheres to its surface, and thus by a
gaseous covering shields it from the further action of the ncid.
Then immerse in the fluid a rod of any metal standing above zinc
in the list (764), as a piece of copper or silver : no obvious action
will occur until it touches the surface of the zinc, when in an in-
stant a torrent of bubbles of gas is evolved from the copper, as
though it were undeming solution, no evolution of gas from the
sine taking place. Tne copper, however, remains chemicallv un-
acted upon, and the zinc is alone dissolved^ and consequently
mere chemistry is incapable of affording a satisfactory solution to
the curious phenomena just described. From the facts already
stated, it is seen that the copper and zinc, being placed in contact,
assume opposite electric states, from the chemical action of the
fluid on the more oxidizable metal.
The origin of the action itself must be referred to an exalted
attraction of the zinc for the chlorine, which becomes at last so
intense as to enable it to take the latter irom the hydrogen with
which it was previously combined. But the hydrogen is evolved
at a distant part of the fluid, viz. from the surface of the copper,
which may be even several feet from the zinc plate, and the inter-
mediate portion of fluid undergoes no visible cnange of any kind
during this transfer of hydrogen from the zinc to the copper plate.
This IS explained hy the fact that at the moment the atom of
hydrochloric acid is decomposed at the zinc surface, and the chlo-
rine combined with the latter, a current of positive electricity
leaves the zinc, and by a kind of convective force carries with it
the atom of hydrogen which was deserted by the chlorine. This
atom, instead of being itself carried onwards, decomposes the first
atom of hydrochloric acid in its path, uniting with the chlorine ;
this second atom of hjdro^n still uiged onwards by the convec-
tive force of the current in its turn seizes the chlorine of another
HATUSS OF A TOLTAIO KLEWSITT.
Ail
427
atom of hydrochloric acid, caiuiiig its hydrogen to he eyolyed, and
this action continues nntil the electric cnirent reaches the copper
plate where it leaves the last atom of hydrogen, which, hecoming
paasiTe, is here set free. As these changes occur instantaneously
and inTisibljr, they altogether escape detoction by our senses. The
following diasram, Fig. 447, will perhaps render these changes
more intolligible, in which c is the copper, and s the zinc plate,
^.447.
^.418.
connected by a wire, d ; and h and cA, respectively represent the
atoms of hydrogen, and chlorine, the arrow showing the direction
of the positive current through the fluid.
766. The metals employed need not be in actoal contact in the
flaid, for if connected by a conductor out of the fluid, the effects
above described take place. This conductor may be a wire, as in
fig. 447, or be constituted by the plates themselves, by so iDclin-
ing them that they may lean against
each other as in rig. 448, where c is
the copper and z the zinc plate. The
arrows represent the direction of the
electric current /rom the positive to-
werrdi the negative element voithin
the cell, and in the contrary direction,
toiihoiU it.
The pointe at which the active
metallic plates are connected with
any conducting matter are called jx)2m,
or eUetrodee; and as the current
peases externally from the copper to
the zinc plate, a piece of metial at-
tached to the copper plate is the
pomtive electrode: and uiother attached to the zinc plate, the
negOtioe electrode : of this fact, the existence of the letter (p] in
copper, and of (n) in zinc, will serve as a " memoria technica."
It must also be remembered that the positive ^ate is connected
with the nM;ative polCt and vice vers&.
That such a current really existe will presently be shown to be
beyond a doubt. As a tolerably satisfactory proof, however, the
well known calorific effecte of electricity may be observed bv sepa-
reting the plates c, z, at the upper pairt and connecting them by
428 VOLTAIC ELECnUCITT.
a piece of very fine pintinnm wire, half an inch in length. This,
if the plates be about four inches long and two broad, will become
brilliantly ignited, from the resistance presented to the electric
current passing through it, so long as chemical action continues.
In repeating these experiments, onlinarj rolled sine may be sub-
stituted for the amalgamated metal, but the phenomena described
will be masked by chemical action ensuing at the zinc surface,
independently of that due to the current : this is usually termed
local action.
767. The electricity thus evolved, although weak in intensity,
is considerable in quantity ; and for many important experiments
a pair of zinc and copper plates, excited by dilute sulphuric acid,
constitute a valuable source of electricity. These eusctroTnotort
as they are termed, are readily made by placing a piece of sheet
copi)er, a foot long, and six inches wide, having a copper wire for
an electrode, soldered to it, in the inside of an earthen jar ; a
piece of sheet zinc, nine inches long and six wide, furnished with
a similar electrode, is bent into a cylindrical form, amalgamated
(765), and covered loosely with a fold of linen, so that, when
placed in a jar, metallic contact between it and the copper may
be prevented. The jar being nearly filled with dilute sulphuric
acid, containing one part of acid to six or eight of water, the
Slates are immersed, and the current of electricity evolved is
irected by the electrodes to any point the onerator pleases.
76tt. In all cases in which electricity is evolved by the chemical
action of a fluid on one of two metals in metallic connexion, and ex-
posed to its influence, it is necessary, as already stated, that one
of the metals should be more oxidizable than the other, or, in
other words, more positive in its electric relations. We may thus
conveniently separate the metallic elements of a voltaic circle into
a generating^ and a conducting plate ; the former being alone
active in determining the evolution of electricity, the latter acting
chiefly as a BUi*face on which the convective energy of the current
may be impressed. Unless the fluid in which the metals are im-
mersed, is decomposable by an electric current, it has not the
power of exciting one ; hence it must be a compound, consisting
of at least two elements. Thus, water acidulated by any of the
mineral acids, or in which an alkaline salt is dissolved, is power-
fully active in these circumstances in evolving an electric current.
A second precisely similar zinc plate can never act as a conduct-
ing plate, necause it will itself teml to generate an equal current
whicn will oppose the first in direction.
769. If, instead of immersing the zinc and copper plates in
dilute acid (765), they had been placed in water only, chemical
action and the evolution of electricity would have ensued, but
with much less energy : electricity of very small potential being
evolved, in consequence of the very low intensity of the chemical
action of water on the zinc. In a solution of common salt, the
BBLATXYK BBS OF POSITIYE AND KXOATIVE PLATES. 429
effects are more obvious^ the chloride of sodium being decomposed,
and chloride of zinc fonned, the chlorine being the negative, and
Bodinm the positive element (764); and electricity is evolved
from the decomposition of the salt, in the same manner as it was
from that of the water, b^ hydrochloric acid.
The quantity of electricity evolved increases with the snrface
exported to the chemical action of the fluid in which it is im-
mersed ; and hence gigantic plates have been constructed for the
puipose of obtaining an inmiense quantity of electricity. Mr. Pepys
Dad an electromotor made for the London Institution, consisting
of a copper and zinc plate, each fifty feet long, and two feet wide,
rolled into a coil, witn horse-hair ropes between them to prevent
their tooclung each other. About fifty gallons of dilute acid were
required to act upon these plates, and the amount of electricity
evolved was truly immense.
770. Having ascertained that the excitation of electricity bears
a direct relation to the amount of chemical action exerted on the
more positive metal employed, and that it increases with the ex-
tent of surface acted upon, the evolution of electricity may be
increased to a considerable amount by a proper arrangement of
apparatus. It is found from experiment that a considerable ad-
vantage is eained by causing the conducting or negative element
to surround the generating or positive, so as to present a surface
opposed to both or all sides of the latter, a fact depending^in all pro-
bability upon the greater extent of the conducting surface ensuring
the whole of the evulved electricity assuming the form of a cur-
rent. For it is fully possible for an enormous quantity of elec-
tricity to be excited, and yet but little to appear m the form of a
current, either from excessive local action (766), or from a defective
arrangement of apparatus. On this account, in well-constructed
electromotors, the zinc or exciting element is generally placed in
the centra with regard to the copper.
It has been stated that an increase in the quantity of the
evolved electricity ensues when either the zinc or copper exceed
each other in size, and that the quantity of excited electricity is
a minimimi when the metals expose an equal extent of surface.
If the fine plate be the lai*gest, the maximum efiect is said to be
obtained when it is seven times larger than the copper ; and if the
latter be the largest plate, that the maximum evolution of elec-
tricity occurs when it is sixteen times latter than the zinc plate.
In the former case the quantity of electricity is three, and in the
latter four and a half times greater than when the plates of copper
and zinc are of e(^ual size. Tlie late Prof. Dauiell, however,
proved that if the diameter of the mean section of the active fluid
remains the same, and all interfering causes arising from depohition
on the conducting plate be removed^ it mattera but little, so &r as
the resulting current is concerned, whether the generating or the
conducting element be of laiger dimensions.
430 VOL'
771. Smt^i Battery. — A coDTsnieot and oerUinlj^ poweiAil
smngement Lai been proposed bj Ur. A. Smee, coniiitiiig of two
pUtM of anulgoTUftted (765} nnc, ei, Fis. 449, clumped to > piece
of wood, B, bj mflaiiB of ft bent piece of brau, c,
'«■ **"- liimitbed with a binding screw ftt i.. Between
\*. the pUtes oF dnc is fixed > thin plate of uItot
I connected at its upper end witb snotber binding
I screw. This plato ofBilverii coTpred witbathin
lajer of platinum, \iy immersing it Ibr a sbort
time in a soiution of cbloride of plAtinnm whilst
'■ connected with the negati-re eloctrodo (766) of
B TOititio batterj. The platinum is depodted oo
the plate in tbe form of a fine powder, and from
the myriads of conducting points thus formed b;
tbe inconceivably minute particles of reduced metal, the evolutiuii
of the hydrogen goi is greatlj facilitated. An arraiiEement of
this lei nd, placed in a pint jar of dilate salpburic aci^ becomes
an excellent and efficient soarce of electridtr-
772. In tbe arrangements of apparatas abore described, a con-
siderable loss of electricitj occurs during; the evolution of the
hydrogen. To prevent this, certaio means nave been had recoorae
to, for the purpose of absorbing the bydrc^en, bj employing it to
reduce metallic oiides, or by combining with the oijgen of any
highly oxidized fluid, as nitric acid. If the plates of sine and
copper, instead of being acted upon by a dilute acid, be immereed
in a solution of sulphate of copper, cbemiual decompoaitinn and
consequant evolotion of electricity will occur. No gas is in this
case evolved, as the snlphate of copper is alone decomposed ; the
sulphuric acid and oxygen acting on the lino forming the sulphate
of that metai, which ii dissolved by the water ; the copper being
deposited, in a metallic stale, on the surface of the oopper plate
used. Thus the battery, or alectiomotor, may be advantageoosly
excited with a solution of sulphate of copper instead of dilute add.
773. In all these arrangements, both plates are inunetved in
the same exciting fiuid ; but considerable advantage is gained by
employing two different fluids. This mode is founded on facts
longknowD but first applied to tbe conatruction of electn>motors
by Daniel!.* The theoretical action of those arranecments is
readily explicable: let a, Fi^. 460, be a vessel filled with a solu-
tion of common salt {chloride of sodium] ; b, a glass tube im-
mersed therein, fiuniahed at its lower part with a diaphragm
farmed of a piece of bladder, and filled with a solution of sulphate
of copper ; a plate of copper, c, and one of sine, i, connected by
the wire, d, are immersed in the two flaids. The geDoikting
or positive element, i, decomposes the chloride of sodium, uniting
with the negative chlorine, forming chloride of liuo, and teleaung
^1
FSg,4B0.
£i
I
the positiye Boditim, vLich pMses through the bladder-diaphragm
under the convective influence of the excited current, to reach the
ne^tive plate, c; here it enters the eolation of sulphate of copper,
which it decomposeR, uniting with the sul-
phuric acid and oxygen to form sulphate
of soda, and setting free copper, which ia
deposited on the plate o, and the positive
current which accompanies the chemical
action passing along the wire d to z, de-
composition goes on as before : the atoms
of Bodiam and copper first set free, are not
thoee which are ultimately active m effect-
ing decomposition, or in being deposited
on the conducting plate ; the same series "
of molecular changes occur as in the case
of the decomposition of hydrochloric acid
already described (766). In this appa- ■'^ ^
ratus, after the current has continued passing for a sufficient
time, the fluid in a will be found cunverted partly into chloride of
zinc, and that in b into sulphate of soda ; whilst the beautiful
crystals of copper deposited on c, will be found to bear that rela-
tion to the quantity of zinc dissolved to form the chloride, which
the atomic weight of copper does to that of zinc. If the wire d
were cut across in the middle, chemical decomposition and the evo-
lution of electricity would cease, until they were united by being
placed in contact, or connected by means of a pood conductor.
774. As in this apparatus the inductive action of the two plates
on each other is limited by the area of the base of the tube b,
through which alone a current can pass from the ^nerating to the
conducting plate throogh the flmd, the evolution of electricity
will be increased by replacing the tube b by a porous reservoir of
animal membrane, as bladder ; and this constitutes a form of ap-
paratus frequently employed. A piece of sheet copper is bent
into a cylindrical form, and placed in a bladder fastened round its
upper part by a piece of stnng ; the whole being placed in a jar,
containing a concentric cylindrical roll of sheet zinc ; a metallic
wire, or electrode, is soldered to each plate. A solution of sul-
phate of copper is poured into the bladder, and one of common salt,
or sulphate of soda, is placed in the jar, exterior to the bladder, so
as to act upon the zinc plate.
DanieU^H Battery.— ihe inconvenience of this arrangement
arises from the zinc being placed on the outside of the copper,
which necessarily produces, as Daniell has shown, a certain loss
of power ;* accordmgly the arrangement which he proposed, con-
sists of a cylinder m amalgamated zinc, z. Fig. 451, placed in
the centre of a hollow cylinder of copper, o ; the former being
Phil. Trans. 1838, p. 41, et Mq.
Ifg. «L aorronnded by a luW of porous earthen ware,
cloKd at the bottom, or, which aOBwera the pur-
pon exceedingly well, a cylindricul bag of oom-
pact aail-cloth, preriouilj soaked in water. The
exciting fluid acting on the zinc, ia a mixture
ef one part Bulpharic acid and eight of water,
the copper cylinder being filled with the same
mixture saturated with sulphate of copper. On
connecting the two platoa, by meani of a wire,
the zinc ptate decompoaes the water, ilihydn^en,
influenced by the convective force of the posi-
tive current, passes tlirouiih the membranoua
bag, towards the copper plate, where it is not
evolved, hut aids the decompoijition of the sul-
phate of copper ; uniting with the oxygen of the
oxide to form water, and setting free the copper
to be deposited in beautiful crj-stals, on the
surface uf the copper element.
775. The cnpper deposited in these eiprimenta upon the nega-
tive pUte Is found, irthe electric action be not !<» mtense, to be
compact, firm, Hiid even malleable ; nnd on sepamiiug it from the
surface on which it has been deposited, it will be found to present
a perfect fac-siiuile of every mark and scratch exiiting on the sur-
face of the negative plMa._ This hna led to the discovery of the
beaatifid art of electrotyping, by which exact copies of almoet
anything of which the surface is capable of being rendered a
tolerably good conductor, may be made in copper. Many con-
trivances hafe been made fur the purpose of
■*%■ «>• &cililating the deposition of copi»r from its
solutions, and will be found described in the
numerous popular treatises on the subject.
I I I I I The siDiplest apparatus consists of an
I il I JL^ I earthen or vsminlied wooden vessel, ^ n
^1 ' I (3) I* Kg. 452, divided vertically by means of »
I I — I I ^^ I poniusdiapbrngm,c,ofwoodurearlhenware,
^^^^^^^^J thus fiimiing two cells ; one of these, as a, is
filled with a vei; weak solution of common
nit, the other, a, with a solution of sulphate of cupper. In the
generating cell, a, is immersed a plate of zinc, i, connected by a
wire with the medal, &e., to be copied, which is placed in h. lliig
tnedal should he coveted with a resinou" varnish or some noii-
conduclor, except on the surface to be copied. An electric cuit^ol
is soon set up, and l>oth solutions are ducumposed (773), miitalljc
copper being deposited freely on the face of the medal ; and when
the deposit has attained aufhcient thickness, it will, if adruilly
removed from the surface of the metal, prevent a moat accurate
■nd beautiful copy of the original. It is scarcely necessary to
say that the cell, b, should be supplied with fresh crystals of sol-
VAXIOira FOBm of SLBlCSHTt. 433
pbate of copper, in proportion as the flnid loses its colour by depo-
siting its copper.
In this manner, by careful manipulation, most accurate copies
of engraved copper plates can be readily made, and those beau-
tiful products of art DO multiplied to an almost unlimited extent.
Even the inconceivably delicate tracings of Daguerre's exquisite
pictures can be copied by the electrotype. Where the object to
be copied is not metallic, it may be rendered a sufficiently good
conductor by covering it with a thin layer of finely powdered
plumbago, and thus casts made of wax or sulphur can be readily
copied in copper.
776. Orove't Battery. — By far the most energetic voltaic ai^
rangement, in which the hydrogen is
absorbed, is that proposed by Mr. Grove. ^. 458,
Various constructions of this excellent
contrivance are met with. They consist
essentially of a slip of platinum-foil, p,
Vig. 453, furnished with an electrode, a,
immersed in a cylinder of porous
earthenware, c, filled with strong nitric
acid (sp. gr. 1'33). This cj'linder is
surrounded by a roll of amalgamated
sine, having an electrode, s, soldered to
it, and placed in an earthen or glass jar,
B, containing dilute sulphuric acid (1 acid to 6 water). The
hydrogen separated from the water decomposed, when ▲ and s
are connected, is not evolved as g^, but combines with some of
the oxygen of the nitric acid, reducing it to deutoxide of nitroeen,
which partly dissolves in the acid, giving it a green or blue
colour ; the rest escapes, and produces red fumes, by combining
with the oxygen of the air to form nitrous acid. According to
Jacobi, with equal surfaces of platinum and copper, the apparatus
of Mr. Grove is about seventeen times more powerful as a source
of electricity than that of Daniell (774). With a nitric acid element
capable of oeing contained in a two-ounce jar, fine platinum wire
may be brilliantly ignited.
The superiority of Mr. Grove*s arrangenient is owing, not only
to the SMorption of hydrogen, but to the excellent conducting
nature of the fluid employed, and to the remarkable facility with
which nitric acid undergoes decomposition, and parts with a portion
of its oxygen.
777. IhuiseiCs Battery, — ^The expense of platinum is a serious
drawback to the use of Grove's battery; to obviate this, Prof.
Bunsen has proposed substituting cylinders or plates of carbon
for the platinum* He made these, by strongly and repeatedly
heating a mixture of pulverised coal and coke, and thus obtained
a porous mass capable of being easily worked into any required
form. The porous mass was subsequently consolidatea by oeing
F F
484 YCLTAIO ELBOTBIdTT.
immened in strong syrnp, and dried, and the mggr then cb^
bonized by exposure to a white heat in a closed TesseT. These car-
bon-batteries are said to be equally powerful with thoae of platinum.
According to the experiments of MM. Liais and Flenry, the
internal resistance of the cell may be oonsiderably diminished, and
oonsequently the power of the battery increased, by the omission
of the diaphragm, proyided the carbon be kept saturated with
nitric acid. For this purpose the carbon cylinder must be hollow,
and cemented into the bottom of a glass vessel a little larger than
itself, the interrening space being filled with nitric acid. This
form of nitric-acid battery is much used on the Continent, but has
-not found favour in this country.
A carbon-battery may be constructed by means of the best
black-lead crucibles, after strongly igniting them for a short time.
For this pur]^ose a cylinder of amalgamated zino,
^' ***• z. Fig. 464, is placed in a porous cylinder con-
taining dilute sulnhnric acid, and immersed in
the crucible, c, filled with nitric acid ; a wire
coiled tightly round o acting as a conductor.
Such an arrangement, although powerful, is,
however, certainly far inferior to Mr. Grove's
apparatus, probably on account of the earthy
matter which is always present in these cru-
cibles, rendering them imperfect conductors.
' 778. Schlhibein^t Battery. — Prof. Schonbein has suggested an
eflSective combination, in which the platinum of Grove's, and the
carbon of Bunsen's battery, are replaced hj panive iKSk, This
is a peculiar state assumed by iron under several circumstances,
especially after momentary immersion in a mixture of strong nitric
and sulpnuric acids. It then retains its metallic lustre, but has
.lost its power of being readily oxidized by exposure to air, and of
.being dissolved in strong nitnc acid. Schonbein places in a vessel
of passive iron, a mixture of three parts of concentrated nitric and
one of sulphuric acid. In this is immersed a porous earthenware
jar, filled with dilated sulphuric acid, and containing a plate
of amalgamated zinc. This forms an economical and powerful
apparatus.
• 779. The MaynootK BaJttery. — ^The arrangement to which this
term hcM been applied was devised by Dr. Callan :* it consists of
a water-tight cast-iron cell, containing a porous cell, within which
is a plate of amalgamated zinc. The iron cell is charged with a
mixture of nearly equal parts of strong nitric and sulphuric acids,
and the porous cell with a mixture of two parts of sulphuric acid,
one of nitric acid, and eighteen parts ot water. This form of
battery is stated by Dr. Callan to be nearly one and a half times
as powerful as Grovels, when of equal dimensions.
780. RcherWi Battery. — A simple and efiective modification
• Phil. Mftg. Tol. XTTiii.
TAJUC^B F0&1I8 OF ELEXENT8. 435
of the zinc-iron batter/ has been introduced by Mr. Boberts :*
this consistg of an alternate series of plates of passive (778) iron,
and amalgamated sine, placed in a wooden frame, and kept in
equidistant and parallel positions hy slips of wood. The connexion
of the plates is peculiar : let a, b, c. &c., represent the iron, and
a, b, c, &c., the zinc plates, then in tne series
A a, B &, c e, D d^ &c.,
A is connected with 6, a with c, b with e, b with d, and so on. lu
this arrangement the intervention of two plates between each pair
in metallic connexion prevents the loss of electricity by conduction
through the excited fluid, and supersedes the use of ])artitions ;
the whole is immersed in dilute sulphuric acid contained in a
wooden box with water-tight joints.
Mr. J. Watson recommends the iron plates to be prepared by
immersion in dilute hydrochloric acid ; and the zinc pUtes to be
amalgamated in M. Bizot's bath, which is thus prepared : — One
oz. of mercury is dissolved in a mixture of one oz. of nitric and
three of hydrochloric acids, and three oz. more of hydrochloric acid
are subsequently added. He states the relative electromotive forces
to be, if clean copper be used, 7 ; if sheet iron, 9 ; and if iron
thus prepared, 13.
781. Leewn^g Battery. — In this batteiy pairs of zinc plates, one
of which is a little longer than the other, are grooved into the
sides of a trough, and are connected by resting on pieces of zinc
at the bottom. Within these zinc cells, as they may be called, is
placed a porous cell containing a plate of copper, bent over at the
top and clamped to the higher side of the next zinc cell. The
porous cells are charged witn a solution of one part of bichromate
of potash in ten of water, and the trough with dilute sulphuric
acid. In this form of battery, as in the preceding, water-tight
partitions are omitted, as being useless.
782. Platinum and potassium being at the opposite extremes of
the electro-positive series of metals (764), constitute a most ener-
getic voltaic arrangement. The decomposition of acidulated
water, and the divergence of a gold-leaf electroscope, have been
effected by the energv of a single element : but the expense of the
materials precludes tne practical application of this powerful com-
bination. A convenient mode of employing it has been proposed
by Mr. Goodman.f
783. A verv efEec^ve, but rather costly form of the zino-carbon
battery may oe constructed by charging the siuj^le cell with a
saturated solution of sulphate of mercury^ contaimng some of the
nndisBolved salt at the bottom. When this salt is decomposed,
the disengaged mercury adheres to the zinc plates and keeps them
fully amaJgamated, thereby effectually preventing any local actioiL
• Proc. Elect. 8oc.jp. 357.
t Mem. ManokeMer lit. and Phil. 8oo. toI. Tiii.
P F 2
436
VOLTAIC BLECTRICITir.
Fy.465.
This form of battery is especiallj convenient wlien considerable
electromotive foroe is reqmred, and when the battery is also re-
3 aired to remain charged and ready, but out of action, for coofi*
erable periods of time.
784. It is not absolntely necessary to use two different metals
to obtain an electric current, for if two portions of the same metal
be employed, the surfaces of which are so constituted as to be
unequally acted upon by the fluids in which they are immersed,
electricity will be evolved ; the portion of the metal most acted
upon becoming the positive element. Thus, a plate of rolled, and
one of cast zinc, constitute an actual but feeble voltaic arrange-
ment ; as does also a plate of new clean zinc, with one which has
b^en previously corroded by an acid. A new and a corroded plate
of copper acted upon by nitiic acid, will also evolve electricity.
785. If e(|ual surfaces of one metal be used, no electricity will
be evolved, unless acted upon by fluids
exerting different chomical actions
upon them. Thus, a plate of smooth
inin acted on, on one side by dilute
sulphuric acid, and on the other by
water, sulphate of copper, &c., deve-
lops a voltaic current. Let a plate
of copper, A B, Fig. 455, be placed
in a glass vessel, and a saturated so-
lution of sulphate of copper poured in
up to the line c d, so tliat about one-
third of the plate may be immersed. On the surface of this fluid
slowly pour some very dilute sulphuric acid, or weak salt and
water; takine care that the fluids do not mix. Under these cir-
cumstances the upper part of the plate, a, will be slowly acted
ujpon by the sulphuric acid ; the lower end, b, becoming the nega-
tive element, decomposition of the sulphate of copper smwly takes
place, and the metal becomes deposited in a crj^stalUne form, on
that part of the copper plate whicn is immersed in the sulphate of
copper.
786. If the electrodes of a single pair of plates be famished
with platinum terminations, and instead of bemg in metallic con-
nexion, be immersed in the same solu-
tion as the plates themselves, no current
will pass, llie electricity excited in the
cell, A, Fig. 456, will not be able to pass
from p to p in the cell b, because the
current is too weak to overcome the
affinities of the elements of the liquid in
B fur each other, and it cannot traverse
the fluid save by effecting molecular
changes in the elements of the com-
pound present. Tlie current may, how-
Fig, 469,
QUJLITITY ASD FOTBVTIAL. 487
eretf be induced to pass, eitber by replacing tbe fluid in b bj
one wbicb ie more readily decomposable, or bj calling in the aid
of the afifinitj of the positive conducting wire for one of the
elements of the liquid in b.
To illustrate the first case, let a and b be both filled with dilute
sulphuric acid, when the current will not pass through b. Beplace
the contents of b by a solution of iodide of potassium, a salt of
Ipeady decomposition ; the current will now readily pass, decom-
posing the iodide, evolving iodine at the surface of the platinum
plate connected with the copper in ▲, and if a little starch be
added to b, the evolution or the iodine will readily be detected
by the formation of a splendid blue precipitate of iodide of
amidine.
The second case may be illustrated by filling ▲ and b with
dilute sulphuric acid, and letting the conducting wires be of
copper, with their naked terminations immersed in b. The current
wul now pass, and bubbles of hydrogen gas will be evolved at the
end of the wire connected with the plate z. Here, although the
corrent per Be could not effect a separation between the elements
of the water in b, yet when aided b^ the affinity of the copper
composing the positive conducting wire for oxygen, it succeeded
in decomposing water, and traversing the fluid.
787. In investigations on voltaic electricity, it is necessary al-
ways to bear in mind a distinction between the quatUity ^'nA. inten-
ftity or potential of the electric current ; the former bearing, caUeris
pmhuMf a relation to the size of the plates, or to the number of
plates combined, and the latter to the number of alternations. A
pile or other voltaic arrangement of fifty pairs, excited by pump-
water only, will readily cause the gold leaves of the electroscope
to diverge, and will produce a sensible shock ; but will scarcely
decompose even a small portion of water in a space of time
sufficient, when the battery is excited by an acid, to rapidly
resolve a much larger quantity into its gaseous elements
(754).
The following facts will place the effects
of the combinations of voltaic elements in a ^' ^' '
clearer light. If in two elements the conduct-
ing wires of the sine, and those of the cop-
fer plates, be respectively joined together,
ig. 457, the current excited by the action
of the acid in a on z is opposed in direc-
tion to that similarly excited in b. Tbe
consequence is, that they mutually inter-
fere, and no circulating force is developed,
imtil the connecting wires be themselves
connected by fo, which will transmit the currents due to the
electromotive forces of both elements, as shown by the arrows.
But if, instead of allowing the currents in a and b to oppose each
488
VOLTAIC BLBCTRICirr.
Other, we canse them to pass in the same direction, we greatlj
increase the potential of the erolved elec-
tricity, and enable it to overcome a much
greater external resistance. Thus, in
Fig. 458, the currents in d and e travel
in the same direction, and, as it were,
urge on each other, so that by their com-
bined inflaence they can traverse a fluid
which wonld insulate the current of d or
E separatelv.
788. Volta^sPiie. — A considerable quantity of electricity may
be evolved by the various electromotors now ((escribed, but unless
the number of elements be considerable, the current is of small
potential energy. To Prof. Volta of Pavia science is indebted for
the discovery of the means of indefinitely augmenting the poten*
tial of the current, by increasing the numbler of elementa employed ;
he thus placed in the hands of philosophers an instrument of
analvsis and investigation, greatly exceeding, in its effects, anv
of the means of experimental research previously discovered.
Omitting the earlier experiments of Volta, or the mode of reason-
ing by which he was led to this discovery, as out of place in a
work of this description, it will be sufficient to observe, that, by
combining the action of several pairs of plates, a great increase of
energy and power is gained. The following is the construction
of the voltaic pile : place a plate of copper, a,
Fig. 459, on the table, and on this one of
zinc, z ; a piece of thick flannel, f, moistened
with a dilute acid, brine, or even water, la
placed on the zioc ; a plate of copper on this,
and so on ; copper, zinc, wet flannel — copper,
zinc, &c., until any reouired number of alterna-
tions is employed. Place the whole pile on an
imulating stand, and connect the lower plate, c,
with the condenser (738), connected with an
electroscope ; the gold leaves will divei^e with
negative electricity. Then connect the upper
plate, z, with the condenser, and the leaves will
diverge with positive electricity. In an insulated pile of anv
number of alternations the potential of each form of electricity u
observed to increase from the centre to the extremities.
The direction of the current between the terminal plates of the
voltaic pile is here apparently at variance with that previously
stated (766) ; but, in point of fact, in the arrangement just de-
scribed, the terminal plates are merely electrodes, and not eleo-
tromntors ; the lowont zinc plate z, and the hic^hest copper plate, c;
are the terminal active plates in the pile. The arrangement here
described was based on the contact theory: namely, that the
evolution of a current resulted from the contact of different metals.
If a pile or batteiy be constructed like the one originally
Iig.U9.
PIJLTBBHAGHBB*8 OHAHI FILE. 439
eontriYed by Yolto, but comprising at least thirty alternations,
and the top and bottom of it be touched at the same instant with
the moistened hands, the electricity accumulated at each end of
the pile will discharge itself throagh the arms, producing an
dectric thoek. If a piece of well-burnt charcoal be placed QjKin
the uppermost plate of the pile, and a wire communicating with
the lowest be brought in contact with it, a series of faint sparks
will be visible on drawing the wire over the surface of the
charcoal. *
789. The source of electricity in the ToUatc pile may be easily
traced to chemical action, for in the lowest pair of plates in Fig.
459, the zinc is attacked by the flaid in tne wet flannel, and a
current is determined through the flannel from the sine to the
second copper disc, and the lowest disc, c, is in a negative state.
In the second couple, the positive state of the copper plate neu-
tralizes the negative state of the zinc plate, arising from the
current determined upwards through the second flannel ; and this
series of actions is repeated to the top of the pile, no greater
fuanHty of eketrieity being obtained from a piUf than from a
nnple pttir of plaUef* the potential of the eUctrie cvrrent cdon4
hetng inereaeea: for the chemical action, and disturbance of
electric equilibrium, in the intermediate plates of the pile or
battery, are exerted onlpr in urging on the electricity to the ter-
minal plate, and thus increasing the electromotive force of the
current evolved.
790. An ingenious modification of Volta's pile has been made
by M. Pulvermacher. In this, pieces of gilded copper and of zinc
wires, arranged side by side, but not in contact, are wound on a
piece of porous wood, each wire terminating in little hooks. This
constitutes one element of the chain (as it is called), and is con-
nected with a second by means of these terminal hooks ; this with
a third, and so on. Tuis apparatus is excited by immersing it
fbr a moment in distilled vinegar. Enough of the acid adheres to
the pieces of wood in each link to excite a current of electricity
by acting on the zinc wire. The whole forms a most convenient
source of electricity, a chain of 100 links giving a most painlul
shock, and producing the ordinary chemical phenomena.
791. A still more in^nious and effective apparatus has been
contrived by Mr. Stnngfellow, of Chard. Each el«^ment of this
apparatus consists of a plate of zinc 2'5 inches long, and 0'3
inch wide, on which are wound, in a spiral manner, 30 coils of
flattened copper wire, as close as possible to the zinc, but separated
irom it by a non-conducting medium. Four of these alternations,
merely moistened with a sponge, dipped in common water, pro-
duce a current sufficiently powerful to decompose distilled water,
evolving a copious stream of minute bubbles of oxygen and hy>
drogen from the platinum points immersed in the water. With
twenty-two alternations (which after being moistened with dis-
• Farsdty, Fhfl. Tnuui. 1894. Exp. Bwetxohei, dth Series, 991. •
440 TOLTAIG BLBOTRIOlTt.
tilled Tinegar, are placed m a caae the mn of a common card-
case), a carrent of electricity is evoWed, capable of producing
distinct shocks, and rapidly aeoomposing water ; and it remains
scarcely diminished in mtensity after half an hour's action. For
physiological purposes, this is certainly the most useful apparatus
which has been hitherto contrived. It owes its remai^bb
power to the iii^nious manner in which all resistance to the pas-
sage of electricity is removed, by the soldering together of the
metallic elements, and to the very small bulk of fluid required to
excite them. Hence, neariy all the electricity evolved is thrown
into current, instead of being partly lost from imperfect conduction.
792. Marie Davy^e Pile. — ^This yery energetic and eflfective
arrangement, for many experimental purposes, consists of a series
of ciroular iron dishes, to the bottom of each of which a plate
of zinc is soldered. In each dish is a layer of salphate or chloride
of lead moistened with water: of these salts the chloride is
much more active, but it is likewise more costly. 83 parts of zinc
will reduce 144 of sulphate of lead, and will yield 104 parts of
lead. A pile of 40 of these elements has been in use at the cen-
tral telegraph office.
793. The power of the yoltaic pile decreases, and finally ceases,
with the neutralization and evaporation of the fluid moistening
the piece of flannel, and with the oxidation of the plates. These
constitute sources of considerable inconvenience in experimental
inyestigations ; to diminish which yarious means have been pro-
—^ ^^ posed, as b^ fixing the pairs of nnc
^' ' and copper lu a trough of wood, Fig.
^^^ 460, and replacing the wet flannel by
^^ a fluid poured into the cells thus
^ I formed : constituting Craikshank*s ar>
V I rangemenU This is not inconvenient,
^ sgj^^aggg ^ especially when a solution of sulphate
of copper is used for the ezcitine fluid ;
which, as Dr. Fyfe has shown, increases the electromotive rorce of
the current, as compared with that evolved by dilute sulphuric
acid, in the proportion of nine to two.
The positive electrode, whether of a single element, or of a
series constituting a voltaic battery, is always that which is con-
nected with the last active copper or platinum plate, and the nega-
tive, that connected with the last active zinc pUte. Much un-
necessary confusion, with regard to the expression of the negatiye
or positive side of a battery, has been introduced in many works^
from the want of a role like that ^ven by Faraday, of connecting
their sides with a g^ven direction of the current. Thus, in
Oruikshank's battery, the positive electrode is that which is con-
nected to the last zinc, and the negative, to the last copper
plate. This difference is only apparent, as will be evident by
referring to the original yoItaic pile (788). Remove the terminal
plates, and then all obscurity will vanish, for the positive electrode
YAUOUS FOSM8 OF BATTBBIES.
441
J^.46L
wiU be in actual contact with the last copper plate, when the
Bnperfluous and truuking plate is removed. In a Graikshank
trough, excited by an acid or saline solution, the positive elec-
trode will be that which is fixed to the end towards which aU the
zme plates look : and the negative, that fixed to the end towards
which aU the copper pUUes look.
A great improvement in the construction of these batteries was
effected by Dr. Wilkinson, who fixed the zinc and copper plates
to a wooden beam, and immersed them, when required for use, in
an earthenware trough, furnished with partitions of the same sub-
stance, and filled with the exciting fiuid. This arrangement i»
rendered still more effisctive by causing each zinc puite to be
completely surrounded by the copper plate of the next pair, as
sogiested by Dr. Wollaston.
Earaday nas proposed an excellent arrangement,* in which
the metals are brougnt as close to each other as possible, the alter*
nate zinc and copper plates being separated, not by partitions of
earthenware, but oy pieces of stout cartridge paper, or card.
794. A series of elements constructed on Daniell's principle
(774), aflfords a most valuable
source of potential electricity, and
has, moreover, the advantage of
being constant in its action for
several hours ; whereas, the others
above mentioned, although very
eneigetic on the first immersion of
the plates, become rapidly weak-
ened bv the continual action of
the fluid employed, an eflect but
partially preventea by amalga-
mating the zinc plates (^765). Ten
pairs on Professor Daniell's ar-
langement, the zinc cylinder of
one being connected with the copper of the next, and so on, con-
stitute a most valuable and poweitnl voltaic battery. Fig. 461.
A very efficient arrangement is made by connecting in a
similar manner, a dozen pairs of zinc and copper cylinders, sepa-
rated by means of bladder
di^hraipnsy Fig. 462, the *^- ^'
zinc bemg acted on by
common salt, and the cop-
per bv sulphate of copper ;
this has the advants«e of
cheapness, and of being
readily constructed. The
zinc and copper plates are
most conveniently con-
nected by copper wires,
• FUL Trans. 1833, lOth Scries, Bzp. Beiearchea, 1123.
442 VOLTAIC ELBCTRICTTT.
fixed bj a binding-Bcrew soldered to each plate, and tfie electrodes
can be readily attached to the screws of tne terminal plates.
795. As bladders and other membranons diaphni|^8 have the
disadvantage of becoming rapidly corroded, and pierced, by the
action of the exciting fluids, and of being torn by tne sharp edges
of the crystals of metallic copper deposited on the copper plate ;
various attempts have been made to substitute for tbem cvlin-
drical vessels of porous earth. Vessels of this kind have been
nsed by Daniel], and are now made sufficiently thin to prevent
their opposing much obstruction to the transit of the electric
ourrenl^ so that their use has become very generaL As already
stated, bags of firm sail-cloth well sewn, or still better, pieces of
the tubular woven hose used for garden engines, form excellent
diaphragms, and withstand for a long time the action of acids.
796. The most poweiful battery is made bv an alternate series
on Mr. Grove's arrangement (776). Six of these elements, each
having a platinum puite three inches wide, placed in thin rectan-
g|ular cells of porous porcelain, so as to bring them^ as near the
zinc as possible, constitute a most powerful and efficient arrange*
ment. The largest hitherto constructed is that made by Professor
Jacobi of St. Petersburg; it contains platinum plates, each having
a superficies of 36 square inches. Even with very small pirates, a
powerful battery may be made with very little expense. For this
puroose, procure the'bowls of six tobacco pipes, and stop up with
seaun^-wax the holes left by breaking off the pipes. Place on
the teble six small glass tumblers. Fig. 463, each an inch and a
half or two inches high, like those used by children as toys, plaoe
in each a piece of amalgamated zinc, bent so as to form a hollow
cylinder. In each of these cylinders let a pipe-bowl be placed,
and id each of the latter immerse a piece of thin platinum foil
1^ inch long and half an inch wide, connected with the next
zinc cylinder by platinum wire. Fill the pipe-bowls with nitrio
acid and the tumblers with dilute sulphuric acid, and an eneigetio
current of electricity will be set free, capable of rapidly decom-
posing water (821), igniting wire (815), charcoal pointo (808), &c.
797. It must not be supposed thfit all the electricity which is
excited by the chemical action of an acid, or other fluid, on the
generating or positive metal, even in the best arrangements,
ohm's thbort. 443
appears in the fonn of a carrent. Various causes, modifying in a
- renuuicable manner the quantity of electricity which appears in
the current, exist in the best constructed apparatus. These liavo
been mathematically investigated by Prof. Ohm of Nuremburg,
and the results are developed in what is known as his formula.
The accuracy of this has been submitted by Wheatstone, Daniell,
and othersi and in a most successful manner, to the test of expe-
riment. The following is a brief explanation of the more simple
results of Ohm B investigations.*
Let E be the electromotive force, equivalent to the affinity of
the exciting liquid for the generating metal, and corresponding
with the amount of electricity which would appear in current, if
all opposing causes were removed ;
i?, the internal resistance, or that opposed to ^ b^ the contents
of the cell, ariiong for the most part (rom the affimty of the ele-
ments of the exciting liquid for each other ;
r, the external resistance, arising chiefly from the imperfectly
conducting nature of the wires used to convey the current : and
C. the current force, or the amount of potential which actually
reaches the end of the conducting wire ; then
E
The theoretical value of ^is diminished materially in practice by
the affinity of the conducting plate for the ingreUienc of the ex-
citing fluid which tends to combine with the generating plate ;
this affinity, however weak, is still seldom absolutely nuU. The
mutual affinity of the separated elements of the fluid, evolved at
the surfaces of the plates, also diminishes the intensity of E,
The internal resistance, R^ varies directly with the distance D,
between the two plates, and is inversely as the area of the section,
8f of the exciting liquid, or n ^ ri-i
r, or the external resistance, so far as it is dependent upon the
conducting wire, varies inver$ely as the section oi' the wire, #, and
directly as its length Z, or I r^i
8
798. If the circuit be closed without any external resistance,
then rso, and n ^ ^^
hence a single voltaic element produces the same effect as a
* For tho furtbar darelopment of this theory, and its Tuioos importmnt
■ppUofttionSr the alodent is referred to the eU borate" Chemicsl Fbiiosuphy**
oi the lata Frof. Dwuell — % work that ought to be iu the bands of every
•todent; also to Fntf. Obm'e original work, *' Die GalvanlBcbe Kette mathe-
matiech bearlwitet," a digest of whiou baa appe«red in the 2ud vol. of
Tajlor'a ooientifio Memoirs ; and to a p^per by Frof . Wbeatstoue, in the
PlttJ. Traoa., Part IL for 1043.
444 VOLTAIC BUBcniciTr.
l>atteTT oonnstiDg of an}[ number of preciMlj similar domenta,
provided no external reeittanee be interpoeed in the dretni (789).
Also a thermoelectric element (Ch. aVL), and a voltaic ele-
ment will prodace the same effect, when the greatly inferior eleo-
tromotive force of the former is compensated by a correapending
decrease in its resistance ; in a thermoelectric arrao^ement the
resistance is in general small, because the circnit is entirely nwtal^
lie, while in a voltaic element^ the resistance of the liqind is al-
wavs considerable.
It appears from [a] that when r has any value, it will diminish
that of C: that is, any interposed extenui resistance will weaken
the force of the current, bat less so as it is smaller in proporcioa
to the other internal resistances in the circuit.
If n elements be united together, then B becomes - , and
JS nE ,
— 1
nr
^'=-5— :=3rF
- +r
n
but if the n elements be arranged in series, then B, J?, become m E,
n 5, respectively, and ^ _ nE
The value of C^ will evidently increase rapidly as « ii
when r is veiy small compared with M; this explains the advan-
tage of employing several small elements combined, or else lam
elements, when uie resistance to be overcome is small: and it is
equally evident that the value of (7, will increase with «, when r
is large compared with B ; this again explains the neceeaity of
emploving a series of voltaic elements, in order to ovcreume con-
siderable external resistances. The same remarks will apply to
the comparison of a voltaic with a thermo-electric circuit.
799. Suppose that r^m B^ and that jc of the a elemenia be ar-
ranged in series and - of these series united, then by snbatitiiting
these values in [a] we obtain _ __ nE
^-T^)
The value of C. will be a maximum, when the value of x-l- —
X
is a mtntmum, and this is the case when
1 — -y =0, whence «= v m n ;*
• BytheDiffeNotial Csloalus, il. /(«)BO,wbeB/(#) is»:
<«.(«+V)-»-7'
wumiitTon'i BaiDos. 4411
I preMDts the moat adnata^na mode of Riranpn^ a
•.T of alemeDts, when the nno of the eitenuJ and m-
itiEea u known : it Bppeara that the niunber to ba
BoiieH ahoald be the neareit integer bi a mean ptt^•
:ween the aamber of elrmenta, and the ratio of ei-
itcrnal reiiatances, tliat is a diTisor of the number of
rhns, sappoae it were reqaired to ascertain bow »
H'b Utter)' (794) of 12 cells should be arranged in
d the strongest cnirent throngb a given quantity of
(coiled round an electro-nugnet [877] for example),
le or which has been ascertained to be dnuble that of
he batlerT ; hem x would be a mean propoiiiooal be-
12, or the equaiv root of 24. which is 48 nearlj:
' the greatest eRect will be produced hj miiling three
consisting of four elementi.
er important deductions nuT be obtained, aa corol-
Ohm's formula ; but its application to the solution of
liooB relating to electromotive force has. It is hoped,
:nt to facilitate its further apphcation to anj otbar
loy be required.
■jiUtone't Briftne.—ln all the Tariooa and importatit
plications of roltaic currents a correct estimate of the
ulancea of Tarinos bodies (or, an the conrenie of r»-
Htmelimes called, their condDctiTities") is india-
icewarj, in cnler that the abore and many other
lulting from Ohm's law may be made uae of The
ly which these reaiaCancei arc determined ii Wktot-
igt, the principle of which is this : — a Toltaic current
between two conducCoia m arranged that the partial
.ypasa in opposite directions through a galTanometer
t IB evident that if the two currents are exactly equal,
nil remain at rest. If then the resistance to be eati-
nlerposed in one of the circuits, and in the other a
nouni of known resistance to bring the needle to tero,
icd resiHtancea must be equal, and the value of the UO-
■epresenfs this inBtromeaC in ita original and riuplest
446 TOLTAIC ELECTBICITT.
tance from esch other, in the middle of the ndee a xt* d ■, Me
placed two other pairs (^binding-ecrews, b, r ; o, h ; the conneiioni
between the binding-scTews shown in the figure are made with
copper wire, namely, a-c, on, Art, f-d, d-o, h-b. When an ezneri-
ment is to be made, a and d are connected with the tonmnaJs of
a battery, and b, o, with those of a galvanometer, m, whidi for
the sake of explanation is represented in the diagram, aa being
interposed between c and d. In order to adjust tne bridge two
equal small resistances, as two equal short lengths of the same
copper wire are interposed at b, r, and a, h ; and the diObreotial
current in the galvanometer is then reduced to aero, by means of
the joined arms d k, which being of laige section compared with
the wire connecting the binding-screws, will take off auch a por-
tion as may be required of the resistance of either fd, or do.
The required resistance is now interposed between e and r, and
a known resistance between o and h, which is varied nntil ^e
differential current in the galvanometer is again reduced to
The course of the currents over the bridge is rather oomi ~~
but by a little care may be readily traced out. 8np]foae the
tive voltaic current to enter at a, it will then be divided bet^
A c, and A E F D ; the portion arriving at o will be a^ain sobdivided
between c b, and c m d, and that arriving at d will be aimilariy
subdivided. The current will therefore have four distinct oomaea
between a and n, vis. a-o-b, a-e — p-d-q — ^h-b, a-om-d-o — b-b^ and
A-E — F-D-M-G-B ; the first two of these wiU dearly have no eflect
on the galvanometer, and of the last two, the portions
and F-D-M-oB, also a-e and h-b are by cons^nctioo e^nal in
■ ■ the ■
resistance ; if therefore the remainders, the interpoaed
B — F and G — ^H, are equal, the whole must be equsl, and the diffe-
rential current, which alone influences the galvanometer, most « 0^
and the needle will remain unaffected ; henoe the amount of known
resistance a — ^h will be a measure of b — ^f, the resistance sought.
This form of bridge is convenient for estimating small resist-
ances, but it is not suitable for large ones ; for it IbUowa from
Ohm^s law, that if a current be divided between two channels,
the quantities passing through them will be inversely propor-
tional to their resistances ; suppose then that the resistanoe ■ — f
were 10,000 times that of the wire ac, it follows that obIv
nrirv^^ part of the current will traverse the galvanometer, and
the aifferential current being again a smaU fraction of this^
is liable to become imperceptible. To remedy thia inoonvenieooe;
two more pain of bmding-screws must be interposed between
A, c, and c, b, between which it will be most advantageona to
interpose equal resistances, and also neariv equal tc thoae at b, f,
and o, h; as in that case nearly one-fourth of the whole cnrrent
will pass in each direction through the galvanometer, and the
differential current will then have its maximum value.
801. There is, however, another fonn of the bridge now fre-
quently employed, which, besides doing all that the form above
rAHDABD wot OF BLICTBIOAL BBBUTillCB. 447
capable of, has alao the advantage of ihoiving approzi-
ratio of the interposed resistances, when their aiifer>
Qsiderable. This instrument consists of a stent
oard, about forty inches long, on which is a scale of a
d into millimetres, a 6, Fig. 465. Parallel with the
scale is stretched a piece of German silver wire, be*
ds of two flat copper oars e, d^ fixed to the ends of the
J^. 466.
laving binding*i«crews at their proximate ends. Be-
ids of c, df are three equal copper bars, e, /, g, each
nding-acrew in the middle, and one at each end : the
g ones are marked with the same letters as in the pre-
"am. The points k, l, h, o, correnpond with the pro-
als in the sides, a o, o b, of Fig. 464. ▲ is a blocic of
ict with the stretched wire, furnished with a binding-
doveable along the scale a b. llie battery is inter-
en ▲ and B, and the multiplier or galvanometer, m,
id D, as shown in the figure. The counter-resistanoea
g always equal, the ratio of the resistances s — f : a — h
uximately that of the portions of the scale, xb: AC,
resistance of the wire is very much greater than that
' the apparatus.
dard unit of Electrical BeiUtance, — Some definite
aparing resistances being indispensably necessary for
ion of electromotive forces, and the amounts of current
became necessaiy to adopt some unit, like the units
e, and weight, in terms of which anv given resistance
(pressed, and for this purpose arbitrary units have
ed. Wheatstone proposed a given length of pure
of a given weight ; this is not, however, quite satii«-
3ils of such wire have been found to undergo consi>
taneous changes of resistance. A column ofdistilled
^ven dimensians was proposed by Dr. Siemens: this
d is more free from spontaneous change, and more
irodncible, but it is subject to considerable variation
from change of temperature. Many electricians have
k more, desirable to establish an €U>Jiolute instead of an
ait, that is, one depending on abstract considerations
,ce, and force, or velocity. A determination of this
it made by Weber,* which it is needless here to de-
has been snperseded by a method devised by Sir W.
* Poggsndorff, Annslsn, Izzxii. p. 83.
448 VOLTAIC BLSCTOcrnr.
Thomson, and carried out by a committee appointed by the
British Association for the determination of electrical constants.
The investigations of this committee have led to very satisfactonr
results, of which only a bare outline can here be given ; bat aU
requisite information on the subject may be obtained from the
various reports of this committeci published in the ** fieports of
the British Association."
Ohm's law, 0-=-^ (798), establishes a relation between elec-
tromotive force, resistance, and the amount of current, which may
be thiis expressed in words: — An unit current i$ produced by an
unit electromotive force in a circuit of unit retietance : and a
relation between TFthe work done, and current, resistance, and
time <, namely W=C*Bt,
has been experimentally demonstrated by Joule ; which may be
thus expressed — an unit ofioork is done hy an unit current pasS'
ingfor an unit of time through a circuit of uniX reeistance.
A third relation, involving electro-magnetic force, may be thus
established : — ^Lot/be the resultant electro-magnetic force exerted
by a current, C, at the centre of an annular coil, through which it
is passing, and of which the radius is k, and the length, L ; then
it IS evident that CL
and if a short magnet, having a magnetic moment, m, be sus-
pended in the centre of the coil (which must now be supposed to
be placed in the plane of the ma^etic meridian), the deflecting
force upon the magnet in a direction peipendicular to the plane of
the coil will be CLm
and if the axis of the magpiet be deflected to an angle B with the
plane of the coil, the deflecting force will then be
CLm ^
• . , cos©.
But if ^be the earth's horizontal magnetic force, the force acting
on the magnet, in the direction of the magnetic meridian, wiU be
Hm sin 9,
and if these forces counterbalance each other, then
-_— cos 9 = Jim sin 0,
whence C = — r ^ = — ^ tan B :
L COS0 L
from which it appears that in a given coil, provided the earth's
horizontal force remain constant, the current is measured by the
tangent of the angle of deflexion. This, it may be remarked, is
the principle of the tangent galvanameteTf to be described here-
BTAVDABD UXIT OP ELBCTBICAL RESISTANCES. 449
after. This formula is absolutely true only for a coil of no sensible
thickness, acting on an indefinitely short magnet : and approxi-
mately true, in proportion as the len^h of the magnet, and the
transverBO width of the coil, are botn small compared with its
diameter.
It may be deduced from the precedine formuln that when the
coil rotates on a vertical axis passing through the centre of the
suspended magnet with a linear Telocity v, and the currents
generated in the coil, bv its several turns being moved across the
lines of force (592) in the earth's magnetic field, deflect the mag-
net to an angle 0, the couple (81) exerted on the magnet will be
j^lTmcos^,
and the opposing couple due to the earth's magnetism will be
Hm sin 0 :
equating those values, we obtain
4i;5=tan0,
whence -^Tp ^ot^;
from which expression the magnetic moments, both of the sus-
pended magnet and the earth, have been eliminated, and the
resistance of the coil is expressed in terms of its dimensions and
velocity, and the resulting deflexion onlv ; and thus an absolute
value of resistance may be determined. For a detailed description
of the apparatus by means of which the numerical determinations
have been made, and the ingenious means devised for obviating
very various sources of error, the reader must be referred to the
reports before mentioned. The nature of the electrical action of
the rotation mav, however, be thus explained : — suppose direct
motion of the coil to commence, then the north side of the coil is
moving from west to east, and experiences an electromotive force
tending to produce an vpicard current ; but the south side of the
coil is moving from east to west, and in that portion a doumward
current is generated ; and if the circuit be closed, these currents
will recur at each coincidenoe of the plane of the coil widi the
magnetic meridian.^ This intermittent deflecting force produces
minute oscillations in the suspended magnet, but they are of quite
insensible magnitude.
If one metre per second be taken as the unit value of v, the
metre-second unit of resistance is a verv minute quantity ; it has
been determined to construct material standard resistance-coils
each containing, or being equivalent to, 10,000,000, or 10^ metre-
second units of resistance, oy a careful comparison of the actual
resistance of the rotated coil with that of the proposed standard
ooils, by means of a suitable modification ol Wheatstone's
450 VOLTAIC BLECTBICITY.
bridge (800). It is desirable that these should possess two re-
qaisites, — ^^ermanency of resistance, and the least attainable
variation, due to change of temperature : these desirable objects
appear to have been best attained by the copper alloy known as
''German silver," and by some alloys of the precious metals.
These standards may now be obtained from Mr. rleeming Jenkin,
the secretary of the committee.
As Messrs. Siemens, and some other electricians, appear to
prefer the mercury standard of resistance, it is very desirable, for
the sake of uniformity, that there should be no " break of gauee,"
but that they should adopt such dimensions as would make their
standard commensurable with that adopted by the British Asso-
ciation, and now very extensively employed in practice.*
The best means of deducing from that of the resistance the
values of the other electrical constants have scarcelv yet been
fully determined, but little uncertainty or error is likely to arise,
when the value of any one of a series of dependent constants has
been satisfactorily established. On this subject also much in-
formation may be obtained from the " fieport " for 1863.
803. Besistanee CoUt. — It has already been shown (800), that
an tmknoum may be measured by comparison with a knoum re-
sistance, by means of Wheatstone s bridge. The known resistance
consists of a series of coils of insulated wire, wound round bobbins,
and for convenience arranged on a stand. The first of these may
be assumed to be the B. A. standard, and the best means by
'which any multiple may be made up out of the smallest number
of coils is to have die three succeedmg ones multiples of the unit
coil by 2, 2, and 5 respectively, then the next four, multiples by
10 of 1, 1,2, and 5, and so on. Thus with a series of twelve, any
multiple of the unit coil up to 1000 may be obtained; vrith 16
coils, up to 10,000, and so on. The whole of these are generally
joined up in series, that is, say the lower end of the first coil is
connected with the upper end of the second, and so on ; and any
coil is excluded from tne circuit, by joining its upper end to that
of the succeeding coil by a conductor of ample magnitude, which
the apparatus is furnished with ready means of effecting.
804. The Rheoetat. — Whenever large resistances have to be
measured, such as those of a line of telegraph-wire, or of a snb>
marine cable, and in which minute accuracy is not required, the
resistance coils suffice for the purpose ; but if it be required to
measure more accurately any smaller resistances, the object is
most readily attained by the use of the rheostat, whicn, with
luanjr other highly convenient electrical devices, is due to the in-
genuity of Mr. wheatstone. This simple but effective instrument
consists of two parallel cylinders, of equal diameter, which rotate
* If meroory be employed^ it 10 sofrratad Uiat re-distillation mifht ooatri-
bate matcriAlly to it« aniformitjr; t£« writer having found tlut tba pQii^
of water it notably aogmented oj sooocMiTe dialiUataon.
COXDUCTIVITr OF METAM.
451
on their axes ; one of brass, the other of wood, having the thread
of a screw cut over its snrrace. Each cylinder has a brass plate
at the front end of it, against the edge of which rents a spring
connected with a binding-screw ; and a thin wire attached to the
plate on the wooden cylinder, after being wound in the groove of
the screw, is attached to the contrary end of the brass cylinder,
and may be unwound from one and wound on to the other at will,
by means of a small winch. It is evident that all that portion of
the wire, which is wound on to the brass cylinder, is taken out of
the resisting circuit connected with the binding-screws ; and if the
Talne of the resistance of the unit coil in turns and parts of a turn
of the wire be known, the comparative value of any smaller amount
of resistance may be readily ascertained.
805. The conducting power of metallic substances differs re-
markably, but the worst condueting metal is many hundred times
more powerful in this respect than the best conducting liquid.
The following table shows the conducting powers of different
Metals.
Beo.
qoerel.
Ohm.
Leu at
0°C.
Leniat
100°.
Lenzat
200°.
Copper . . .
1000
1000
100-00
7300
64-82
Gold . .
93-6
57-4
79-79
65-20
54-49
Silver
73-6
35-6
136-25
94-45
68-72
Zinc . .
28-5
33-3
—
—
—
Platinum
16*4
171
1416
10-93
900
Iron . .
15-8
174
17-74
10-87
700
Tin . .
16-6
16-8
30-84
20-44
14-78
Lead . . .
8-3
9-7
14-62
9-61
6-76
Mercuiy
3-46
—
—
Potassium .
1-83
—
— —
—
metals according to Becquerel, Ohm, and Lenz ; it likewise shows
that the conductivity is inversely proportional to the resistance,
as determined by Sir W. 8. Harris, from the amount of heat deve-
loped by the electric discharge (733).
The conductivity of metals is considerably diminished by
elevation of temperature ; this is shown by the observations of
M. Lenz at the temperatures of 0^, 100**. and 200° C, in the pre-
ceding table, from which it appears that at a temperature of
200* U., metals lose about half of their conducting po^ver.
The following conductivities of several alkaline and earthy
metallic bases have been determined by Mr. Matthiessen, that of
silver being taken a#100 : —
Sodium, 37-43 ; Magnesium, 25*47 ; Calcium, 22*14 ;
Potassium, 20*85 ; Lithium, 1900; Strontium, 6'71.
806. Having a battery of sufficient power, which if of Crnik-
o o 2
452 VOLTAIC ELBGTBICITY.
shank's or WoUaston's arrangement (793) should oonsist of at
least 40 pairs of four-inch plates, or of 10 or 12 cells of Smee*8 (771),
orDanielPs (774) battery, orof 6 or 6 cells of Grove's battery (7 96),
let the electrodes be connected with the two moveable rods ▲, b,
of the universal discharger (729), and having unscrewed the
knobs, tie on each rod by means of thin copper wire, a pencil of
well-burnt boxwood charcoal, or still better, of the plumoago-like
substance found lining the interior of lonff-used coal-gas retorts.
On movinf? the rods of the discharger, so that the pieces of char-
coal may lightly touch each other, a vivid light will appear be-
tween them, igniting their extremities, and heating the air so
intensely that on allowing the charcoal points to be withdrawn a
little distance from each other, the discharge will continue with a
most dazzling light through the intermediate space.
If the battery be of smaller extent, as a single trough of Wol-
laston's construction, of ten pairs of pUites, a piece of the charcoal
should be attached to one of the electrodes, and a piece of plati*
num wire to the other, which should be brought in contact with
the carbon ; at the point of contact a vivid dazzling light will be
evolved, the platinum wire will be ignited, and if tmn, melted
into globules.
807. The discharge of the voltaic batteiy and consequent evolu-
tion of light, either when charcoal or metallic surfaces are em-
ployed, does not take "phcti at . first without absolute contact.
Jacobi* carefully approximated two metallic points terminating
the conducting wires of a battery of 12 pairs of zinc and platinum
plates, excited by dilute sulphuric acid, until they were within
0*00005 inch from each other, and not the sUghtest evidence of
the passage of electricity was observed; the discharge beinar
checked by the small interval of air. Prof. Daniellf repeated
this experiment with his large battery of 70 cells, and found that
no discharge ensued even on heating the closely approximated
electrodes to whiteness. On transmitting the chai^ of an electrio
jar through these electrodes, so that the dischiu'ge might take
place at the point of separation, the battery current became esta-
olished, and luminous discharge ensued. It is evident that the
discharge of the jar, by producing a transfer of particles of matter
from one electrode to the other, thus formed a conducting medium
for the battery-current ; for an intense luminous discham of a bat-
tery is always accompanied by the transfer of some kindof matter.
808. The evolution of light does not depend upon the combuB-
tion of charcoal terminating the conducting wires, for it will take
place with equal splendour m a vacuum. This may be shown by
allowing the wire nolding one of the pieces* of charcoal, to slide
air-tight through the brass neck of a glass globe. Fig. 466, a
second piece being attached to a wire, fastened to the lower part,
and connected with a brass cup, z. On exhausting the globe of
• Pog. Axmalen, xliv. 635. t Phil. Trans. 1839, p. 93.
-»*" rf
LUMINOUS DIBCHABOE. 453
air, connecting the electrodes of the batterj with the brass cups,
c, z, and approximating the charcoal points sufficiently, an eyolu-
tion of intense and dazzling light will ensue.
These experiments are of the most brilliant ^' *^*
kind of any in experimental science, especially ^f^
when performed by the aid of a large battery, W
as one consisting of 30 or 40 of Grove's ele- JL
ments ; or of the powerfal batteiy of 70 elements ^3^
constructed by Prof. Daniell. In this case a /liN
very curious transfer of carbon from the positive f ^&. \
to the negative electrode is observed, the piece I ^^ /
of charcoal constituting the former presenting \JLx
a conical cavity from this loss of substance. This 3— C.
is beat seen bv throwing on a screen an image z V-r^
of the carbon electrodes of an electric lamp, when TJtC^
the transference of particles of incandescent car- ^-"^-^ ^
bon from the positive to the negative pole be-
comes quite evident. The light thus evolved between charcoal points
has been proposed as a means of artificial illumination. Indeed,
some experiments lately performed speak well for the ultimately
■uocessful application of tbis mode of lighting the streets of large
towns. Several trials made at Manchester have been very satisfac-
tory ; the intensity of the light is so remarkable that the burning
gaa-Iamps in the streets are hardly visible in its splendour : but the
expense of maintaining a sufficient current has hitherto been
found too great to admit of an extensive practical application of
the electric li^t.
809. Prof. Daniell* has observed that when the nenitive elec-
trode, or the wire connected with the last zinc plate of the battery,
is furnished with a termination of platinum, and the positive elec-
trode with one of charcoal, and the discharge of a powerful battery,
aa one of 70 elements, is transmitted, an abundance of intense
light and heat is evolved, and the carbon is carried from the posi-
tive electrode and deposited on the platinum point, which becomes
beantifrilly moulded in its extremity. When the current is reversed,
particles of platinum are transferred to the carbon negative elec-
trode, and are deposited on its surface in the form of fused globules.
810. J3elf^egtu€tting Electric Lampa. — The voltaic discharge
between carbon electrodes having been of late years extensively
employed as a source of brilliant fight, not only tor the purposes of
experimental demonstration, but for the more important practical
purpose of illuminating light-houses, it became necessary to devise
some means by which uniformity, not only in the intensity of the
light, but also in its position, might be maintained. The heated
carbon continually undergoes a slow corabastion, besides which
there is necessarily (808) a continual transfer of carbon particles
from the positive to the negative electrode, the consequence of
* Phn. Tr»nf . 1839, p. OS.
<54 V0I.1
which is that m practice the oonaoniptinti of the pogiti™ electrode
» about twice u ntpid hb that of the negHtivo : it is neceasarr,
therefore, that bj some mechaniam the eJectroJes should be lunde
to approach each other at a rate prupurticinal to their conioniption,
i.e„ an 2 ^ I. This is gonerall; effected by two rncki and wheela,
or chorda and pullejg coonected with a train of wheels, the laat of
which ia held b; a detent controlled h; an eleclro-magaet, actu-
ated b^ the current : the train is released, and allotted to run,
and BO to approximate the electrodes, whenever the currcot is
weakened by their being at l^io great a dialance from each other ;
and when the foil current ia re establlahed hy lh« mulaal approach
of the electrodes, the detent acta, and Blops the train. Varioua
plans have beeu devised hy Hart, Ueiach (Murray' and Heath),
and Holmes in this countrv, by Uubosq and Serrin in France, by
Jasper of Liege, Gresaler of Berlin, and others ; of theae that of
M. V. Serrin will alone be described, aa it poaaesses the important
adranlage of being ablu to be lighted at a distance or extinguished,
bj merely turning on or off the current. SeTcral of the others
require manual interference, on the cessation of the current, lo
prevent, in the absence of control, the approximating mechanism
from jamming together the carbon points to their own desiructiou,
and also to adjust them at the commenceDiont of action. IHiis in-
eenionsly contriredspparatua iarepreaented in Fig. 467, in which
ii.. — I ;..._ — -Hmped vertically ovei each other lo two
llie positive electrode, pr, slides, witb
liGcitnt friction to su]jport the weight of
]■, in a tube, kl, which ia jointed lo two
lual and parallel hoHaDDtarhnlis, ab, ed,
and therefore capable of moving verticajlj ;
'ts movement being limited bj a stod on
he ourside, placed between the points of
BO adjusting screws, u, o. The negative
electrode, hh, slides treaty in a tubular
support, c u, and is leminated below bj a
ack, which oiWr gearing with a wheel, r,
inters a socket, e, Id keep it steady. On
the same axis with r ia a pulley of doable
its dianiolcr, auil a still larger wheel iii
h a pinion, o, which has a wheel
xis in gear wilh the pinion of a
click-wheel, h, whicli kst is held hy a
detent, o, attached to the socket, k l. A
o, aud descends lo be attached t
bottom of p. Tlin end e of a lever, eg,
lurniii|; on a fulcrum, / is connected wiih
a soft-iron cure, v, placed inside a bobbiu
irrRATIFICATIOXS XM ELECTRICAL DIBCHAEGBS. 455
of insulated wire, b ; the other end is connected bj a link, gh^ with
a stud projecting from the socket, k l, and the ▼ertical motion of
the core, m, is ifurther secured by a link, 2, parallel to the lever,
eq. The core, ic, just balances p p and its socket k l, to which is
aaded, if required, a little heavy matter in a saucer-shaped cavity
placed at K to receive any proceeds of combustion that may fall
from the carbon points. The current enters the apparatus at p,
whence an insulated wire wound round the bobbin, b, proceeds to a
fixed stud connected by a chain, ^ with the bottom of p p, whence
it passes up through the carbon electrodes + and—, then down
through H]pand away by the wire, n, attached at d. In order
to follow tne action of the apparatus, first suppose the carbon
points separated, and the current not on ; n H wiU descend by its
own weight, and the rack, e, jiutting the train of wheels in motion ,
will draw up pp (and with it the balanced socket, kl, as far as
the screw, v, permits) at double the rate, as required, because the
pulley is double the size of the wheel, v. As soon as the points
come in contact, the slightest further pull on the chain pulls down
the detent, o, and stops the train. Suppose now the current put
on ; it pursues the course already described, and m being drawn
up into B, the carbon points are slightly separated and the light
developed. As the points wear away, the current is weakened by
increased resistance, and m descending by its own ^vity raises
kl and its attached detent, o, and releases the train, which ap-
proximates the points until the current is sufficiently reinforced to
raise m, which aepresses k l and o, and thus stops the train, it
will here be seen that on the cessation of the current the carbon
points are left in apposition, and therefore ready for its renewal.
Thus the lamp may be with ease and certainty extinguished and
relighted at a distance as often as may be required, without further
manual interference than that necessary to replenish the carbon
points, when worn away.
In the lamp of Mr. Heisch the current is further regulated by
ihe diversion of a portion of it into a second circuit, the resistance
of which is controlled by a rheostat (804) ; this is a convenient
addition.
811. Stratifications in Electrical Disclwrges. — The striated
condition of the electrical discharge in vacuo when two wires, in-
serted into a well>exhausted tube, are connected with the ter-
minals of a powerful induction coil, were first observed by Mr.
Grove,* when the vacuum was rendered still more perfect by the
absorption of oxygen by a bit of phosphorus placed in the ex-
hausted vessel. These curious phenomena have been made the
subject of an extensive series of investigations by Mr. Gassior :
he has obtained very satisfactory results with Torricellian vacua
(484), rendered still more perfect by a method devised by the lato
Mr. Welshjf which consists in attaching a small supplementary
* Phil. Trans. 1862. t Phil. Trans. 1S56.
pta. «s.
ijrphonto thenppnrendof thetnbt, into which anjrandiulbobUe
of Mr or gas may be tlirown bj inclining the tabe. In a Tacnrno
thna obt«iii«d, tliii Htjatiti«l appearance of ad electrical ttiachargv
taking place between two platinum wiren, hemietiRaUj ttt£d
into the tube at diatant poinu, maj In
TSTj well obeervod, and is repreacnted in
^g.468. The stralificationa an ob«n«d
e coDCtTe towarde tbe poaitire wire
to be larger in proportiuo aa the dia-
meter or th« inbe i> increased, aa ia tbe
figore. Thej appear to proved &i>D tbt
'live term inaf towatdB tbe negating
I which thej are aepantod bj a dark
band, or space. The negalive tennioal i«
Burroonded bj a brilliaat glow of light.
takes place. The lauM efiecti msj be
produced, when tbe wirts are reapec-
tivel]' connected with the rnbber aird
prime condacloraf an ordinaij electrical
machine, or wh«n the diechorge of a Leyden jar ia efiected throogfa
a wet string, in order to modiij its intensitj.
The Bame observer has obtained similar results with a water-
battery containing 8620 cells, also with a series of 400 calli of
Qrove's batteT7. More recently Mr. Oassiot has emplajed in hi*
rmearcbex a batterf of 4000 nno-carbon elementa, chargod with
inlphate of mercniy (TS3], and carefiiU; insulated, from which a
cnrrent of yery great potential, ar -~" -- "' '•■' — "
nencr, has been obtained.
613. JnjtutMea/JiatUlaiKXOJH
has obtained several verj curious results, bj introducing a
, . . ,.. ., .=_.... .1. ,. , ^ ^^
u of oooiideiable u
duated reeistance in the circuit; this couiiita of a V-aha«i
tabe. the lege of which are 16 or IS inches toog, filled with d^
tillaa water, in which two ptatinam wires are iiisertad, famg
UiToogh corki in the mouthe of the tab«>, so that bv maing or
lowenng one or both of the wires, anv retioired length of watei^
miitaiice maj be iDterpoaed in the circuit.
When a cnn«nt (i«m 1200 of the elements wm tmianiittad
throagh one of the vacnam-tubes, with a (ew inches onlj of wala>
resistance, a series of 12 to 14 well-^leflned Inminona diaci ap-
peared : SB tbe water-retiatanoo was gradnallj increaeed, thrnu
IB layerv appeared to be abeorbed one bj one into the ricrw
'' '-'- ' QDlil all had diaameared ; bi2 oa
„ , „ e resistance, the bands appnaml
Bucceasivel; to flow out from the positive pole.
With another tube a still more Bingutar resolt waa obtained,
which is represented in £1g, 469. With a certain amount of nsia-
nrsuLATios bt ax absolotb tacuux. 457
) a series of 19 well-defined lonated disos, concsTe towardfi the
[ye pole were produced, as in a ; bat when the resistance was
(((((((((((((((UU
tly diminished, the discs became snccessiTelj doubled, as in
cy being again successively resolved into single discs, on in-
ling the resistance. The negative termination of the tube is
ted in the figure.
another of the vacuum-tubes, the luminoas discs appeared to
e each other in batches from the positive to the negative pole :
xpUination has yet been offered of these anomalous and pecu-
phenomena.
3. Innilation by an AbtohOe Vaeuum. — ^The facility with
th an electric discharge of sufficient potential is transmitted
agh a to-eaUed vacuum, by induction on the particles of
ter intervening between the electric poles or terminals, ap-
-s to increase with the tenuity of the matter, of whatever
1 it may be. But recent investigations have shown that
trical conduction requires the presence of $07ne matter, how-
it may be attenuated, as in tiie carefally-prepared Torricel-
vacuum above described, in which nothing probably remains
a minute quantity of mercurial vapour. By the following pro-
an ahtchOe vacuum has been produced.*
large glass tube, in which towards its extremities two pla-
m wires are hermetically sealed, has a smaller tube, six or
it inches in length, attached to one end of it, in which two or
e pieces of fus^ caustic potash are placed, and the open ends
be compound tube are drawn oat. It is then filled with dry
onic acid gas, exhausted by an air-pump, and refilled several
^8, sufficiently to replace the whole of the air contained in tho
; by carbonic acid. One end is then sealed, and the other end
g immersed in a cup of mercury under a receiver, the carbonic
is exhausted to the utmost extent, and the tube immediately
>d. The smaller portion of the tube containing the potash is
I heated to the pomt of fasion of the potash, and when the
;r is fused, the tube is turned round, so that its interior may
»me coated with the alkali. In a few days it will be found
the residue of carbonic acid has been absorbed by the potash,
thus an absolute vacuum is obtained. On connectine the
inum wires, as before, with two electromotive terminals, no
age of electricity takes place, nntil by heating with a spirit
> the tube containing the potash, an attenuated atmospnere
* G«dot, Phil. Trans. 1859.
458 VOLTAIC ELECTRICITY.
either of the vaponr of that inbstance, or of carbonic acid, is dis-
engaged ; the stratified discharge now takes place, and will ooo-
tinoe to do so, until the disengaged matter is again absorbed by
the potash.
814. Diseharae arrested by Mofftietie Force. — ^Mr. Gassiot has
remarked that tne stratifications are very powerfully affected by a
magnet when the discharge takes place from wire to wire, as in ▲,
Fig. 468. If the poles of a horse-shoe magnet be passed conse-
cutively along the tube, the discharge will assume the form of ^^^ ,
in consequence of its tendency to rotate round the poles in opposite
directions. He also observed that when a carbonic-acid ▼acnum
is placed across the magnetic field (592) of a powerful electro-
magnet, the discharge through it of the water- and Grovels bat-
teries, preyionsly mentioned, is entirely arrested.*
815. If the electrodes of a voltaic batterr be connected by means
of a fine platinum wire, and the current ue sufficiently powerful,
it becomes heated to redness, and even melted. A small battery
will heat a considerable quantity of wire of ^A^ inch in diameter,
a single pair of small plates, igniting an incu ; and a battery con-
sisting of ten alternations will neat to redness about eight inches.
The best mode of showing this experiment, is to roll about
eighteen inches of wire into a long spiral, and place it in the
interior of a ^lass tube ; its ends passing through corks, so as to
be readily twisted round the electrodes of a batteiy. If the cur-
rent be too weak to ignite the wire, it will heat it sufficiently to
communicate a very high temperature to the glass tube in which
it is placed, so that phosphorus may be infiamed by bringing it in
contact with its exterior ; and by immersing this tube in a nnall
quantity of water, the latter may be speedily raised to the boiling
point. The heat evolved by the passage of a current increases
with the resistance opposed by the wire ; hence with difierent
metals the heating power of a current traversing them will be in-
versely as their conducting power (805^. When thin metallic
wires forming the electrodes of a voltaic oattery are placed across
each other, the wire terminating the positive electrode becomes
ignited and melted, whilst that connected with the negative re-
mains comparatively cool ; a fact which as yet has received no
satisfactory explanation.
If thin metallic leaves be subjected to the action of the current
of the battery, they infiame and bum with considerable brilliancy.
This experiment is best performed by attaching a plate of tinned
iron to the negative electrode of the battery, and having taken up
a leaf of any metal on the point of the positive electrode, bringmg
it in contact with the tin plate. In this manner, gold bums with
a vivid white li^ht, silver with an emerald g^reen, copper and tin
with a pale bluish, lead with a purple, and zinc with a daszling
white muue.
• Phil. Traoa. I860.
THE OALYARIC KKIFE, AHD CADTERT. 459
816. The Oatvanic Knife^ and Cautery. — ^The heat developed
bj the passage of a current of large potential through an insuf-
ficient conductor has in several wajs been applied to the purposes
of operative sui^iy. One of these is the galvanic knife, Fig.
470. This consists of a sliding platinum bbde, which is sepa-
Flg, 470.
rated down the middle, nearly to the point, in order that it maj
constitute the heated conductor. The handle is also divided down
the middle like the blade, and having on each side an ample mass
of metal in contact with the blade and also with one electrode of
the batterj, the protruded portion only of the blade constitutes
the acting resistance, and tne heat will evidently bear an inverse
proportion to the length in action. The blade is graduated in
numbers, so that with a given amount of battery power, the tem-
perature wiU ran^e from 600' up to 1500' C. it is stated that at
600° tbe combustion of the tissues is so gradual that hnmorrhage
18 completely arrested, which is in many cases of vast importance ;
but that at 1500** the combustion is so rapid, that the action of the
edfCB very much resembles that of an ordinary steel knife.
in the galvanic cautery a piece of thin platinum wire is wound
round the end of a rod of some ^ood insulating material, as
ebonite, and beyond the point at which the heat i.<i required, the
ends of the platinum are soldered to two thick silver wires. A
verv good construction is to carry a thick silver wire through the
midd& of a rod of vulcanite of suitable size, according to the
purpose for which it is required, haviug the thread of a scrow of
30 or 40 turns in an inch, cut for a suitable distance at the end
of it. The end of tbe platinum wire being soldered to the pro-
jecting end of the silver, is wound on the thread of the screw,
which effectuallv insulates the turns from each other. At the
termination of the screw, the platinum end is soldered to ^another
thick silver wire, which may for convenience be laid in a groove
in the insulator, and for use the two thick wires are connected
with the terminals of a battery. This instrument is very
convenient for internal application, in either normal or abnormal
cavities, as it may be heated after its introduction.
817. Under certain peculiar circumstances, the passage of elec-
tricity through metallic conductors will actually reduce, instead
of elevate their temperature. Thus if two bars of bismuth
and antimony be soldered across each other at right angles, and
they be connected with the electrodes of the battery, so that the
positive electricity will pass from the antimony to the bismuth.
460 ▼OLTAIC ELECTRICirr.
the temperatare of the metiUs will be eleyated ; bat when the
current moves in the opposite direction, viz., from the bismuth to
the antimony, the metals become cooled at the point of contact.
If a cavity be excavated at this point, and a dn>p of water pre-
viously cooled nearly to 32° be placed therein, on the current
passing, it will become rapidly frozen.* In this instance it must
be concluded, that in consequence of the peculiar thermo-electric
relations of these metals (Ch. XVI.) a jK)rtion of their heat
{thermic potential) is inductively convertea into electric potential,
and therefore disappears as heat.
818. Dry Piles. — ^A curious modification of the voltaic battery
is found in those arrangements termed drypUes; these consist of
a large number of alternations of some metal in a state of extreme
tenuity, as silver, combined with one more oxidizable, as tin, and
alternated with pieces of writing paper ; the moisture in which
substance appears to act as the excitmg fluid on the more oxidiz-
able metal. Thus, a pile composed of pieces of tin-foil and
silvered paper, if containing about 200 alternations, will act power-
fally on the gold-leaf electroscope by aid of the condenser. The
piles of Zamboni are the most convenient : these are constnicted
by pasting on one side of a sheet of paper, finely laminated nnc,
and covering the other side with finely powdered black oxide of
manganese. On cutting discs out of this prepared paper, and
piling them upon each other, to the number of 1000, takmff care
to press them together, a little pile will be obtained capable of
diverging the gold-leaves of the electroscope to the extent of half
an inch, on touching its cap with one ena of the apparatus, the
other being connectea with the earth. With the aid of acondenfler,
a pile of but 300 alternations will readilv act on the electrosoope.
These dry piles continue in action dunng several yean, and are
capable of yielding a spark by means of the condenser, although
not the famtest shock, nor the slightest evidence of chemical
action, has yet been obtained from them. The electricity they
yield appears to be of high potential, bnt extremely minute in
quantity, and disappears altogether when the paper discs have lost
all their humidity oy gradual evaporation.
819. A very remarkable form of apparatus for the excitation of
electric cnrrents has been invented by Mr. Grove. It is termed
the gas battery, and consists of a series of platinum plates covered
alternately with jars of oxygen and of hydrogen in the proportion
to form water. It has been long since shown by Faraday that
plates ofplatinum will greatly accelerate the oombmation of these
Kases. By connecting the consecutive plates in pairs, Mr. Grove
aiscovored that in proportion as slow combination of the incloded
gases went on, an extremely weak but very distinct current cir-
culated through the apparatus, and which he succeeded in increas-
ing in tension, until it afforded a minute spark, and gave distinct
* B. Lens. PoggendoriT, Annal. zIIt. p. 848.
6£0VE 8 QA8 BATTEBY.
461
evidenco of being able to effect chemical deoompoeition. A series
of ten cells is stmcient to exhibit minute sparksi and even slowly
to decompose water.
820. The foUovriog mode of constracting this cnrioos battery is
recommended by Mr. Grove, as being the most convenient : — ^a. c,
Fig. 471, is a glass tube with a series of tubular legs attached to,
Fig, 471.
* and opening into it ; it terminates at a in an opening closed by a
glass btopper, and at c, in a ^nnel-shaped opening. Into each of
a series of glasses b, two platinum plates are fixed, one long and
narrow, the other shorter and wider, the former being placed lower
than the latter ; the wide plate of one cell is connected with the
narrow one of the next by means of a platinum wire. The glasses
are then filled up to the top of the narrow plates with acidulated
water, and in the vessel z, filled with dilute sulphuric acid, is
placed a piece of zinc supported on a little tripod. The stopper
Doing removed from the tube, a c, the legs are immersed in the
cells so that each narrow platinum plate may be inclosed in a leg,
the wide ones being excluded and half exposed to the air : the
hydrogen evolved in the vessel z will rise and fill a c, expelling
the atmospheric air. The glass stopper is then to be inserted into
A, and the generation of hydrogen will continue until the piece of
zinc becomes uncovered with acid ; then the narrow slips of plati-
num will be exposed to an atmosphere of hydrogen in the legs of
the tube, the wide ones being exposed to the oxvgen of the air.*
A current of electricity will thus oe generated, the electrode con-
nected with the terminal narrow plate being negative, and that
connected with the terminal wide plate, positive.
821. Having learnt that an electric current excited \}y chemical
action may be made to circulate through conducting wires, and
its force thus brought to act upon any intervening compound body
that it is capable of traversing, it becomes necessary to investigate
• Piul. Tnne. 1813.
462
VOLTAIC ELECTRICITY.
Fig, 472.
,0
more in detail the peculiarities of the changes effected in cam-
pound bodies thus traversed bv the current, and to study the
phenomena of electro-chemical decomposition, or electrvlyM,* as
Faraday has termed it.
Let tne electrodes of a batteiy, consisting of
at least ei^ht or ten elements, in good action,
be placed in the cups, ▲ b. Fig. 472, containing
a few drops of mercury, and communicating
with the platinum plates, p, p. The tubes, o, h,
are filled with water, rendered conducting by
the addition of a little sulphuric acid, and in-
verted in the vessel e, filled with tne same
fluid, over the platinum plates, p, p. Directly
connexion i.s made with the battery, the plati-
num plates will become coyered with bubbles of
gas, which being disengaged, will rise in the
tubes 0, H, in unequal proportion, rather more
than twice as much gas being collected, in a
given time, in one tabe, as in the other. These
gases consist of oxygen and hydrogen, the former being evoWed at
the surface of the platinum plate, where the current of electricity
enters the fluid in e, and the hydrogen, at that surface where it
leaves the fluid. As these gases are eyolved from the deoom- *
posed water, their volumes ought to be to each other as two to
one ; the reason why they are not precisely in this proportion, is
to be found in the partial solubihty of oxygen in water; and
hence, its real volume is rather less than it would be, if this source
of fallacy were absent. In this experiment, the gases are evolved
from both plates simultaneously ; and, although at each instant,
but a single atom of water is decomposed, the hydrogen being
evolved from one, and its oxygen from the other plate, the gases
are not observed to pass from p to p, the fluid between these
electrodes being free from bubbles. This circumstance may be
explained in a similar manner to the electrolysis of hydrochloric
acid (765) : let the two platinum plates be represented by the
letters P, N, the former b«ing that by which the current is sup-
posed to enter, and n that by which it leaves the acidulated
water ; a, b, c, d, are supposed to be four atoms of water lying
between the plates, p, n, each consisting of an atom of oxygen, o,
and one of hydrogen, h : thus
A B O D
^^A^ ^^A^ ^A^^ ^A>^\
P . . . OH, OH, OH, OH, ... IT
The positive electricity entering the fluid at p, decomposes the
atom of water a, with the evolution of oxygen, and causes the
hydrogen to pass towards n ; and this being carried forward by
• *UkiKTpw, and XvM, soIto.
FOSMATION OF OZOKE. 463
the inflaence of the current, decompoBes the atom, b, uniting with
its ozjgen, and repelKng its hydrogen, which in its turn aecom-
poees the atom c, and so on ; at last, the hydrogen of the atom d
ia set free, and is evolved at the surface of the plate h, as the
electricity, by influence of which the decomposition of the atoms,
A, B, c, D, was effected, leaves the fluid at this point. A similar
explanation is applicable to other cases in wbieh eledrdyteB (826)
being decomposed, the elements are evolved at distant portions of
the fluid traversed by the current
822. If, instead of platinum electrodes being employed, the
copper wires themselves be plunged into the dilute sulphuric acid
(821), water is, as before, decomposed, hydrogen being evolved at
the negative electrode ; whilst at the positive, the oxygen com-
bines with the metal of which the wire is composed, forming an
oxide which is dissolved in the acid present.
823. During the decomposition of water by the voltaic current,
a ])owerful phosphorus-like odour of ozone will be evolved. The
evolution of this matter, now recognised as au allotropic form of
oxygen, has been already noticed cuiring the action of the common
electrical machine (691, 2). The odour of this ozone has been
long recognised, but its cause was first traced to the formation
of this peculiar bodv by Professor Schonbein of Bale. The same
substance is evolved under many other circumstances, as when a
stick of phosphorus is allowed to remain for a short time in a laree
glass bottle fiill of moist air ; or, still better, bv placing a litue
ether in a large glass bottle, and then holding in it a previously
heated glass rod, so as to reach nearly to the surface of the ether.
Ozone is a most ener^tic oxidizing agent, a piece of silver leaf
on beinff exposed to its influence, crumbles almost immediately
into oxiae : it is also a most remarkable deodorizer, almost in-
stantly removing the offensive smell evolved by a piece of tainted
meat. Ozone frequently exists in the atmosphere, especially in
the air blowing from the sea, and, in all probability, plays a most
important part in the laboratory of nature. It acts on iodide of
potassium, like chlorine, setting free the iodine : hence a piece of
paper, moistened with a mixed solution of iodide of potassium and
starch, turns blue when exposed to its influence, and thus becomes
a delicate test of its presence in the atmosphere.
824. Ozone, on account of its powerful antiseptic properties,
might perhaps be advantageously employed as a remedial agent
in medicine and surgery : but for this purpose some simple and
effective means of prtnlucing it in large quantities become neces-
sarv ; these are provided in a simple little apparatus represented
in Fig, 473. This consists of a glass tube about two inches in dia-
meter, and 10 or 12 long, terminated at one end by a much
smaller tube, d. Into the other end of this a rather smaller tube,
widi a closed end reaching nearly to the end of the large portion,
is hermetically sealed, the mouth of the inner tube being left open.
Near the point of anion another (nece of emftll tiib«, o, is joinsd
latarsUvinto tbe lu^er one, opening ioto the epaee between the
tnbec. The intide of tbe inner tnbe receives a metallic coating hy
being moistened with gnm-
"S-*n. ^g^tet, aad then haTJng
I some gnaular metal scat-
tared orer it ; and the out-
lide of tbe outer tube ii
ooated with tin-foiL Tio
apparatus is damned on a
Btsad bj two blnding-
BcrewB, A, b; ofvhicb a.
is in metallio connexion
with the outer tin-foil, anil B with the lining of the inner tuba.
When a and b ara respectivBlj oonnucted with the iiaulftted rubber
and prime crnductor of a macblne in action, s powerful iuductin
discharge will take place between the opposed sorfacea of the two
tubes, attended bj a copious evolution of ozone, which will issna
from c, if the noude of a pair of bellows be applied to d. The
same result will beproduced, if the terminaie of a poweifol ss-
condar? coil (Ch. XIV.) be connected wiih a and a.*
82d. If several pieces of apparatus for the deoompodtion of
waler (621) be arranged, so that the cnrrcot of a battery may paa
thmugh each in succcssian, the quantity of gases evolved in each
will tie found to be precisely equal. And if the current, beudea
passing through one of these apparatus be also made to traverse a
metallic solution, as sulphate of conper, the quanti^ of copper
procipilaled in a metallic state, will iwar tbe same relation to the
qiiantily of oxygen and hydrogen collected, as their atomic weif^la.
Thus, a current of electricity capable of decomposing 9'01 graioi
of water will decompose 5878 of chloride of sodium, I63'!8 ot
acetate of lead, Td'HS of aalphateof copper, &c This anses fiom tha
definite nature of tUetro-chemieal or tUOroli/tie thcompontioit,
a fart first demonstrated by Faradaj.f
826. Compound bediee, capable of being decomposed by Ibo
agency of electric currents, are convenient^ termaa ebdrolytet.
Before an electrolyte can be decomposed, it is necessary that it
shoald be capable of allowing induction, and consequent con-
duction to take place thrDneb it ; as the latter cannot occur, in the
great majority of cases, whilst tbe electrolyte is in a solid state,
It must be dusolved in water or fused, in which state it generally
readily conducts the current. Thus, the chlorides of tin, silver,
and lesd, nre readily decomposed when the cnirent is transmiltad
throoeh them, whilsl tbev are in a state of fusion. Some com-
pound fluids exist which refiiae tooondnct the currant, and therefore
• Tbli appantni wu HinstTDCtrd b; Ur. W. Iddd.
t FhikMophicd Iruuctioni, 183^ fth Suin, Mvlion 7.
465
ft few othon
conduct It readily, and yet can BcaroelT be said to yield lo electro-
lytic force ; of this cIbu solphuric acid is *.n eiaoiple.
B27. Wlien varioui electrolytes sre Bnbmitted in a diasolved, or
fiued state, to the action of the current from the voltaic battery,
the tUciro-titgalivt elements ore in variably set free at the positive
plate, vhere the current is suppoeed to eutei the fluid; and' the
~'~~'n>w(ilti'e' elements, at the negati re plate. Thus, if cblo-
of sodium, iodide of potassium, hydrochloric acid, sulphate
IV-*'*'
of copper, nitrate of lead, or fused chloride of lead, be submitted
to the action of the current simultaneously, by placing them in
Teasels connected by platinDin wires dipping mtosach, the chlorine,
iodine, mlphuric, and nitrio acida will be set free at that point
where the positive current enters the solation, or fused maaa ;
whilst at the electrode where it leaves them, the soda, potassa,
hydrogen, copper, and lead, will be developed in an isoUted Btat« :
the evolution of the elements of the electrolyte bearing a conabuit
relation to the direction of the cnrrent traversing it.
82B. Ae the only true teat of the smooDt of electricity in circuit
ii its dedTolytie power, the volta-
iN«t«r or Tolla-elwrtrometer, as it
11 termed by Faraday,* becomes
K valuable instnunent in giving an
approximate measure of the power
of a battery or pile. This consists
of an apparatus, in which water is
submitted to the action of the cur-
rant, so that the gases into which
.it is resolved may be measured. — | 1 j r 1 1 i j i.
A convenient form of this instro- ^ iTZ^r ' *■»...
nwnt ooniiats of a glaM Tesiel, a, '
£1g. 474, cemented into a wooden base, having two plalea of
lilatinnm passing into its interior, connected
by wires with mercury cups, or binding Rg-tn.
screws, B. The glass ressel being filled wi£
dilute BQlphnric add, has a bent glass tube
passing through a cork fixed in its mouth,
BO as to convey the gases evolved into a
graduated receiver, standing in a pmumatio
trough. Avolta^lectrometermay beteodilr
constmcted by fixing two pieces of thick
0 the
.foil into a good cork, so as to dip
of a small wide-moathed
bottle, Fig. 472:
peces will enabli
with any apparatus, and a
with any apparatus, and a bent tube past-
ing through the cork will carry off Uie
~ Thil. Tnu. ISH, 740, 741.
466 VOLTAIC BLBOTRICITT.
gaaefi to be measared. The charge of these volta-electrometers
should be one part of salphuric acid diluted with eight of water.
829. The platinum electrodes employed to effect the decompo-
sition of water assume a peculiar electro-polar condition, by which,
on being disconnected from the battery, they develop a secondary
current, passing in a direction contrary to that of the battery
current. This may be detected by connecting the cups b, Fig. 474,
of a voltameter with a delicate galvanometer (855), after re>
moving the battery electrodes ; when the needle will immediately
traverse, from the action of this secondary current. The electrodes
do not entirely lose this property by pouring out the acidulated
water in which they are immersed, and replacing it by fresh, or
even by washing them with hot water. If a rod of amalgamated
zinc be plangnd into the acidulated water contained in a volta-
electrometer, the platinum plates of which have been previously
connected with a voltaic bifittery for a few minutes, and wires
twisted round its upper end be connected with the two cups b,
decomposition of water will, of course, ensae, and hydrogen will
be evolved from both platinum plates, but in unequal volumes,
nearly twice as much being evolved from one, as from the other,
as has been elsewhere shown.* This curious polarized condition
of the electrodes in all probability arises from the fixation of small
portions of oxygen and hydrogen on their surface ; a view counte-
nanced by the experiments of Schonbein,f who has found similar
propeKies to be assumed by platinum plates, after immersion in
oxygen, chlorine, bromine-vapour, &c.
830. The secondary current here mentioned is produced by the
affinity, or reuniting tendency, of the atoms of oxygen and hydro-
gen adhering to the platinum plates, and is identical with the
action of the gas battery (820) : and a similar contrary electro-
motive force is always generated by the affinity of any chemical
elements disunited by the force of a voltaic current. The exist-
ence of this contrary force may be readily shown by connecting
three or four decomposing cells, or voltameters, arranged in series,
with a battery of moderate power, consisting, for example, of
6 or 8 of Smee's (771), or Daniell's (774) elements, when it will
be found that the contrary electro-motive force of the platinum
plates will considerably retard, if not entirelv arrest the decom-
position of the intervening water ; but if the voltameters be united,
and the electrodes of the same battery be connected with one plate
of each voltameter, an immediate disengagement of the gaseous
elements of water will ensue.
831. It is necessary that all parts of the circuit should be
formed of as good conductors as possible, in order that the whole
electrolytic force of a voltaic current may be effectually exercised,
as the amount of decomposition bears a ratio to the facility with
• Phil. M»Kume, 1839.
t Poggen jor^ Annalen, zlvii. p. lOA.
ELECTB0LT8IS B7 1. SINGLE ELBIfEMT.
467
Fig. 476.
which the cmrent passes. Hence, a flaid not readilj acted upon
by a current in consequence of its resistance, often yields reaaily
to the current, when made to conduct it more freely : thus, pure
water conducts badly and is decomposed with extreme slowness ;
on the addition of sulphuric acid it becomes an excellent conductor,
and is decomposed with I'acility. This is an example of the man-
ner in which the value of G in Ohm's formula (7^7) is increased
by diminishing that of Ji,
832. Although compound batteries have been referred to in the
above remarks, as necessaiy to produce chemical decomposition,
yet it must not be supposed that they alone are efficacious ; for a
single pair of plates properly constructed, is capable of effecting,
by the current evolved, most important decomposing actions in
bodies the elements of which are held together with the greatest
force. Faraday decomposed iodide of potassium (a salt capable
of very ready decomposition by a small force,) alkaline chlorides,
and sulpbates, hydrochloric acid, and even water, by means of a
single pair of plates. M. Becquerel,* by availing himself of weak
cnrrents, aided by " well chosen affinities," succeeded in effecting
the reduction not only of the more readily reducible oxides of
copper, lead, or tin, but even of the refractory
earths glncina, alumina, and silica. This phi-
losopher obtained these interesting results by
means of a single pair of plates, placing the
solution of the metallic salt in a glass tube. A,
Fig. 476, closed at one end by means of a plug
of moistened clay, and immersed in a weak
solution of common salt: on placing a com-
pound metallic arc formed of zinc and pla-
tinum in the solutions, in such a manner tnat
the platinum plate p may be immersed in the
tube containing the metallic solution (to which
M. Becquerel applies the ^neral term of " ne-
gative tube"), whilst the zmc is placed in tbe solution of salt, de-
composition ensues, and after a lapse of time, varying from a
few nours to some weeks, the metal in solution is generally de-
posited on the platinum plate in a more or less crystalline form.
6ecquerel did not attribute the reduction of the metal to the elec-
tric current alone, but conceived that three distinct causes, at
least) concurred in producing this effect. The decomposition of
the water and of the common salt by the electric current set in
motion, and the transferrence of hydrogen and soda through the
clay diaphra^ to the negative tulie, where the alkali unites with
the acid holding the metal in solution, causing the deposition of
its oxide, which, while in its nascent state, is reduced by the
hydrogen, and precipitated in its metallic form on the negative
plate ; thus regarding the hydrogen funiished by the decomposi-
• Traits de rEUoiriolt^ et da Magn^tUme, vol. iii. p. 228, et uq, 1836.
H H 2
tion oF the water bb the Bctaal TsdncinK agent. la wine catea,
a Toartli cause it Bupposed to be aiiperuHded to these, aa vben »
bodj IB lued for the oegiitiTa pjate, for which the metal in aola-
tion has a certain degree of aiSnitr; a well known example of
vhich is fonnd in the redaction of potaaiium from a solution of
pat»ssa,whea sabmittedtocomparatiTelyweakToltaic action whit«
m contact with mercnry. Mercury ia not the only metal applicable
to this porpoae, Becqnorcl having frequently osed iron with suc-
ceBS. He found that the Bolutions of the pure chlorides of air-
coninm, glucinum, titanium, silicon, &c., rat'uacd to yield to the
reducing action of weak electric currentu, nnlil after the addition
of a BDiall quantity of chloride of iron : this the cnrreDt readily
decompoeed, precipitating the iron in a crystalline form on the
platioam plate, which deposit speedily inducMthecommencemeut
of the decompoailion of the more refrsctory salts. This circmm-
stance he attributeit to the affinity of the iroa for the odier metal
tending to the formadou of an alloy, a:id eipressly statei, that
when perftctlu pure the ahoTe mentioned chloiidea diit nit in-
der/fo the tlighiat dMoiapoiition.
833. From a series of experiments on this snligect, it appeared
that the quantity of electricity was not so essential as a contiDuoiM
weak current, and the late Author was induced to prefer the follow-
ing apparatus (which howercr
is out a slight modification of
Daniell's), in cooaequenoe of its
aSbrdiag a constant and ragnlsr
currvnl of electricity of feiy
weak tension, continuing (brie-
Teral weeks or even longer,
without any fresh addition of ex-
citingfluid, Aglaaaoyliiider,D,
'=- 477, 4 inches in length,
diameter, was closed
xit-tm.
Fig. :
audi'
at one end by means of a ping
of planter of Paris O'T inch in
thicknesa :* this cylinder wm
fixed by means of corks inside » cylindrical glaaa vosmI, a, abont
5 inches deep and 4 inches in diameter. A piece of sheet copper,
6 inches long end 3 inches wide, haTinga copperconducting wiie,
F, soldered lo it, wng loosely coileil qp. and placed in the small
cylinder, with the pUstar bottom; a piece of abeel anc,!, of equal
size, was also loosely coiled up, and pUced in the larger external
cylinder, being famished like the copperplate with a condacting
wire, a. The larger cylindrical glass Deing then nearly filled with
weak brine, and the smaller with a gnturaterl solution of sulphate
rf copper, the two fluids being prevented from mixing by the
plailer of Taiia diaphragm, the appaiatui is complete i and if cms
• Phil. Trui. 1S37.
■- »t-i^w«tf^ - jj- -kar «
BECQUSBSL's BJITTBBT. 469
be taken that the fluids in the two cylinden be at the same level,
it will continue to afford a continuous current of electricity for
some weeks, the sulphate of copper being very slowly decomposed.
834. If the ends of the conaucting wires of this apparatus be
immersed in a solution of nitrate, or acetate of lead, no immediate
action ensues, but in about fijfteen minutes, or even less, some
elegant and delicate feathers of metallic lead, which rapidly
increase in sixt, appear at the negative electrode. This effect does
not occur when both conducting wires are of platinum; but when
the negative electrode only was composed of that metal, the re-
duction of the lead continuod with apparently increased energy.
From these experiments, as well as many others of a similar kind
wluch it is unneoessaiy to detail, it appears that to render effective
the reducing agency of a feeble current, or at least of that
elicited by a single pair of plates, it is necessary that the positive
electrode should be a readily ozidizable metal : thus using a kind
of battery of tioo cells, in which the wires forming the electrodes,
and the fluid submitted to experiment, constitute the elements of
the second cell.
835. But few metallic solutions yield so readily as thoee of lead
to the reducing agency of weak currents ; and where a longer
time and continuance of action is required to effect the reduction,
the decomposing apparatus of M. Becquerel will be found a neces-
sary addition to the little battery, with the substitution of a
plug of the plaster of Paris for one of clay. This apparatus is,
in met, a counterpart of the battery itself, and is represented in
Fig. 477, connected with the wires f, o; it consists, like the
former, of two glass cylinders, one within the other, the smaller
one having a bottom or floor of plaster of Paris fixed into it : this
smaller tube may be about half an inch wide and three inches in
length, and is intended to hold the metallic solution submitted to
experiment, the external tube, in which it is immersed, being
filled with a weak solution of common salt. In the latter solution
a slip of amalgamated zinc is immersed, for the positive electrode,
soldered to the wire coming from the copper plate of the battery ;
whilst for the negative electrode a slip of platinum-foil, fixed to
the wire from the sine plate of the battery, passes through a cork
fixed in the mouth of the smaller tube, and dips into the metallic
solution which it contains.
836. When a solution of the chloride or nitrate of iron, copper,
tin, zinc, bismuth, antimony, lead, or silver, is placed in the
smaller tube, and connexion made with the apparatus in the
manner already described, action is almost instantly apparent,
water is decompoeed, and torrents of minute bubbles of hvdrogeh
are evolved at the surlace of the platinum plate (negative elec-
trode), which generally continue for a short time, sometimes, in-
deed, lasting for hours; a circumstance depending apparently
upon the degree of facility with which the metal under expen-
f^^mr^^^gtam-^ _ m^^} ■ "w^.* .<> . ■»
470 .YOLTUO ELECTRIdTr;
ment is reduced. Thus with Bolations of copper, scarcely a'
bubble appears, the metal being almost immediately redaced, all
the hydrogen being probably employed for that purpose from the
instant ot completing the circuit : with solutions of lead, tin, or
silver, the evolution of hydrogen continues for a short time only,'
and ceases as soon as the minutest portion of reduced metal ap-
pears on the platinum plate ; bnt with solutions of iron and man-
ganese, the evolution of gas frequently continues for six, -eight, or
ten hours, or even longer ; the evolution of hydrogen thus seem-
ing to bear something uke an inverse ratio to the ease with which
metal is reduced. After the hydrogen has ceased to appear at
the negative electrode, striaa of the reduced metal, which rapidly
increase, are deposited on the surface of the platinum.
The metals tnus reduced generally, but not invariably, possess
a perfectly metallic lustre, are always more or leas crystalline, and
often very beautifully so, affording a considerable contrast to the
imegnlar soft spongy masses obtained from the same solutions by
means of currents from compound batteries. The crystals of
copper obtained by the process just detailed, rival in hardness and
malleability the finest specimens of native copper, which they
much resemble in appearance. The crystallization of bismuth,
lead, and silver by these means, is veiy beautiful, that of the
former being lamellar, of a lustre approacning to that of iron, bnt
with the reddish tint peculiar to this metal. Silver may be thus
obtained of a snowy and indeed dazzling whiteness, usually under
the form of needles, or cicicular cr3rBta1s.
837. The metallic solutions hitherto mentioned as yielding to
the action of weak currents, are, as is well known, equally acted
on by voltaic batteries, consisting of a considerable number of
alternations, the metal being reduced in a spongy form, often des-
titute of a metallic appearance. But there are some metals which
are deposited from their solutions as oxides only, when acted on by
currents from large batteries, and yet are deposited in a brilliant
metallic form, if submitted to the action of toe currents from the
little apparatus already described. Of these, nickel is an example :
a solution of its chloride or sulphate, when placed in the smaller
tube of the decomposing apparatus, yielding after some hours a
crust of metallic nickel on the negative electrode, often of a sil-
very lustre on the surface immediately applied to the platinum,
that portion of the crust more in contact with the fluid being
generally black, and frequently covered with a layer of hydrat^
and gelatinous green oxide.
838. For the reduction of silicon, let a solution oF fluoride of
silicon in alcohol be prepared, by passing a current of the gaseous
fluoride into strong alconol. On filling the decomposing tube with
this solution, and making the connexion with the battery in the
manner already described, bubbles of hydrogen were copiously
evolved at the surface of the platinum plate (negative electrode),
^*-_ ;;^»> ■iw"
REDUCTIOX OF ALKALIHE METALS. 4?!
oantinning from eight to ten Lours, when the platinnm appeared
to be tarnished, and in twenty-four hours a copious deposit of sili-
con had taken place on the platinum, to the surface of which it
firmly adhered. Around the reduced silicon, and suspended in the
fluid, was a dense gelatinous cloud of silicic acid. On quickly
withdrawing the slip of platinum, dipping it in water, and then
pressing it between folds of bibulous paper it was dried, and
freed from any adhering solution. The silicon was nearly black
and granular, under a ^ns, exhibiting a tendency to a ciystalline
form. It was not deposited on the platinum in a confused and
irregular manner, but in longitudinal striae, which appeared to
follow the direction of certain lines of minute eminences on the
surface of the piece of platinum, produced apparently by scour-
ing it with fine sand and a piece of cork before being used for the
construction of the negative electrode.
839. Potassium and sodium may be readily reduced by these
weak currents, and obtained as amalgams by using a modification of
the decomposing apparatus before described. Let the smaller tube
containing the metallic solution be replaced by a small glass funnel,
A, Fig. 478, the beak of which has been ^^ ^^g^
carefvdly filled up with plaster of Paris :
&x on this plaster floor a piece of glass
tube closed at one end, about 0*5 inch in
length, and 0*2 inch in diameter, and half
filled with pure mercury ; this tube should
not be placed vertically, but inclined so as
to form an angle of about 40** with the
plaster floor of the funneL The external
cylinder communicates as before with the
copper plate of the battery, by mesns of a slip of amal^mated
sine, z, dipping into the brine it contains : a solution of chloride of
potassium is to be poured into a, and a piece of platinum wire
connected with the zinc plate of the battery being twisted into a flat
spiral at one end so as to present a larger surface, immersed in the
mercury contained in the little tube submerged in the saline con-
tents of the funnel. The circuit being thus completed, electric
action soon becomes apparent, bubbles of hydrogen being evolved
from the surface of the mercury (which now formed the negative
electrode) in a very curious manner, not in confused and rapid
streams, but in large and distinct bubbles, which very slowly
appear, and perform several gyratory movements on the surface of
the fluid metal before they are detached. In about eight or ten
hours the mercury will have swollen to double its former bulk, and
if it be removed from the little tube as quickly as possible, and
poured into distilled water, an evolution of hydrogen gas takes
place from its whole surface, and the water becomes alkaline from
the formation and solution of the oxide of potassium or potassa.
The film of mercury adhering to the platinum wire remains on
I l/t m-
472 YOLTAIC ELBCTBIGITT.
•
it for Bome days, giving it the appearanoe of haring been amal-
gamated.
840. Of all the saline solationa that the late Author sabmitted
to experiment, none afforded soch conclusive and interesting re-
sults as those of ammonia. The compound ammonium being
reduced with almost as much ease as copper or tin, when a solu-
tion of the chloride (hydrochlorate of ammonia) is submitted to
the action of the voltaic current in contact with mercury, in tho
same manner as chloride of potassium or sodium, the same adhe-
sion and creeping up of the mercuty along the wire is observed,
and after a few hours the fluid metal swells to five or six times its
former bulk. On removing it quickly and drying it, b^ allowing
it to fall on bibulous paper, the amalgam of ammonium is obtained
of a buttery consistence, possessing a dull silvery colour, and
yielding a peculiar crackling, or (if it be allowed to use the expres-
sion) an emphysematous sensation to the finger on pressing it : on
being immersed in water it very slowly gives off hydrogen, and
yields a solution of ammonia.
841. By far the most satisfactory method of obtaining this
amalgam is by using for the negative electrode a piece of platinum
wire coiled up at one end, after it has been amalgamated by dip-
ping it into the aromoniacal amalgam obtained by the last de-
scribed process (840). A minute quantity of mercury is thus made
to adhere to the wire, which being connected with uie zinc side of
the battery, is dipped into a solution of hydrochlorate of ammonia
contained in the smaller tube of the apparatus used in effecting
the reduction of silicon (838). The circmt being completed, a few
bubbles of hydrogen are disengaged from the amalgamated wire,
which soon cease, and in an hour or two, a leaden grey spongy
mass is observed adhering to the wire, which is sometimes suf-
ficiently bulky to fill the tube, and putting on much of the external
appearance of a mass of cellular galena. This mass consists of a
spongy amalgam of ammonium, containing a very minute proi>or-
tion of mercury; it is lighter than the solution in which it is im^
mersed, for on adroitly separating a portion of it, it rises to the
surface and rapidly decomposes water, hydrogen being evolved and
ammonia formed.
It is a very curious and interesting fact,' that although this
spongy ammoniacal amalgam cannot be kept immersed in water
even for a few instants without the formation of ammonia, yet as
long as it is connected with the negative electrode of the battery,
it may be preserved without change for days and weeks. The
instant the connexion with the battery is broken, a mass of this
amalgam, as large as a walnut, appears to vanish in a few seconds,
torrents of minute oubbles being given off, and a scarcely appre-
ciable quantity of mercury being left on the wire. On again
closing the connexion with the battery, decomposition recom-
mences, and the amalgam is reproduced.
SLIOTBOLTBD BT A XACBIHE-CUBBBHT.
473
JV-479.
842. The deoompomtioii of seyeral electrolytes, as sulphate of
soda, iodide of potassiam, &c^ may be effected by means of a
cmTent of franklinic electricity, from the electrical machine. For
this pnrpoee, place upon the table of the unixersal discharger a
piece of bibmoiu paoer, soaked in a solution of some alkaline
combination, as iodide of potassium ; fix to each of the sliding
rods of the apparatus a piece of fine platinum wire, to serve as
electrodes, which must rest
lighily upon the paper, about
an inch from each other. Con-
nect one of the rods, n, Fig. 479,
with the rubberof the machine,
or with the earth, by means of
m chain, and the other, p, with
the prime conductor by a wire,
or, still better, by a |»iece of
wet string: on workmg the
machine, the salt will oe decomposed, iodine being set free at
that wire which is connected with the conductor, or at the point
where the current of electricity enters the compound ; and the
alkaline base at that which is connected with the rubber, or
where the current escapes. The alkaline element may be detected
by placing a piece of turmeric paper, moistened with the solution
employed, on the table of the discharger, in place of the ordinary
bibulous paper.
843. It is not necessary to use metallic conductors to effect
electrolysis by franklinic electricity. To show this, take two tri-
angular pieces of paper, a, b, Fig. 480,
B being coloured with litmus, and a
with turmeric, place them base to base
on a glass plate, and moisten them with
a solution of sulphate of soda ; let a
pointed wire, p, proceeding from the
prime conductor of an electrical machine
in action be placed a few inches from b,
the sulphate of soda will be decomposed, the acid will be set free
at B, where the electricity enters the paper, and will turn its blue
colour to red, whilst the soda will be set free at a, staining it
brown. This elegant experiment of Faraday is conclusive against
the old notions of the electrodes inducing decomposition by acting
as attracting surfaces, or poles.
844. Electricity is evolved not only during chemical decompo-
sition, but has been supposed to attend chemical combinatum / a
statement first made by BecquereL The truth of this opinion
has been, by many, either aJto^ther denied, or limited to the
case of the combination of nitnc acid with alkalies. That an
electric current, certainly of extremely low tension, is really
evolved during the combination of sulphuric, hydrochloric, nitric,
Fig.iSO,
474 VOLTAIC ELECTKICITT.
phoBphoric, and acetic acids, with the fixed alkalies, and even with
ammonia, is readilj demonstrable, bnt what the immediate cause
of this evolution of electricity may be, is (questionable. In the
case of electricity evolved during the combination of nitric acid
and potassa, or Becquerel's battery, as it has been termed,
Daniel] *8 view of the composition of salts enables a tolerably
ready explanation to be proposed. The npparatus consitita of a
tube closed by a plug of pipe-clay filled with a solution of potassa,
and immersed in a vessel of nitric acid. Plates of platinum fur-
nished with conducting wires are immersed in the acid and alkali.
As soon as these conducting wires are twisted together an electric
current takes place, oxygen rising in bubbles from the plate im-
mersed in the alkali, whilst hydrogen is evolved in the acid and
immediately acts on it, tinging it yellow from the formation of
nitrous acid, the hydrogen abstracting a portion of oxygen from
the nitric acid. Meanwhile combination of the acid and alkali
occurs through the clay diaphragm, and nitrate of potassa is
slowly formed.
. On Daniell's hypothesis, nitrate of potassa, considered in its
electric relations, is a compound of N O^ + E, and not of N 0^ + K 0,
and hence was termed oxynitrion of poteusium. Similarly
aqueous nitric acid would be oxynitrion of hydrogen consisting of
N Oe + H, instead of N 0^ + H 0.
In BecquerePs apparatus, the following elements are therefore
arranged on each side the porous diaphragm, represented below
by the double vertical line || :
K0|1N0, + H= I NO« + k| +H0.
Thus an atom of oxygen is set nree in the alkaline solution, and
one of hydrogen in the acid, so that in this case the evolution of
electricity may be really traced to chemical decomposition ; con-
sequently Becquerel's arrangement does not present any exception
to the general rule.
845. The statement, that in cases of electro-chemical decomposi-
tion, the changes which take place in the electrolyte are continuous
through a line of molecules, and not limited to those in contact
with the electrodes (765), meets with an interesting illustration in
the well-known experiment in which an alkali appears to traverse
an acid without combining with it ; and which has been erron^usly
regarded as a case of suspension of the laws of chemical affinity.
Let three cups, a, s, b, Fig.
■^* ^^' 48 J , be placed side by side,
and connected by means of
pieces of lamp-cotton moist-
ened with a solution of sul-
phate of soda. Let a and b
oe^ filled with a solution of
this salt, and the central cup,
TRAX8FERRENCB OP CHEMICAL ELKHENTB. 475
By with dilute salphnric acid. Let a pomtiTe platiniim electrode, c,
dip Into A, and a negative electrode, z, dip into b. The voltaic
current will now enter the fluid in A, and escape from b through z,
traversing s in its course. Electrolysis of the sulphate of soda
will take place, its acid with oxygen heing set free in a, and the
sodium witl ]^ass through the sulphuric acid in s, and reach b, so
that a quantity of free soda irill soon be found in b ; the sodium
being oxidized at the expense of the water. It is evident that
this alkaline body must have traversed the acid in s, with which,
indeed, it for an instant combined, and the resulting sulphate of soda
being decomposed by the current, the soda ultimately appears in b.
846. That in experiments of this kind, the base really combines
with the acid it is made to traverse, is proved by using a salt with
the base of which the acid forms an insoluble combination. Under
these circumstances it is removed from the influence of tbe current,
and does not reach the third cup. Place in a and b, solutions of
chloride of barium, and in a, dilute sulphuric acid ; on the current
Dassing, the contents of a are decomposed, chlorine is evolved, and
oarium set free ; this is conveyed in the manner before described
to the middle cup, and here it is arrested in its course by the acid
which, in combining with it, forms an absolutely insoluble salt, the
solphate of barytes, which falls to the bottom of the vessel, and then
neither barium nor its oxide reaches the cnp b. Hence the salt
chosen for experiment must be one of which the base fonns a soluble
combination with the acid in the middle cup 8 (Fig. 481).
847. When water containing a very minute proportion of saline
matter is subjected, in two cups connectoil by threads of moistened
lamp cotton, to the action of the current, not only are the elements
of tbe water set free, but the traces of saline matter are decom-
posed into their constituents, so that the acid will appear in one
cup and the base in the other. It has been observed by Daniell,
that if a solution of sulphate of soda be thus treated, a voltameter
being included in the circuit, not only is the Quantity of mixed
gases collected in the voltameter the same in bulk as that set free
in the solphate of suda solution, as might be expected (825}, but a
quantity of the sulphate is itself decomposed, equivalent to the
gaseous elements evolved from the decomposition of water in the
voltameter and in the solution of the sulphate. Thus, the current
which decomposed an atom of water in the voltameter at the same
time decomposed an atom of water and one of sulphate of soda in
the apparatus connected with it; forming an apparent exception to
the general law (825). To meet this difiBculty, Daniell has sug-
gested that the elements of the water in which the salt is de-
posited, are separated by a secondary action. According to this
view, sulphate of soda consists of S 0^ + Na, instead of S 0^ + Na 0,
being in the prooosed nomenclature an oxysulphion of sodium.
Then, when a solution of this salt is decomposed by an electric
current, S O4 is set free, and immediately acts on the water, taking
476 YOLTAIO ELECTBICTTT.
ftn atom of hydrogen to form the aqueo-acid, and thus an atom of
oxygen is evolved from the water. The sodium then acts on an-
other atom of water to form soda with oxygen, and sets free its
hydrogen ; and thus the decomposition of an atom of water and
one 01 sulphate of soda by a current, which is alone capable of
decompobinff one atom of water when the salt is absent, is attri>
butable to the secondary action of the assumed elements of the
salt on the water. The same ingenious explanation applies to the
electrolysis of all solutions of oxy-salts.
848. It has been already observed that salts materially differ in
the facility with which their elements aro evolved uniier the in-
fluence of the voltaic current. This difference is attributable to
the varying amount of intensity with which these elements are
united. Thus, as has already been shown, the current from a
single pair of platinum and zinc plates is capable of decomposing
a solution of iodide of potassium ; chloride of sil ver kept fused in
a glass capsule is readily resolved into chlorine and metallic silver
by the same weak current. On the other hand, a solution of sul-
phate of soda, and nitrate of potass in a state of fusion, resist the
action of this current, but if its intensity be exalted by the addi-
tion of a little nitric acid to the exciting liquid, it is then capable
of overcoming the force which binds the elements of these salts
together, and they are readily evolved at the surface of the respec-
tive electrodes. In the following list of electrolytes, the firet three
are decomposed by the current from a single pair excited by dilute
sulphuric acid, while the last four bodies do not yield until after
the addition of nitric acid to the exciting liquor.
Chloride of lead, fused.
Iodide of lead, fused.
Hvdrochloric acid.
Dilute sulphuric acid.
Iodide of potassium, dissolved
in water.
Chloride of silver, fused.
Frotochloride of tin, fused.
849. The only difference between franklinic and voltaic eleo>
tricity consists in the low potential or intensity of the latter, as
compared with the former, which it vastly exceeds in quantity
(767) ; their identity has already been demonstrated (732). By
availing himself of the law of the definite nature of electro-chemicid
decomposition, Faraday has, by a series of very ingenious experi-
ments, succeeded in demonntrating the enormous quantity of elec-
tricity naturally associated with the elements of a grain of water.
He found that when two wires of platinum and zinc -^ inch in
diameter were immersed to the depth of {■ of an inch in a mixture
of one drop of sulphuric acid and four ounces of water, as much
electricity was set free by this miniature battery in about three
seconds of time, as was yielded by an electric battery (724) having
3500 square inches of coated surface, and charged by thirty revo-
lutions of a plate machine 50 inches in diameter. The quantity
of potential electricity yielded by the machine, and sufficient to
FBAHKLIHIG AJID TOLTAIC CUBSBKTB COMPJLBED. 477
kill a small animal, was thos evolved bj the solution of an almost
inappreciable portion of zinc wire. By an extension of this reason-
ing, it would appear that 800,000 charges of the electric battery
would be required to decompose a grain of water, a quantity
capable of being supplied at an infinitely lower potential by a pair
of platinum and zinc plates, sufficientljr excited by an acid to keep
Ignited during rather less than four minutes, a platinum wire y}^
inch in diameter.
In a voltaic battery, containing any given area of exciting sur-
face, it is found that, in proportion as the number of elements is
increased, and their size diminished, the current will become more
assimilated to that evolved by friction, and the discharge between
the two electrodes will be more disruptive. It may be remarked
that in a battery containing a large number of elements, 400 for
example, the divergence of the gold leaves of an electroscope in
connexion with one electrode, may always bo observed ; and that
divergence will be doubled when the other electrode is connected
with the earth.
Rbferbhces.
To the no less excellent than laborious Traite de TElectricit^
et dn Magn^tisme, of Becquerel, the student is referred for an
elaborate account of all that is valuable in electrical science. The
papers of Faraday, in the Philosophical Transactions, now fortu-
nately collected into a separate work, cannot be too attentively
studied by those who wish to acquire a thorough acquaintance
with this beautiful science. Nor ought the writings of Pouillet,
Coulomb, Poisflon, De la Bive, and many other Continental philo-
sophers, as well as those of our talented countryman, Daiiiell, to
be overlooked by the student. Noad's Manual of Electricity is an
elaborate treatise, from which much useful information may be ob-
tained ; but it is unfortunate that the old " fluid " notions are still
retained. Ferguson's electricity (a volume of " Chambers' Edu-
cational Course'') is an elementary work of great merit, and
Inrought up to the knowledge of the present time.
478
CHAPTER XIV.
ELEGTR0-DTNAMIC8.
850. The direct influence of the discharge of franklinic e]ec-
tricit,v on magnetic needles, was studied long ago by Franklin,
Beccaria, Wilson, Cavallo, and others ; the power it exerted of
communicating, destroying, or reversing polarity was also pointed
out. But it was reserved for Prof. Oersted, of Copenhagen, to
announce to the world the existence of a new and peculiar force
reciprocaUy exerted between magnetic needles and the connecting
wires of a voltaic battery ; a fact, to a certain extent, theoretically
anticipated in a work, b^ the same philosopher, published twenty
years before his great discovery, which was made in 1820.
851. Let a copper wire, connected with the two poles of a ▼ol-
taic arrangement, be stretched parallel to a magnetic needle, sup-
ported on a pivot, and free to move in a horizontal ^lane. The
magnet will instantly leave its position in the magnetic meridian,
and after a few osciiiations will
^' assume, and retain, a position
at, or approaching to, right angles
to the wire, so long as the cur-
rent continues to pass. To show
this, let a thick brass wire be
supported by two pillars, ▲, b,
Fig. 482, passing through their
long axes, and soldered to the
binding screws, c, z. The mag-
netic needle, n s, is supported by
a pointed wire, w, fixed in a hollow stem, i), in which it may to
placed at any required height by means of a screw, n is the north,
and 8 the south pole of the needle (593).
A. Screw the positive electrode of an electromotor (767^ into
c, and the negative into z, then the current will pass in the direc-
tion A B, as shown by the arrows ; and the needle n a, placed in
the magnetic meridian, will move from its previous position; its
end, M, moving towards the voest.
B. Ix)wer the wire w into the socket d, so that the needle irs
may be beneath the conducting wire. On making c<mnexion with
the electromotor as before, the end m of the needle now moves
towards the east.
C. Remove the wire w, and the magnetic needle, replacing it
IXTEBSOS. 479
with one arranged as a dipping needle, parallel to, and on the
same horizontal plane with the condocting wire a b. On making
connexion with the electromotor (A), the end n of the needle wiU
he elevatedj provided its poles he in the same position as hefore,
and it he placed on the west side of the wire a b.
D. Arrange the apparatus as before (C) , but let the dipping
needle be placed on tne east side of a b ; its poles retaining their
former direction. The polo n will then be depreised.
If these experiments be repeated, and the connexion with the
electromotor oe reversed, so that its positive electrode may be
connected with the screw z, and its negative with the screw c, the
direction of the magnetic needle will in each case be reversed.*
852. To impress on the memorv the directions of these devia-
tions, the following formula devised by Ampere is extremely useful :
let any one identify himself with the current, or let him suppose
himself to be lying in the direction of the positive current, his
head representing the copper and his feet the zinc plate, and
loolnTig at the needle; its north pole will always move towards
his right hand. This will be readily apparent if the student will
suppose himself to be lying on the wire a b (Fig. 482), in the
direction of the positive current and looking towaras n s, and will
then repeat the above experiments.
853. The amount of action exerted on the maenetic needle by
the electric current appears from the researches of Biot and Savartf
to diminish with their mutual distance, its intensity being in the
inverse ratio of the square of the distance of the needle from the
wire, when considered as applying to a small section of the con-
ducting wire ; and of course proportional to the sine of the angle
of deviation. But as the length of the current may be considered
to be infinite with regard to the needier, its intensity is in the in-
verse ratio of the simple distance, when considered as being exerted
by an indefinitely long conducting wire.
654. To avoid the trouble and delay of reversing the direc-
tion of the battery current in electro-magnetic experiments,
several kinds of apparatus have been contnved, most of which
are very inconvenient, from their requiring mercury to fit them
for use. The late Author devised an instrument which, when used
to connect the battery with any apparatus, allows the direction of
the current to be readily changed, without using that fluid metal.
This consists of an elevated curved ridge, a b, Fig. 483, composed
of three stout pieces of brass, a, p, b, separated at the dark por-
tions by wood ; a and b communicate by means of a thick wire
passing under the base of the instrument. Two thick quadrangular
* A simpler form of this apparatuB is made by Messrs. Elliott Brothers,
in which the needle w s is attached to the wire a b, which is capable of
reTolving round its own axis.
t Bio(« Precis de Phjsiqae, tom. ii. p. 707. Paris. 1824.
• 480
SLBCTSO-DTirAMIOB.
ban of brass, b, b, pass throngh a circular piece of wood, p,
and terminate in the binding screws, g, h. The piece, f, moves
on a centre, the bars, d, b, being made to press npon the cnired
ridg^ A B by means of a screw
^. 48S. at the centre of motion, r.
Two other binding screws, l, m,
are connected with a and p.
If the bars be placed as in
the figure, the positire elec-
trode of a batterj being con-
nected with G, and the neg*-
tiye with h, the voltaic cor-
I rent will flow from l to ic, if
thej be connected bj means of
a wire, or any conducting apparatus. Let the bars be then moTcd
until the end of B rests on b, d will of course be on p, and the
current will move in the opposite direction, or from m to l. When
this instrument, which is called an inversor^ia used, a drop of
oil should be placed on a p b, to allow d b to glide readilj over it.
855. From a consideration of the above experiment, it is ob-
vious, that if a conducting wire be bent into the shape of a rect-
angle, the needle being placed between its two horizontal branches,
the action of a cnrrent traversing both will be to move the needle
in the game direction ; for although one branch is above, and the
other below the needle, yet as the current moves in each in oppo-
site directions, its effects on the magnet will be the same.
In this manner the means are obtained of increasing the action
of a current on the needle to an extraordinary degree, and conse-
quently a mode of detecting traces of potential far too minute
to act on the gold-leaf electroscope: for these valuable contrivances
we are indebted to the ingenuity of Schweigger. ^ The commonest
form of these instruments, galvanometers, muUipliers^ or reome-
ter$, as they are termed, consists of a rectangular coil of copper
wire BBS, Fig. 484, containing
-^* 4S^ about twenty convolutions, the
wire being vfuuloited by cover-
ing it with cotton or silk, to
prevent the transmission of the
current from one coil to another,
by actual contact The cups,
G, s, are connected, respectively,
to the ends of the wire coil, b b s. A magnetic needle supported
on a pivot, is placed in the centre of the coil, and a card is at-
tachea to the board, a, on which the coil rests, which is graduated
in degp'ees from a line coinciding with the plane of the coil. On
connecting any source of feeble electricity with the cups, c,s,
which contain a little mercury, and are thence called merewry-
cups, the current will traverse the coil, and the needle will move
OALVAKOMBTEK.
481
to the east or west, according as the direction of the current (851)
is from n to a, or from b to n.
856. This form of multiplier will, it is obvions, detect the exist-
ence of a current only when it is sufficiently intense to overcome
the directive action of the earth, which tends to retain the needle
in the magnetic meridian. If the current be too feeble to produce
this effect, its existence cannot be detected without using a much
more delicate instrument. To the late Chevalier Nobili science is
indebted for the application of the cutatie needle to the multiplier,
thus giving the means of detecting the existence of currents
of the lowest potential, by nearly annulling the directive action of
the earth on the needle. The following is a description of one of
the many forms of multipliers that have been proposed, and which
is preferred, on account of its extreme sensibility, and the facility
with which it is used : it consists of a firm base of hard wood,
Fiff. 485.
LL, Fig. 485, excavated
in the centre, and sup-
})orted by three level-
ing screws, of which
two are shown in the
section. The coil, a b,
is formed of copper wire
one-sixtieth of an inch
in thickness, and about
two hundred feet in
length, carefully insu-
latra (855), to prevent
lateral contact. Tins
wire is wound on a thin
wooden frame two inches aqtuxre, the upper and lower portions of
which are about one inch apart ; this frame is attached to a cir-
cular piece of wood passing through the board ll, and ending in
the grooved wheel k, connected by means of a piece of cord with
the pulley f, moved by the handle k, so that when the latter is
turned, tne frame and coil a b are moved round their vertical axis.
The ends of the coil, after being twisted into a loose spiral, pass
through the board, and are soldered to the binding screws g o.
The magnetic needles are thin, light sewing needles, about one
inch and a half in length, possessing very nearly the same de^e
of magnetic intensity, and fixed about three-<^uarters of an inch
apart, on a piece of aluminium wire, as shown in the small figure,
with their pules opposed in direction. The connectingpiece is placed
in the vertical axis of the coil, so that the lower needle may be
between, and the upper one above, the convolutions of wire ; the
connected needles being supported by a filament of unspun silk, or
fine human hair, from the arm c, are readily raised or depressed
by means of the screw d. A circular piece of card graduated in
degrees is placed on a b, and before the instrument is used the
II
fuldg of wire on tlie frame ihould be placed e»«:llj pknilel totha
needles by moving k. A glnea ehsde is pl«ced over the appuatiu
to prevent »ny diBlurbince ensuing from cutrenU of ur. U may
Bource of an electric carrent te connected with the Krrmt O, o.
the needles will imniedialely deviilo from tiieir pretiou* poeitka,
the intensily of the current being, in gene™!, u the nne of llie
ande of deviation, eipeciallj aa the needles used always !»«*•
Bomo slight directive power. To illuslrste the delicacy of thiain-
etr^iment, place on the ton of one of the braa screw, a, a. a (hop
of spring water, and having a ifSece of anc connecled to the ocWr
screw initner«e its eilremity in the drop of water, the needles will
inimediatoly be moved by the weak currant thus eicited. TV
calvanomoter conatitutei one of the most valuable inshmmenU in
olectro-chemioal teBear.;hea that we are aequaiuted with
867. The Aitalie Needle.— A» this fonua a very important p^
- ■ ' ■ '■ - — ' meter, it is necesaaiT- *"**''■ ""*-
be claarly uuderstai
connected needles a
poddoD, M in Fi^. 486. Let theoi
be sappxaed to be inclined at a sduII
angle with each other, as ^ B, Ci>.
aod let A and u be the north pole* of
the two needles. Then (ntfqndcg
them to be suspended perpendicularly
them to be suspended perpendicukriy
to the plane of Che papei^ tbe earlh'n
foree will act on them in the direc-
I tions of the arrows.
■ suppoMug the msgne
the needles to U unequal, let that of a a be In that rf c » a.
cu-OA- then the resultant of Ihe parallel momenta of < and c
wiU be a moment equal to Iheir sum acting in the aame direction
at o (761 Join k, the interaoction of a a and c u, witli o, and
draw KP biaecting, and also perpendicnkr to, a c. Then tl>e mi
effoct of the moments at A and C wil! be equal to that of a .mgle
moment actioR on the short, half-mspnet k ... «.d the p<«.t»n of
equilibrium of the system ip ■'■-• ■" "'"■■'■ -"» >"«■»* t mi-
isition E y ; that is, th"
i-aEwilhlhe niaBn«l''= n....-.— ..
are equal, then to vvill coi.icide with . r, and the s
rest exactly across the meridian. IPthe momentaare _ . ,_ .
tKe nrKTlo A E D decreasBs, E 0 will approach and would nlUmalely
■ -X -.;.l, „r- \wr,ee if the moments of the needlea are on-
in iW matnictic meridiun. If ihe needles bo quite puaUel, and
th«r moments also equal, then « r wiU b«oino cT^ac«it ;_ then,
and then only, the system will be ab»luMly ortrt.^*™ m
fixed posilion of eoutlibnum, and the n«dle. if deflected by ■
cuncutTiill remain when ths deflecting fon-e ccaaes to act, aad
Tenieut. In practice tbe nt
if Dot very nnt-qual, will n
the Btrongi>r, but will have s Tsir ai
>r sensiliilily; na they
:,C;
[1 quite equal, nliich they gcnerully ai
in iagtrumeiils of the greatest delicituy, ihey will rest exHClly |jer-
peodicular to the mrriiliau : aail tlie length of the virtual mo^el
■ FbeiQg very Biuall, the oscillation uf the Bystetn will be very alun.
869. Du LfoiK-IleymoDd, whose t-laburate rexeitrchee on the cur-
renli of electricit; exisUng lu aniiuul stniclurea have attracted bu
much iotereltj baa carried Iha pcrfecUun of galvBDuniEteni to an
almost incredible sxlent. One of bis ioitrumenta, nsed to detect
the musuular currents, has a coil of line ineuhiled copper wire.
3360 feet long, and -0067 inch ia diameter, wound 4650 ti
roond the frame. The other, the sensibility of which '
that it detects the electricity floniug through the i
wire 16,760 feet, or more than three miles long, wound 34,1 1
tiniee round the ^me. The Epnce in which the lower needle u
■uspeaded is but ODe-leDthofna inch in height. Tlietwo needles,
which are 1'5 inch long, with llitir cuiinecting piece (made of tor-
toise-sliell), weigh only 4'9 grains, and are bu equally magnetised,
that they perform a single libralion in 33 beconds, A minute
correcting maguet was applied to the astatic needle in this inatru-
ment, but its employment is unnecessary. lusLruitieats of equal, if
not of greater ecmtibilily, have been constructed hy Mr. Becker, in
Londuii, in wbicli tbe subetituiion uf aluminium tor turtoisS'Sbell
saves a small portion of tbe weight.
839. TItomion'i Stfiteting Oahianojaeter.^Tim peculiar fea-
tures of this itistniment, designed by Sir W. Thocnsoii for tele-
graphic purposes, are that the galvanometer o, Fig. 487, consists
of a circular coil. In w .„
the centre of which ^*- **'-
a very small magnet,
attached to the back
of a small circular
pended. A small
^Qcil of light pass-
ing tlirougb a tiole
in the chimney uf u
lump, t, fulls upon
is theuce reflected
on to a scale, s, on
which the deviationB
of the magnet are read, with the advantage of the angular dis-
placeoient of the reHected pencil of light being doohle that of the
*^.«».
484
Ftf.taa. magnet itself. An a^uBline magnet, H, >!iile«
I up and down on the Tertical st&ia, and moves
horiKint»llj_by_ the tangent rerew.T. Thi« is
designod principnlly for telegraphic porposea,
and la at) excreuingly naeful instrument. The
actual size of the magnet, lit, and mirror em-
ployed is represented in Fig. 48B ; being
made of inioroecopio glass, the mirror weighs
only a few grains.
860. Thonuon'i Marine O^mmomeler.—Sh W. Thomson has
ingeniously adapted bis preceding inatrumcnt to manne ponioses :
in fact, the signals constantly transmitted to tbe Ortat Ea»tem
during her recent Bucccasfulpragresa in iayiag the cable across the
Atlantic, were received by means of this ingtrument. Aa tbe un-
certain and incessant motions in all diractiona render the use of on
ordinaiy galvanometer impracticable on board ahip, tho mirror
and magnet, instead of being
freelv sngponded, «re attached to
Ma sliding frame, a, Fig. 489, by
meana of a slender Bloment
III' stretched along it, in a line pasH-
"" ing Ihraugh tho centre of graiily
of the magnet and mirror \ the
poattiou of the miTTOr is therefore
Dever oltered by its own gravity.
The coil, e e, is surromided by ft
stout aolt-iron cylinder i t, which
conaidemblv diminiahea the in-
fluence of the carth'a magnetlam
on tho auspended magnet; and
this is furtbU' modified by a soft-iron box, oonHiatingcd the pieces
i., B, C, D, in which the ^vanometer is enclosed. The snapended
.magiiet ia adjuati:d by meana of a aemicircular magnet, ir, placed
ontaide the aoft-iron cylinder, and capable of being moved hori-
lOntaliy by the tangent-screw^ t. This instrument ia admirably
contriviid, and perfectly etScacioua.
8S1. Oaugmn't Tangent Otdeanom^tr. — It has already been
shown (802) that if a cireular coil be placed in the veiticaf plane
of the magnetic meridian, and a needle be placed at ibo centre
of it, the length of which is small comparetl with the diameter
of the coil, then tlie needle will be deflected by a ct
ing through the coil, and the potential of the cum
proportional to the tangent of the angle of deflexion,
nient constructed on this principle is called a tangent goXnano-
mfttr. In Gaugaiu'a inatmment, Fig. 490, instead of a single
coil in a plaoe passing through the needle, two coils, D, B, are
employed, both of which He on the surface of a donbls cone of
I2\y, the common apex of which is at the centre of the needle.
t wifl^
Thrae are three Isjers in each coil, whi<
and the termiDals are hrousht to four b
inther one, two, or all threelayen may
be brought into cireuit at pleasure. ■''■ *"■
There are bIbo tno single coils of rery
thick wire, nnited i>f e. transveraa
portion becealh, vhich terminate in
two binding-BcrewB, x,b; these are
designed for the meaaurBment of cur-
rente of large polential.
862. If the ma^eti be fixed, «id
the conducting wires nioreable, thus
reTereine the coodilionBof tbegalvano-
meter, the wires will be acted upon bj
the magneta, and assume a constant
position with regard to the direction of
the current, and the position of the
magnetic poles.
8(i3. Let a thic^ curved wire be connected with an electromotor
M that the current maj traveis« it ; divide it iu the middle, leav-
ing ahont an inch between the divided ponions, and re-connect
them by means of a piece of fine copper wire. On dipping this
thin wire, whilst the current ig passing through it, into iron filings,
Ihej will be attracted and adhere to it as if it had suddenly ac-
quired magnetic pro^rties. The filings will he attached to the
wire in the form of nuge, about one-twentieth of an inch apart,
and will drop oS the instant the cmrent ceases to be transmitted.
864. Wires conducting electric cnrrents aitraet taeh Other akat
the currtsU are moving in the laiae, and reptt each other, trhen
they more m contrary directiom. To show this, let a frame of
491, be fixed to a piece *
of light wood, D, moving
on a jiivot, the ends of
the wire dipping ioiolwD .
cnncentric annular cells, <■
filled with mercury, and
connected by wires pau-
ing through the item e
totheacrewa, p,a. These
•civws are connected by
wires with the innerior
(854), which by tho
wires, tf, z, is itself con-
nected with the two plates of an electromotor. A current thai
trareraea the frame, A e c, in a diroclion varying with the position
of the bars of the inversor; and let the current move in the
486 ELECTRO-DYNAMICS.
direction shown by the ritowb, and let a thick bent wire, m l y,
be placed in communication, by means of cups of mercury, with
the two plates z", c^', of a small electromotor. Lei this wire ap-
proach towards c ; the positive currents will be descending both
m c, and in l m, and thus moving in the game direction, the frame
of ABC will move on its centre to meo^ h m , mutual attraction
ensuing. Then move the bars of the inversor. so that the positive
current will ascend in c, instead of descending, and immediate
repulsion will be observed to occur.
If two flat coils, each consisting of a single spiral layer of in-
sulated wire, be suspended parallel and near to each other, and a
current be transmitted through each, they will be strongly
attracted mutually when the currents are in the same direction,
and equally repelled, when the currents are opposed.
865. By means of Roget's electrical spiral, the mutual attrac-
tion of conducting wires conveying currents
moving in the same direction can be easily
demonstrated. This consists of a loose coil
of thin copper wire, a b, Fig. 492, suspended
from a metallic support, z, connected with
one electrode of a battery : the end b just
touches the surface of some mercury commu-
nicating by the wire c with the other elcc
trode. On establishing connexion with the
battery, the wire coil will contract longitiidi-
nally, the successive coils attracting each
other, and the lower end being raised out of the mercury, the
connexion with the battery is thus broken : the weight of the
wire then causes it to fall into the mercury again, and the passage
of the current iH restored ; and these actions being repeated succes-
sively, a rapid longitudinal vibration (376) of the wire is produced.
866. The action exerted by a conducting wire on a magnet
(851), is obviously not a single force, but a couple (81), by which
the opposite poles of the magnet tend to recede from the con-
ducting wire in opposite directions, and assume a position of
equilibrinm when the opposite actions of the wire on both poles
become equally balancea by the earth's directive force. Reason-
ing on this fact, Faraday concluded, that if the action of the
current could be confined to one pole only of the needle, a rotary
motion might bo produced, provided no opjyosing forces interfered.
After a series of experiments on this subject, lie succeeded per-,
fectly, and thus developed one of the most interesting and extra-
ordinary phenomena in electrical science.
The most convenient apparatus for illustrating the rotation of
magnets round a conducting wire, consists of two slender magnets,
N s, N a. Fig 493, fixed vertically and equidistant from each other,
with their poles in the same directioni in the piece of wood, a,
H
Fig. 402.
|2
"^
B
^ —
e
c
BLECTRO-MAOKETIO BOTITIOV.
487
supported hj a pointed wire, b, bo as to rotate freely on its centre.
Tne middle of the piece of wood, a, is excayated, and contains a
Fig.4Sa,
drop of mercury, which communicates by
means of a curved wire dippins" into it,
with the external circular trou^ of mer-
cury, E. A pointed copper wire, supported
by a screw at o, dips into the mercury in
A ; and is fumisheci at its upper end with
a cup containing mercuiy, so as to be
readily connected with an electromotor,
by means of the inversor (854). llie
cup, c, and trough, s, are then connected,
the former with the copper, the latter
with the zinc, plate of tne electromotor.
So that the current descends from c to a,
and then reaching e through the bent
wire, escapes to z. It thus acts only on
the poles n, k, of the magnets, and the
forces acting on these poles being e<|ual,
and in opposite horizontal directions,
constitute a couple, which will cause the
connected magnets to rotate round the conducting wire c, from
left to ri^ht, or in a direction like that of the hands of a watch.
By shiftmg the bars of the inversor, or otherwise changing
the direction of the current, the direction of the rotation will
immediately be reversed : the same thing also occurs, when the
position of the poles of the magnet is reyersed. Let the mag-
nets or currents be arranged as they may, the direction of the
rotation always corre^'ponus with the formula of Ampere (852).
It may here be remarked, that in this, as in all other experiments
in electro-magnetism, where wires dip into mercury, tneir ends
should be cleaned and amalgamated, by being dipped into a solu-
tion of nitrate of mercury, to ensure perfect contact.
If a fiat bar-magnet be substituted for the pair of magnets and
conducting-wire in the above experiment, the same results will
ensue, and the magnet will rotate round its own axis.
867. If the magnets be fixed, and the conducting wires move-
able, the preceding conditions will only be reversed, and the con-
ductors will now rotate round the fixed magnets. U his may be
readily shown by means of a horse-shoe magnet, n s. Fig. 494,
placed in a vertical position, with circular troughs, a, b, clamped
upon its legs ; a light wire frame, supported by a fine steel point from
each pole of the magnet, is so arranged that its vertical branches
just touch the surface of the mercury in a and b. Each of the wire
frames terminates in a cup containing a drop of mercury, into which
the ends of the cross wire from b dip. Connect the cup of mer-
cury, B, by means of a wire, with the positive electrode of an
electromotor, either directly, or by means of the inversor, and let
the virea, c, t. coming from tb« dninlar troneh*, a, b, be belli
connecteil witb ibe negative electrode. Uoder theae circnm-
— ^jj_ BtaiiceB, a cnrrent of electricity will enter
the cup E, nnd there, being divided into
-'' twn portions, will deacend the Tertteil
brancheB of the wire fnuoea, and retch-
ing the trongbi A B, witl leave the *pp*-
ratua bj the wiree o, t Directly the cur-
rent IB in action, the wire frame roapended
from the north pule of tbe magnet besini
to rotate rapidly in a direction from leU to
right, and that ronnd ibe sonlb pole, in a
Contrary direction, from the action of tho
filed magnet on the corrent in the
moveable conducting wiren. If the direc-
tion of the current M reversed, either by
altering the craineiione with Uie electro-
motor, or by abiRing the bara of the iorer«or, the diicction of the
rotations niU alao be revaraed.
If the electric current be transmitted tbroogh the wires c, t,
from B to A, or vice vertd, it will travarsa the vertical bara of the
two wire ca^s in oppotite directions, and consequently they will
both rotate in the lamt diraetinn.
fiimilur results may bt obtained, bh was Gi«t shown by Ampere,
when the annular troughs a, b, are converted into clectromotora.
For this par[>oee tbe wire cages are double, tlie outer ones resting
by needle points in little mercury cups at the top of tbe inner
ojies. Short cjUndert of eheet zinc are aoldered to the ends of the
wirea of the inner cages, and of copper lo those of the outer onea.
The tron^bs must be Riled wilb dilate sulpbnric acid, or still
better, with a saturated solution of sulphate of copper, ■ little
acidulated (TT2), and be lar^ enoo^h to admit of the immeivoa
of the nnc and copper cylinders without contact either at tlio
aides or bottom of the trough. While the cylinders are immersed
in acid, curreote will travel tip tbe wire« of the outer cage, and
dovm those of the inner one, and they nitl ho found to rotate in
opposite directions.
868. The rotation of a conducting wire may be also conve-
niently shown, by bending two wires into helical ouila like cork-
screws, and allowing each to rest by one extremity on the depi«t-
aion on each pole of the horse shoe magnet. Fig. 494, the other
end dipping into the mercury in tbe circular tmughi A, B. On
coonecIinE one of tbe latter witb the positivs, and Uie other with
the neffstive electrode of the electromotor, the current will ascend
through one belii, descend the pole of tbe magnet which supports
it and ascend the other pole, and reaching the second helix will
deacend alonfi il, and thus by the mercunal trough into which it
dips, reach the sine plate of the eici^g apparato*. In thia
ROTATIVO DUCa.
489
Fig. 405.
▼ariation of the experiment, the helical coils of wire will rotate
round their reepective poles in the tame directUmy because whilst
the current ascends in one, it descends in the other.
869. It; instead of submitting a conducting wire to the action
of one magnetic pole only, it be so arranged as to be exposed to
the influence of both poles, it
tends to move in a plane equi-
distant from both, and if it be
smtabW attached, a vibrating,
insteadof a rotatory, motion maj
ensue. Let a light wire, w,
Fig. 495i, be suspended from
a brass rod connected with the
cup of mercury, c, so that its
lower end just dips into a cavity
cut out in the base of the in-
strument, filled with mercury »
and connected by a wire with
the cup E ; and let a horse-shoe magnet be placed, as shown in the
figure, so that the end of the wire, w, may be midway between
the poles of the magnet. Connect c with the copper, and z with
the zinc plate of an electromotor ; the current of electricity will
descend w, and being acted upon by both poles of the magnet, the
wire will tend to rotate to the right, round the pole, m, and to the
left round the pole, s. As it cannot at once obey both these
forces, opposed in direction, it takes an intermediate course, as
wonld be expected, from the law of composition of forces (284V
and is thrown forwards out of the mercury, in the direction indi-
cated by the arrow. Connexion being thus broken with the
battery, the wire by its gravity falls into the mercury, and, thus
completing the circuit, is again thrown oat, keeping up this
oscilJating motion as long as a sufficient current traverses it. Let
the direction of the current, or the position of the magnet be
reversed, and a vibrating motion of the wire, in an opposite
direction, or backwards, will ensue.
870. If the electric current be made to pass through a spur
^g* 406.
wheel, w. Fig. 496| instead
of a wire, a rotary move-
ment between the poles of
the magnet ensues. Thus,
if the current passes from
the cup c, to the axis of the
wheel w, it descends through
that spoke which happens
to dip into the mercury, and
passes from thence to z, and
to the zinc plate of the elec-
tromotor. As soon as the current descends the radius of the
490 BLBCTBO-DTirAMICS.
wheel, tbe portion dipping into the mercnrjr ib thrown onf, as in
the Tihrating wire; another spoke of the wheel dips into the
mercuiy, and is thrown out in its turn, and so on, a continued
rotary motion ensuing. If the position of the poles of the magnet,
or the direction of the electric current, he reversed, the wheel will
rotate in an opposite direction. The wheel w may he replaced
hy an entire disc of metal with advantage, as the motion is then
more uniform and continuous.
871. If a horse-shoe magnet he hronght near to a suspended
rectangle of wire, a c, Fip. 491, through which an electric current
is passing, it will he forcibly attr:\cte(^ whilst the current is pass-
ing, and the poles of the magnet placed, in one direction ; and
repelled, if either of these positions he reversed. This apparent
attraction is really owing to the same cause which determmes the
vibration of a wire suspended freely between the poles of a
magnet (869). The rectangle having a tendency to rotate, in
common with all conducting wires (867), round the poles of the
ma^et, in opposite directions, it is compelled, by the law of com-
position of forties, to advance between, or move from, these poles,
according to tbe position's in which they are respectivelv placed.
872. If the freely suspended rectangle before aescribed, through
which an electric current is moving, be left to itself, uninflnenoed
by any opposing cause, it will be acted upon by the magnetism of
the earth, and will assume a definite position ; which it will, if suffi-
ciently mobile, regain, when disturbed from it by any apj^lied
force. That /oee of the rectangle through which the positive
current is moving in the direction of ue hands of a watch,
aiwavB turning towards the south, whilst the other, or that in
whicn the current of electricity appears to move from right to
lefl, or in a retrograde direction, will assume the properties of
a north pole, and will consequently face the north. u1ius in Fip.
491, that face of the rectangle ac, which is there represented, will
regard the south pole of the earth, the current of electricity
moving in it, having a direct (p. 122, note) motion. If the conduct-
ing wire be bent into a circular or other figure, it will present
the same phenomena as the rectangle ; tbe shape not influencing
its properties.
873. If, instead of a single loop of wire, a coil of several con-
volutions be employed, its polar phenomena will be proportionately
lig. 497. increased, lliis may be very satisfac-
torily shown by means of the little ap-
paratus contrived by De la Rive, con-
sisting of a plate of zinc, z, Fig. 497,
about an inch square, placed between
the folds of a bent plate of copper of
the same size, and separated from it by
bits of cork, or wood. A piece of copper
wire, covered with silk, is soldered to tbe
491
ecmper pTiite. and after being twiited into aboat twaoty circular
ooil»,B,kept close together by means of thread, ia filed by its other
eitramity to the zinc plate. This apparatus ia placeit in a >faaIlon
wooden cnp, A, filled with dilute anlpbunc acid, and, on alloning it
to float in a veaael of water, the coil will, after a few oscilUtiona,
arraDge itaelf acroaa the magnttio meriilian ; the action of the
ftcid on the plates, c, i, deTeloping aufficient electricity to cants
the coil B to present magnetic phenomena : that face in which the
current ie moving from lelt to right, regarding the loutherD
betniiphere of llie earih. On presenting a magnet towarda the
coil of wire b, whilst the apparatus is in action, attraction and
repulsion will enaue, as if the wire iteolf had really became a
nwgnet. If otie of the ends of a bar magnet (according to the
direction of the cntrent) be introduced into tbe ring, b, it will
tTOTel slowly to the extremity of the magnet, and then turning
inond, will again embrace it ; this ia a curious and interesting
experiment.
874. The peculiar polar properties of this coil of wire may be
well illustntfld by Giing it oti a piiot, in the centre of a ehallow
circular trough of mercury, diiided into two portions by a little
wooden partition, aa shown at A i>, Fig. 498. Tbe ends of the
wire coil are pointed, and so long as just _^ ^^^
to touch the surface of the mercory in the
dirided box, a u, as it, by capillary repul-
rion (41), rises above the level of the pa^
tition without overflowing. On connecting
the two cella of niercaiy, bj meaua of the
wires, c e, with the two plates of an electro-
motor, and placing the whole between the
poles ofa horse-shoe magnet, tbe wire coil, B,
will rotate rapidly, from the two faces uf
the coil being idlemately attracted and re-
pelled by tbe magnetic poles, k, 8, and tbe
direction of the current traveraing it being
reverseil at each half revolution. 'Iba^
diaphragm must be so placed that tbe poliirity of the coil may be
reversed just after it has faced Iht attracliag poles.
875. Ibe coil of wire used in the preceding eiperiments may
be regarded, aa long aa the current traveraea it, oa sjiat mugnet |
but il tbe convolutions, instead of being nearly in the same plane,
be drawn out, ao as to represent a long helix, aa A R, Fig. 4^9, ila
apparent maenctic properties become much more distinct. Let a
wire, covered with cotton or sillc, be coiled on a bIrbs tube, in
a direction froio hit lo right, forming a right-handed helix {Wi),
and be supiwrted on a pivot, aa at c, ill two ends, D B, hanging
down, and just dipping into two concentric troughs of mcrcnry,
connected with tbe screws, i, t, aa in the support of the rectan-
gular cohdnclor before described (6641- Cm connecling these
492 BLBCTRO-DTHAM1C8.
■crevB Trith the two plates of an electromotor, tbe electricitj
will traverse the helical conducting wire, which, after a few
He 499 oscillations, will arrange itself in the
'^' ' magnetic meridian; that end in which
the current moves from left to right,
pointing towards the south pole of the
earth. The two extremities of this helix
are respectively attracted or repelled bj
the poles of a magnet, as long as the
electric current traverses it, as completely
as if it were a permanent steel magnet.
If the extremities of this helix be attached
to the plates of a small floating electro-
motive element, as in the case of the flat
coil (873), it will assume the direction of
the magnetic meridian, and will comport
itself in all respects as a floating magnet.
876. Ampere, to whom we are indebted for the knowledge of
the properties of this and other helical conductors, has termed it
the eleetro-dynamic cylinder. The most important pro^rtyof
this helical conductor, is its power of inducing magnetism in aoar
of soft iron, placed in its interior. Thus, if a bar of soft iron, in
which magnetism is readily excited, be plaoed in the helix, ▲ b,
Fig. 499, and a current of electricity be made to pass through the
latter, by connecting its two extremities with the poles of an
electromotor, the bar of iron will instantly acquire the power of
attracting another piece of iron, and indeed present all the pn>-
Eerties of a powerful magnet These magnetic properties are,
owever, transient, and are manifested on^ whilst tne electric
current is traversing the helix, vanishing altogether on the elec-
tricity ceasing to pass through the wire.
As in this experiment the current of electricity does not aUer
the iron, but merely passes round it in the coil of wire, we learn
that an electric current traversing a wire possesses the property
of inducing magnetism in iron bars brought within its influence,
and placed with their axes at ri^ht angles to the direction of the
current. If they be not plaoed m this position, the induced mag-
netism is proportionably weaker.
877. If a bar of soft iron be bent in the shape of the letter U,
and be covered vrith several series of coils of copper wire, insu-
lated bv being covered with silk or cotton, and a current of eleo>
tricity be transmitted through the wire, by connecting its two
ends with the electrodes of a voltaic battery, the intensity of the
induced magnetism will become very obvious. On placing a
smooth bar of soft iron opposite to the poles of this eleetro-magnfet^
it will be attracted, and remain flrmly adherent ; and an immense
weight may be suspended to the bar without separating it from
the poles of the magnet. In this manner, iUdro-tnagneU^ capable
of mppOTtiDg WTeral bnnilTedireigliti, and evva tooB, hara been
constructed. It is remarkable, that if tbe contact with the eleo-
tcotnotor be broken, whilst the poles of the electro-magnet ere nn-
connected vith each other, the induced mscnttiun will, if tbe iron
lie veiy soft, almost entirety ranish : but ifthe poles be counected
by a bar of sort iron, before communic-ation with the source of
electricily be interropted, ■ conriderable magnetic inleoBity U
left in the cnrred iron bar, and ia penaBDent So long as its poles
ara connected, disappearing only ou the remoTal ot the piece of
iron adhering to them.
878. If a bar of hard iron, or steel, be mbstitnled for soft iron,
little 01 no magnetism is developed, so long ai tbe electricitj tro-
Ternng tbe belli, in which they are placed, is of low notentlal.
But ir a current from a powerful Toltoic battery, or tbe discbarge
of a Leyden jar, be transmitted throngb tbe coil of wire, the in-
claded bar becomes permaneBtty magnetic, ill polar properties
Dot disappearing, as in the case of soft iron, on the cessatioD ofthe
inducing corrfuit. In every case, the dirtetion of the poles of
the indoced eieclro-magnet Iiears a conatant telatioo to the cootm
taken by the electric current, and is the same as that in the eleo-
tnwljnomio cylinder (876).
879. Tbe phenomena of the electric indoclion of monietism may
be well illoatrated by means of a contnTonce of the late Dr.
Sitchie, consistine of a bar of soil iron, supported by a pi*ot, and
covered with a coil of insulated copper wire, the two eitremiliea
of which just (ouch the aurikce of the mercury contained in acir-
calor trongb, divided into two cells by a transverse slip of wood.
In Fig. 600, H B is on nprigbt horse-ehoe magnet, bavii^ the bar
of iron, x, covered with a coil of insnlated copper wire, supported
hy ita [uvot over the tw»celled vessel of
mercory, b. On c<mBecting the latter by ^^ ''"'■
the wires, C, a, with the two plates of an
electromotor, the bar x becomea a temporary
magnet, and, if the conneiioDa be properly
a« the poles, b, a, to which they are opposed ;
of conrse, repulsion enmes, and a peribima
half a nvoludon : hero its wires paae over
the wooden partition, and dipping into the
opposite cells of memury, its polarity be-
comes reversed, and so on : tbe bar a revolv-
ing with immense rapidity, and having its
poUritT reversed twice during each revolu-
tion. During the action of this apparstas,
as well aa that of the routing coil of wire (874), a load humming
noise, often amoonting to a loud musical souad, is excited by the
rapid vibratory motion aBSumed by the fiied magnet during the
rapid revolution of the electro-magnet, or wire coiL This mnaical
N
494 £LBCTRO-DYlfAMlC8.
sound is remarkably well observed when the magnet is mpporleJ
by three levelling screws, on a smooth table ; and if the appaianu
be large, it much resembles the drone of the bagpipea.
880. If the electro-magnet (879) be about four or fiv« inches in
length, it will rotate by the masnetism of the earth, independent
of any steel magnet in ita neighbourhood. Care must in tnia case
be talcen to place the bar in the magnetic meridian, and to allow the
electric current to traverse the wire coiled round it, in such a direc-
tion that the poles of the temporary magnet may be such as will be
repelled by that hemisphere of the globe to which they are oppcnte.
881. It has been shown that a conducting wire and a magnet,
by their mutual reaction, tend to arrange themselves in a directioo
at right angles to each other (851), and that if the action of the
current, or, what comes to the same thing, of the wire oonveying
it, be limited to one pole only of a magnet at a time, tliey will
tend to rotate round each other in a given and constant directkm
(866, 7). VN'ires conveying currents, it has been shown, also
possess the properties of mutual attraction, or repnUion, aoconlii^
to the directions of the currents (864), and of lieing acted upon
by the magnetism of the earth, orof a permanent magnet, arrmng-
ing themselves in a constant direction, with regard to the poles of
either (879, 880). Ampdre has extended these frets still hirther,
by showing that twoeli'Ctric cun^nts, properly arranged, will even
tend to rotate round one another, provided their direction be ai
right angles to each other. ThIu^ if a current of electricity
travei-se a fixed horisatUal wire ▲ a.
P^. 601. Pig 501^ j^n^j another current {ms
through a moveable^ but always vertieal
wire c d, respectively in the directioM
of the arrows, then attntctioo will take
place between the current e b and c n,
m the angle c e b ; for if c d were b-
dined towards b b, the cunnentM in each
would be moving in tlie same directioD.
Eepulsion will be exerted in the angle ▲ k c, between a k and c u ;
for if c D be supposed to be inclined towards a b, the convnts ia
each would move in opposite directions. If then the coodoctar
A E B be circular, the moveable cunent c d will tend to revul^c
round an axis passing through its centre.
882. This may be proved by surrounding the circular copper
trough V V, Fig. 502, with some thick insulated copper wire, c«a-
nected with the binding screws e, c. The metallic anppiirt s is
connected, by a wire, with the screw or cup^ c, and the trough r
itself with the screw or cop z. A light wire frame, a b D, fur-
nished with a hoop or circle of thin copper, is jwovided with a pivot
at B, by which it ma^ rest with as little friction as possible on the
support, B. Fill V with a saturated solution of sulpnate of cop{;er,
place A B D on 8, so that its hoop may just dip in the aoSatioci in
MAOHfiTlC THEORY OF AUP^BK.
495
V, and connect cz, andez, with the electrodes of two electromotors.
Under these circumstances, currents of electricity will traverse
the wire round the
trough 17, and along the ^* 602*
frame abd, in the
direction pointed out
by the arrows : and the
horisontal circular cur-
rent in the wire acting
on the descending ver-
tical currents in a b d,
will cause the latter to
revolve in a direction
depending on the course
of the current in the
wire surrounding the
Teasel, v.
883. From the phenomena detailed in this chapter, a hifichly in-
genious theory of magnetism was proposed by Ampdre, differing
altogether from the conventional hypothesis previou»^ entertained,
in denying the existence of any magnetic "fluid" as distinct
from electncitv, and considering that all magnetic phenomena are
Imt the visible effects of invisible electric currents, permeating
the iron bars or other substances in which they exist. According
to this theory, every molecule of a magnet must be regarded as
being surrounded by a currentof electricity constantly circulating
around it ; and that the only difference existing between a magnet
and an inert bar of iron, is simply, that in the latter no current
electricity is present ; whereas, in the former, it is in a state of
rapid rotation around each ultimate atom, or molecule of iron. Ail
the effects produced by these elementary cun-ents may be theo-
retically represented by a set of resultant currents surrounding
the mass, as shown in Fig.
503. llie end m of such a ^ j^, 503,
liar will be the north pole,
and will point towards the
uurthem nemisphere of the
globe, because there the cur-
rents of electricity repre-
sented by the arrows, are
moving in a direction from
right to left, or retrograde
(872). llie opposite end
will, consequently, be the south pole ; for, on looking at the face
B, as shown at s^, the currents will appear to be moving from left
to right ; for the same reason that a word is seen backwards, on
looking at it through the paper on which it is written.
884. The attraction between dissimilar, and repulsion between
\
'tdXTO
496 ELBCTBO-DTNAMICS.
aimiUr magnetic poles (594) are thns explained, by anpponng-
that, in the former case, the elementary currents are moving in
the same, and, in the latter, in opposite directions (864). The
rotation of a conducting wire ronna a magnet (867), becomes also
reduced to the simple case of the rotation of a yertical round a
horizontal current (882) ; for all magnets, it must' be recollected^
.are, on this hypothesis, supposed to have myriads of currents
traversing them m a direction at right angles to that of their mag-
netic axis.
On this theorVi also, the magnetism of the earth is explained,
by supposing the existence of currents of electricity constantly
traversing it in a direction from east to west. It is unquestion-
able that, opposed as this view is to all previously received con-
ventional theories, it is entirely in accordance with every dis-
covery in electro-magnetic induction, as well as with all other
observed facts in electricity and magnetism.
885. Manj attempts have been made to render the attractive
foroe of electro-magnets available as a source of mechanical power,
but it appears that although it is possible to obtain any required
amount, no method has hitherto been devised by which the expense
of generetinff electric power does not so far exceed that of labour-
ing force denved from the ordinary sonrces, as to render it totally
unavailable in practice. In the year 1839, Prof Jaoobi succeeded
in propelling upon the Neva, at the rate of four miles an hour, a
boat 28 feet long, and H feet wide, which drew about 3 feet of
water, with ten persons on board ; but for this purpose a Grove's
battery (796), consisting of 64 elements, was employed, each
flatinum plate of which presented a surface of 36 square inches,
n 1842, Mr. R. Davidson propelled a carria^ weighing four tons
at the rate of four miles an hour, on the £dinbuign and Glasgow
Bailway, and in 1849, M. Hjorth constructed an engine of ten-
borse-power, one of the electro-magnets being capable of sustaining
a weight of 5000 lbs.
A very common form of electro-mafnetic engine is that in
which the electro-magnets are arranged round the circumference
of a wheel or cylinder ; one of the latter form is^ shown in Fig.
504, in which a is a compound
permanent magnet, and b one of
three paire of electro-masnets,
placed e<}uidistantly round a ro-
tating axis. The confcact breaker
in these machines usuid ly consists
of a brass spring, resting on a
ring or circle of conducting and
non-conducting matter in alter-
nate compartments ; and the current traverses each electro-mag-
net in succession, as it approaches the permanent magnet, and is
interrupted at the instant that they are exactly opposite each
8EC0HDABT OUBBBim. 497
other, 80 that no lepnlaion occnra, the motive power being the snm
of the attractions of the acting electro-magnets.
886. Of all the numerons and successfnl researches made by
Faraday in tho different departments of electrical science, none
are of greater importance, or more worthjr of deep attention and
stady, than the discoyery of electro-dynamic induction, which was
made by that philosopher in 1831. As a brief generalization of
this disGoreiy, it may be stated that, whenever an electric current
commences traversing a wire, it excites a momentary current in
the oppotite direction in a second wire placed parallel to it, which
may for convenience be termed an inverse current ; and on sud-
denly interrupting the primary current, an induced current reap-
pears in a direction contrary to the former, or, in other words, a
direct current results. Also, while a conductor traversed by a
current is movins towards a parallel conductor, an inverse cur-
rent is manifested in the latter ; and a direct current, while the
former is receding.* Whenever, also, a magnet is moved towards
or from a conductine wire in any manner, (but especially when
the long axes of both magnet and wire are at right angles to each
other,) similar induced electric currents are excited in the wire.
These induced or secondary currents are but of momentary dura-
tion, appearing only at the instant the primary or inducing current
either effects its passage, or ceases to pass through the wire ; and,
when excited in a coil by a permanent magnet, or by an electro-
magnet, they exist only during their mutual approach or recession,
and cease the instant they come to a state of rest.
887. Coil on a wooden cylinder, about two inches long, and an
inch in diameter, about eight or ten feet of insulated copper wire
(i. e. covered with cotton or silk thread), and let its two ends pro-
ject ; call these ▲ and b : over this, coil forty or fifty feet of copper
wire, also insulated, and separated from the first coil by several
folds of silk : call the free ends of this second coil c, d. Then con-
nect c, D to the screws o, o, of the multiplier (856), and a, to one
of the plates of an electromotor ; suddenly bring b in contact
with the other plate, and immediately the needles of the multiplier
wiQmove from an induced electric current, traversing the coil c d.
This being only of momentary duration, the needles will soon re-
gain their former position : then suddenly remove b from the plate
of the electromotor with which it was previously in contact, and
the needles of the multiplier will again move, but in an opposite
direction to that in which they first deviated. In this expenment
we see that a current traversing a wire induces a secondary one
in a wire parallel to it (considering the concentric circles formed
by the wires as being equivalent to parallel lines), both at the
instant of making and breaking connexion with tiie source of
electricity. These currents are always opposed to each other in
direction, as proved by the galvanometer, ana must be considered as
• Phil. Trans. 1882, pp. 127-129.
KK
498 ELBCTRO-DTN^MICB.
Midng from induction, and are probably due to what Faraday has
deiiomiDated extrorcwrentsy eziBting at the moment of making
and breaking the primary drcnit.
B^ winding slips of tin-foil spirally and opposite eacli other on
the inside and outside of a glass cylinder, and discharging a Ley-
den jar through one of the coils, Prof. Henry demonstrated the
existence of a similar induced current in the other. And on con-
necting the inner coil of the first cylinder with the outer of the
second, and the inner of that with the outer coil of a third
cylinder, he succeeded in producing induced currents of the third
and fourth orders.
888. Coil on a hollow cylinder of pasteboard, half an inch in
diameter and three inches long, about fifteen feet of inatUated
copper wire, connect its two ends with the screws o, g, of the
multiplier, and then pass into the hollow axis of this helix a
cylindrical magnetic oar : the needles of the multiplier will in-
stantly move, snowing the existence of a current traversing the
coil. Allow the bar to rest in the cylinder, and the needles will
return to their primitive position, the induced current disappear-
ing. Suddenly withdraw the magnetic bar, and the rapid motion
of the needles of the multiplier will indicate the momentaiy ex-
istence of an electric current in a direction the reverse of that,
which appeared on introducing the bar into the helix. If the
opposite pole of the bar be passed into the coil, the induced cur-
rents will be in a direction the reverse of those produced by the
action of the former pole.
889. Wind round a cylinder of soft iron, or a bundle of iron
wire, a few feet of insulated copper wire, of which the free ends
are called ▲, b ; over this coil wind about twenty or thirty feet of
insulated copper wire, carefully separated from it, and connect its
iree ends with the multiplier as oefore. On connecting a and b
with the plates of an electromotor, an electric current will pass
through it, and convert the included iron bar into an electro-mag-
net (877). The magnetism thus set in motion in the bar will, like
the movement of the permanent magnet (886). induce a current
of electricity in the outer coil connected with the multiplier, and
its needles will be powerfully acted on. Then on breaking the
connexion of the pnmary coil with the electromotor, magnetism
will vanish from the iron bar, and an energetic current of elec-
tricity in an opposite direction will be excited in the outer coil,
causing the needles of the multiplier to be violently deflected in
the opposite direction.
890. A secondary coil of wire is by no means necessary for the
development of an electric current ; a single length of tntulated
wire, coiled into a tolerably compact hehx, having an induced
current excited in it in one direction, on making connexion, and
another, in an opposite direction, on breaking connexion with the
battery, or other source of electricity. These induced currents,
ELONQATION OP AN IRON BAR IN A HELIX. 499
like those before described, are but of momentary duration; tbey
may be considered as arising from the induction of the piimary
current, traversing each fold of wire, on the adjoining folds. In
this manner is explained the appearance of a vivid flash of light,
observed on hreakina connexion with a small electromotor, by
means of a wire folded into a compact coil, whilst scarcely the
faintest spark is perceived when a short wire, or a long uncoiled
one is used. If connexions be made and broken by means of a
cup of mercury, the vividity of the light is increased by reflection
fTum the brilliant surface of the fluid metal, as well as from the
latter undergoing combustion by the force of the discharge. If the
wire bo coiled round a bar of iron, the induced magnetism will
increase the inteHbity of the secondary current, and consequent
splendour of the spark, on breaking contact with the source of elec-
tricity. In this manner are explained the vivid sparks observed
during the rotation of a flat coil (874), and of an electro-magnet (879).
891. If about sixty feet of thick insulated copper wire be wound
into a short compact coil or helix on a short wooden reel or
bobbin, the effects of these secondary currents may be beautifully
observed. The battery employed mav be an electromotor of a
single pair of plates ; let these plates be called z and c.
A. Connect one end of the helix with z, and fix on c a cop of
mercury ; introduce the other clean and sharp end of the helix
into the mercury, and withdraw it with a jerkmg motion, a vivid
flash of light wifi ensue. The heat evolved is sufficient to inflame
ether, or gunpowder, when placed on the surface of the fluid metal.
B. Connect one end of the helix, as before, with z, and attach to
c s clean steel file, draw the other end of the wire over the I'Urface
of the file, and a succession of brilliant sparks from the combus-
tion of the steel, will appear.
C. If connexion with the electromotor be broken by means of
the helix arranged as before (B), but with one end of the wire
fuminhed with a piece of leaf-gold or silver, combustion of these
metals, attended by the evolution of their characteristic light
(815) will ensue.
892. A curious result has been obtained by experiment, which
well illutitrates the powerful influence of the currents induced in a
bar of iron, placed within a cylindrical helix, on the molecular
arrangement of the bar itself. Let an iron bar, half an inch or
more m diameter, and four or five feet long, be placed in a helix,
so that the middle of the bar and helix may coincide : and let the
middle of the bar be encompassed by a nng, on which the bar
may rest in the helix, so as to prevent contact with any other
than its middle point. On suddenly completing the circuit, the
bar will emit a feeble ringing sound, and a much Tonder tone, when
the circuit is interrupted. These sounds arise from the develop-
ment of longitudinal vibrations ^376) in the bar, by the sudden
polarisation of the particles within the influence of the helix.
KK 2
500 ELECTB0-DTNAM1C8.
when the contact is made ; and hj their sadden release from con*
straint, when the influence of the inducing current is remoTed bj
breaking contact.
It has been demonstrated bj Prof. Joule that an elongation of
the bar takes place at the moment of its magnetization ; this is
shown by means of a lever resting on the end of the bar, to which
a small mirror is attached : the motion on a screen of a small
pencil of light reflected from this mirror indicates a slight elonga-
tion of the bar.
An hypothesis has been proposed by De la Hive in explanation
of this molecular change in the bar; namely, that magnetic in-
duction tends to set the longest axis of the molecules parallel to
the axis of the bar. This theory obtains some confirmation from
the fact that if a tube containing water, in which minute particles
of the magnetic oxide of iron are suspended, be placea in the
centre of a helix, the fluid becomes more translucent lengthwise,
while magnetism is induced by a current passing through the coil.
893. Let about 200 or 250 feet of insulated copper wire be
J.. -Q. coiled on a hollow wooden bobbin or reel.
*^' * Fig. 505, about four inches lone; and
each end of the wire furnished with brass
or tinned iron cylinders, a, b, terminat-
ing in metallic points, c, d. Grasping
' these cylinders with the hands, immerbo
c in a cup of mercury connected with
one plate of an electromotor, and i>, in
a second cup, connected with the other
plate ; on suddenly withdrawing one of
them, as d, the secondary current thus
excited rushes through the arms of the
person who grasping a and a forms a short circuit between
them, and a severe electric shock is produced. If the hands be
moistened, to render them better conductors, and connexion be
made and broken with the electromotor, by connecting c with one
plate, and drawing d over the surface of a file connected with the
other r891 B) a rapid succession of very painful electric shocks will
pass through the arms and chest of the operator. By placing in
the hollow axis of the reel a bar of soft iron, or, still better, a
bundle of insulated soft iron wires, e f, the intensity of the in-
duced current, the vividity of the sparks, and strength of the
shocks, will become remarkably increased.
These shocks have been by some persons erroneously regarded
as directly produced by the electromotor, whereas they really arise
fipom a secondary induced current, quite independent of (except
that it is excited by it), and far exceeding in intensity, the current
originally generated. The electricity of the wire appears to be
constrained by the inductive force of the battery current, and
to be kept in a state of coercion so long as the current continues
to pass : but when the coercing force is removed by the cessation
COSREVT BVOLVISD BY ROTATIOV. 601
of the current, the electricity accnmulated at one end of the coil
rashes back partly through the wire itself, and partly through
any external conductor intervening between a and b, and the re-
lative quantities of electricity traversing the external and internal
circuits, will be inversely as their resistances. Hence it follows
that the severity of the shock will, eceterU paribus^ be augmented
by increasing the length of the secondary coil.
The induction of one coil on another may be readily demon-
strated by the suspended flat coils already mentioned (864) if the
terminals of one be connected with an electromotor, and those of
the other with a galvanometer.
894. In all electro-magnetic apparatus, in which the contact
with the battery is suddenly broken, a vivid spark evinces the
passage of the induced current excited by the action of the magnet
on the conducting wire. This may be seen in the vibrating wire
(869), where each time the moving wire leaves the mercury, a
vivid spark is observed ; although the electromotor itself may be
incapable of affording one, without
the aid of induction. The existence _ ^' ^^'
of these currents may be readily
S roved by means of the revolving
isc. It has already been shown
(870), that the passage of a radial
current through a disc placed be- ^^
tween the poles of a magnet pro- >^^
duces rotation : and the converse ^^
of this is equally true, namely, ^^
that, if a copper disc be made to rotate between the poles of a
permanent maenet by means of a handle, as in Fig. 506, and two
wires, one of which is in contact with the axis of the disc, and the
other with the mercury in a trough in which the edge of the wheel
is immersed, be connected with the binding screws of a galvano-
meter (856), the needle will be deflected by the currents perpe-
tually induced in a radial direction, by the poles of the magnet.
If two discs of tolerably thick sheet copper about nine inches in
diameter be placed vertically one above tne other in a frame, their
edges being Kept in contact by the gravity of the upper disc, and
a powerful compound ma^et be placed horizontally, so that the
point of contact of the discs may be midway between its poles,
the axes of tlie two discs being in metallic connexion with a gal-
vanometer, it will be found by the deflection of the needles on
rotating the discs by a handle attached to the lower one, that
actually a larger quantity of electricity will thus be evolved by
induction, than from a four-feet plate machine in full action ; a
result that appears at first sight scarcely credible : it must, how-
ever, be remarked, that this induced current is one of compara-
tively small potential.
895. The currents thus excited (887—894) are available for all
the experiments in which ordinary voltaic electricity is applied,
502 ELECTRO-DYKAMICB.
and yarious kinds of apparatus, termed magnetihelectric and
electro-magnetic machines, have been contrived for the purpose of
exciting them with ntpidity. These may be divided into three prin-
cipal kinds, in two of which an electric current is employed aa the pri-
mary exciting agent; and, in the other, a permanent magnet is used.
896. The simplest and most convenient form of electro-magnetic
machine is founded on an experiment already described (887), and
may be constructed by winding on a wooden reel, five or six
inches in length, with a hollow axis, sixty feet of insulateJ capper
wire of about the bixteenth of an inch in diameter, its termina-
tions being soldered to binding screws : this is termed the primary
coU. Over this, wind about 1400 or 1500 foet of insulated copper
wire, about the sixtieth of an inch, or even less, in diameter, and
solder its terminations to other binding screws : this con>titute8
the secondary coil. If, then, the primary coil be connected with
an electromotor, whilst the ends ot the external or secondary coil
be held in the hands, espcciallv if tin or copper cylinders be used
to increase the extent of sur&ce for contact with the hands, on
interrupting the primary circuit, all the electricity under con>
straint in the exterior coil by the inductive influence of the primary
current is released, and passes through the body of the operator,
producing a severe shocK. If the terminals of the long wire dip
in acidulated water, or rest on paper moistened with a salt, as
iodide of potassium, electrolytic action results, and the proximate
elements are separated.
897. It is obvious that some means of breaking contact with
the battery with sufficient frequency is necessary to ensure a rapid
succession of electric currents ; and for this puipose various plans
have b?en proposed. Ratchet- and toothed-wheels (265— -267) have
been employed for this purpose ; but as they involve the necessity
of being turned by the hand, they are very troublesome. If any
apparatus of this kind be employed, instead of a toothed wheel, a
cvunder of wood having two bars of metal inlaid, connected with
the electromotor through the primary coH should be used. A
brass spring, connected with the other electrode of the batteiy,
presses upon the cylinder, and on causing the latt«r to revolve by
means of a multiplying wheel, the contact with the battery may
be rapidly made and broken. Connecting the primary coil with
the electromotor through the medium of the vibrating wire (869),
stellated wheel apparatus (870), or still better, of the roiatine
coil (874), or magnet (879), will answer very well, as contact wifi
be effectually broken several times in a second by their action.
The late Author preferred, however, a little apparatus which has
been described elsewhere,* consisting of a light iron beam vibint-
ing between two fixed magnets ; this produces a break of contact
about 400 times in a minute, and consequently affords a rapid
succession of currents of induced electricity.
* Phil. MBgMine. horember, 1837.
COIL-MACHINES.
603
898. The most convenient form of the electro-magnetio machiDO
is, however, the foUowing; it is far superior to that contrived by the
late Author, on account of its certainty of action, and its dispensing
Fig.Wl,
with the use of mercury.* It consists of a wooden bobbin, a,
Fig. 507, on which the two coils of wire already described (896)
are woiud, the ends of the long and fine coil being soldered to the
binding screws b, u. One end of the short and thick (primary)
coil is soldered to the beginning of the copper wire surrounding
the two little vertical bars of soft iron, its other end being con-
nected with the screw o. The other extremity of the short coil
is soldered to the base of the brass column d. This column sup-
ports a slip of elastic brass, bearing at its end a disc of soft iron,
placed over the vertical iron bars. A slender screw k, furnished
with a platinum point, passes through the top of a bent support
of brass, and gently presses on a pLate of the same metal fixea on
the slip of brass below it ; the foot of this support is connected
with the binding screw f : all these connexions are made under
the base of the instrument.
On connecting the electrodes of a single element, s, with f, o,
the iron bars become magnetic by induction (877), and attract
the disc above them. This being drawn down, Dreaxs the contact
between the end of the screw, k, and the brass spring, and of course
the magnetism in the bars ceases. The elasticity of the spring
raises ue iron disc, and at the same time causes it to touch the
end of K ; contact is thus made, the bars again become magnetic,
and break the circuit, and so on. The course of the current from
the plates b to the primary coil on ▲ bein^ thus interrupted and
renewed many hundreds of times in a minute, a loud humming
sound is produced by the vibrations of the brass spring, which,
when they are sufficiently rapid, assumes the character of a defi-
nite tone. Of course at each of these renewals and interruptions
of the primary current, induced currents traverse the secondary
ooil, which become remarkably increased on placing a bundle of
insulated soft iron wires in the hollow axis of the bobbin, a. On
then grasping a pair of conductors connected with b, c, in the
hand, a rapid succession of severe shocks will be experienced.
• thoM particular form of apparatna, wbioh ia peonliaxiy adapted to medical
pnrpoaes, ia extennrely mannfactmed by Mr. Neeree, tondon.
504
BLBCTSO-DTXAiaCB.
J^,80B,
U.
899. From wbat htm been already remarked (886), it is obTitms
that the induced, or secondary currents thus excited will be alter-
nately in opposite directions. Those excited when contact ia
broken with ue batteij being much more energetic than those
excited when contact is m&dn. The following experiments will
be found instructive.
A. Place on a plate of glass a slip of bibulous paper, moistened
with a mixed solution of starcn, and iodide of
potassium ; let the points of two platinum wires
fixed to the screws, b, c, rest on this paper, the
blue iodide of amidine will appear at both wires,
a much larger quantity being developed at one,
than at the other.
B. Let two platinum wires b be thrust through
a cork fixed in the end of a glass tube, a, Fig.
608, filled with dilute sulphuric acid. On con-
necting the wires with the screws b, c, a torrent
of minute bubbles of mixed oxygen and hydro-
gen gases will be evolved from both wires ; one giving of^ how-
ever, much more than the other.
900. The apparatus just described may be conveniently called
the electro-magnetic machine with alternating currents. It is,
however, sometimes important to be able to obtain the induced
currents in one direction only, hence the contrivance of the electro-
magnetic machine with a single current. A very convenient
arrangement of this kind is represented in Fig. 509, in which the
required contacts are made and broken by a contrivance frequently
introduced in electro-magnetic machines, namely, a metallic spring
resting on the circumference of a wheel or cylinder, the surface of
which consists of metal, and wood or ivory, in alternate c<Mnpart-
ments ; it is evident that when the spring rests on any of the
metallic portions, the current may be transmitted, but when in
_. _^ contact with any non-
^'^' metallic portion, it
will be interrupted.
In this machine, the
primary and second-
ary coils, and the
bundle of iron wire, or
eorCf as it is commonly
called, are enclosed in
a box L, on the top of
which the contaet-
«-. . breaker is placed.
This consists of a wooden cylinder a, having pieces of brass inlaid
at either end, and in metaUic connexion witn two brass pivots, on
which the cylinder turns in the brass uprights o, k ; one of the
pivots passing through the upright k, has a handle attached to it.
kuhmkorff's coil. 505
Two brass standards, b, p, are placed near the cylinder a, having
binding screws at their base, and brass springs, resting on the sur-
face of the cylinder, attached to their summits ; consequently, s
and K, or o and h, will be in metallic connexion only when the
conresponding spring rests on a metallic portion of the cylinder. A
little attention is necessary to trace the connexions, which are as
follows : — One end of the primary coil is soldered to the foot of a
binding screw, d, and the other to the brass standard, k ; one end
of the secondary coil to the foot of the standard p, h, and the other
to that of the support^ o ; and the wires terminated by the handles
or plates, p, f, to the binding screw, h, and to another at the side
of o: also, the electrodes of a single element, c, (which is sufficient
for this purpose^, are attached to the binding screws, d, b. The
position of the brass pieces on the . cylinder a is so arranged, that
p and Q may akoaya be in metallic connexion, when contact is made
between b and k, but Tiever when it is broken ; consequently, the
current induced in the secondary coil, on making contact between
the primary coil and thei electromotor, will pass from h, through
A, to G ; but the current induced on breaking contact can only pass
tiin>ughp,F. Whichof the electrodesF, f, ispoaitive, and whichnega-
tive, will depend on the connexions of the electromotor with d and b.
The same obiect may, howeyer, be attained by a simple ad-
dition to the coil-machine already described (Fig. 507) ; this con-
gists in placing two other brass standards bent over at right angles
on the opposite sides of the brass spring that carries the iron
keeper, having^ platinum-pointed screws, which may be brought
into contact with a bit of platijj^um on the upper surface of the
brass spring, simultaneously with the point of k. If the ends of
the secondary coil be connected with these last standards under-
neath, it is evident that the current induced on making contact
will pass through these standards, as the shortest circuit from one
end of the fine wire to the other,* while that induced on breaking
contact will traverse any conductor interposed between b and c.
If the experiments made with the apparatus with a double
current (899) be repeated with either of these last, the iodine and
potassium in the one case, and oxygen and hydrogen iu the other,
-will each be set free at one wire, but not at both.
901. Bukmkorff't Induction' Ajaparatus. — M. Ruhmkorfif of
Paris, brought the induction coil, in the year 1851, to a greater
degree of perfection than it had hitherto attained, by paying great
attention to the insulation of the secondary wire, each layer of
which he covered with a laver of shell-lac varnish. The energy
of this apparatus was considerably increased by the application to
it of the condemer of M. Fizeau ; which consisted of two strips of
tin-foil, each containing four or five square feet of surface, placed
alternately between three wider strips of oiled silk^ and the whole
folded up : the pieces of tin-foil are respectively in metallic con-
* Tins ptooeM is teohnioally termed ahfrt-drcmiiHff a cozrent.
506 ELECTB0-D7NAMIC8.
nezion with the Btandards that support the contact-breaker. The
condenser now generally employed consists .of pieces of tin-foil
laid between larger pieces of oiled silk or thin gutta-percha, and
connected alternately with each other. The function of the con-
denser seems to be in forming a temporary reserroir for the poten-
tial accumulated in the extremities of the primary coil, b^ the
inductive action of the current on the electricity in the coil itaelf.
The disruptive discharge that takes place between the ends of
the primaiT coil, at the moment of breaking contact^ is consider-
ably diminished by the application of the condenser, just as it has
been already shown (736) that the discharge is much enfeebled,
when the charge of a small Leyden jar is transferred to a much
larger one. The inductive charge of the primary coil acts preju-
dicially on the inductive charge of the secondary ; and conae-
Quently it is found that the potential of the inductive charge of
tne secondary coil is greatly augmented by the condenser, and
the disruptive discharge becomes more violent. In this appa-
ratus, the discharge took place through about an inch of air. In
the coil of Buhmkorff the vibrating hammer acts against one ex-
tremity of the core. It is also furnished with an inversor and
contact-breaker: this consists of an ivoiy cylinder, resting bv
two disconnected brass pivots in two brass standards, at which
the primary circuit is interrupted. Two pieces of brass are at-
tached to opposite sides of the cylinder, each of which is in con-
nexion with one of the pivots, and two brass springs in connexion
with the electrodes of the battery, are so placed as to rest against
the opposite sides of the cylinder* When these rest on the ivory
the circuit is interrupted ; and it is completed in either direction
by bringing the corresponding metallic portions of the surface of
the cylinder into contact with the springs.
902. Hearder'g Induction CoiZ.— -Subsequently a more effective
arrangement of this apparatus has been earned out by Mr. Hoarder,
of Plymouth : the principal features of this are, that the primary
and secondary coils are distinct from each other, and consequently
the length and thickness of the primary coil may be adapted to
the battery employed. A short primary coil of thick wire,
actuated by a battery containing a small number of large ele-
ments was found the most effective. The secondary coil con-
tained 3000 yards of insulated wire, and the condenser about
thirty square feet of surface.
903. Liodd's Induction Apparatvs. — Some of the most power-
ful induction coils hitherto constructed are those bv Mr. W. Liadd,
of London. The core is 13 inches long, and 1*6 m diameter; it
consists of a bundle of rather fine iron wire (No. 22), carefully
annealed and insulated. The primary coil contains three layers
of thick copper wire, and the secondary coil is three milea in
length. The utmost care is bestowed on the perfect insulation of
the secondary coil : the ends of the bobbin on which it is wound
FABADAT^S OBIOIKAL IXDUCTION APPARATUS. 507
are thick platos of gutta percha, and several layers of a tliin lamina
of the same most efficient insalator are placed between the suc-
cessive layers of the coil, and hermeticanj sealed to the ends of
the bobbin. The wire is evenly laid in a spiral, so that no turn
of the coil may overlap a previous one, and an inch of thin gutta
percha is left at each end of the coil, which, like the uncoated
margin of a Leyden jar, prevents the disruptive discharge taking
place between contiguous layers of the coil. The condenser con-
sists of fifty sheets of tin- foil, each containing about a souare foot
of surface, laid alternately between sheets of gutta percna. The
vibrating sprine is placed vertically, and the hammer oscillates
between the end of tho core and that of an adjustible screw tip-
ped with platinum. The pressure of the spring for maintaining
contact is regulated by a screw that presses against its middle
}K>int, and by tightening which the primary current is interrupted
ess fre^^uently, a greater amount of induced magnetism in the
core being rei^uired to overcome the spring, and thus the energy
of the inductive charge is increased. With this apparatus, ac-
tuated by a Grove's battery of five elements, a spark of four or
five inches in length in air may be obtained.
The discharge of this machine through a vacuum of several feet
in length produces a torrent of electric light that is truly astonish-
ing ; and all the beautiful phenomena of the stratified discharge
(811), variously coloured by the nature of the attenuated medium
in which it takes place, may be most successfully exhibited. One
of the most striking experiments that has been devised is a lipped
goblet of uranium-glass, placed under an air-pum^ receiver, and
resting on a small metallic plate in connexion with one of the
terminals of the secondary coil. The other terminal is conducted
through a glass tube to a small brass plate placed in the bottom
of the goblet. When the positive current is directed to this, a
stream of electricity flows over the lip (being the shortest way) to
the plate beneath, and the whole vase is Fig. 510.
brilliantly illuminated with fluorescent ^*=^
light. (Vide Ch. XIX.)
904. The simplest means of demonstrat-
ing the existence of a current produced by
the induction of a permanent magnet, is
the original apparatus devised by Fara-
day : this consists of about ten yards of
insalated copper ribbon wound round a
soft iron bar a b, Fig. 510, in contact with
the poles of a magnet n b. Let one end of
this coil be soldered to a plate of amalga-
mated copper, c, upon which the other end,
sharply pointed, is madie'to press with elas-
ticity, to effect which, it is bent into a loop, d e. The bar a b
becomes magnetic by induction, and on suddenly jerking off one
the electric current dsTeloped in the coil U shown bys Tind spark
occnning at the point where b pressea on c, aait beoomea ilightlj
rttiaed rrom the piste b/ the sudden Jerk cominnnicat«d to a ■.
905. Of mogDela-fllectric mschineB, in which a pennuiciit
raaefnet ia the exciting caiue of the cmrentg, there are many
VKneticB. Of these, Suton'g and Clark's arraQgement are nipe-
rior to those of Fiiii and others ; that of Mr. Clark being upon tho
whole more conrenient than Saxton's from its small bulk, il
tensity of action, and its dispensing with the use of meionrj. '.
coDiists of an upright componoa horsa-Bhoe magnet, clan
against a board, d, Fig. fill, by tlie cros^-piece, c. By raeana of
a maltiplying wheel, B, the
■^ ■ onniiture, ABOF, isBiade to re-
ToWe rapidly before the polea of
the fixed mafnet. This arma-
ture consists oT two pieeei of aoft
I iron, coDBectedatrightangle* to
f the bar of iron, aB, byacrewi;
round the legs or branches of
which are wound about 1500
J, One end of the w . .
connected with a collar of brass,
against which the spring B
presses, the other end being
soldered to an insulated brass
collar, I, part of the cifcnmls-
rence of which has been remored,
as shown on a larger scale in the side figure. A thick copper
wire I, presses against i, and is conneclcd by a brasa pilur, r,
with A metallic strap, L, fixed on one side of the wooden block, ■,
whilst a similar piece of metal, n, with which l may be conueclsd
by a bent wiie^tf , is on the opposite side, and supports the spring
m. When v, o, and consequently their iron axes, are opposite lo
the poles of the magnet, the latter, by indoction, couTerts the
included iron into ■ temporary magnet ; at the instant this action
occurs, a cnmnt of electricity is induced in the coil. If the
armature be turned half roand, the ma^etism of the iron piece
becomes reversed, and a second current in an oppotUt direction is
excited ; and as at the moment this takes place, the wire a comes
in contact with the intermpted portion of the collar i, a brif^ht
spark passes between them. On rotating the armature with
rapidity, a succession of ri lid sparks ensues ; and if wires attached
to the braai pieces, i., u, be immersed in acidulated water, de-
oemporitiDn of that fluid will occur, (he oxygen and hydrogen
gases being evolved alternately from each wire; for of the sue-
MAaNBTO-BLECTSIO lCA0HIlfB8. 509
oeasiye induced cvrrents, each is alwa3r8 oppodte in direction to the
preceding, the alternate ones only moving in the same direction.
906. If a copper cylinder be grasped in each hand, whilst wires
connected with them commtmicate, one with the piece l, and the
other with a cavity excavated in the end of the revolving arma-
tare, on turning the wheel b, a rapid succession of currents is
sent through the body of the person grasping the cylinders, pro-
ducing a series of severe and almost intolerable shocks; the muscles
becoming so spasmodically contracted, that he is generally unable
to drop vie conductors.
If tne wires, instead of terminating in copper cylinders, be
furnished with platinum points, electrolytic decomposition of any
conducting fluia in which they are immersed will ensue, as in the
case of the induced current of the previously described apparatus.
907. If an armature, having a ahart helix of thick msulated
copper wire, be substituted for the armature a b, in the machine
just described, the intensity of the evolved electric currents will
be diminished and no shock will result from them. The vividity
of the spark at i will be, however, increased, and pieces of platinum
wire may be readily ignited by allowing the electricity to pass
through them ; also a feeble chemical action may be detected.
The ordinary phenomena of electro-magnetic rotation may be pro-
duced b^ passmg these currents from the short helix through the
appropriate apparatus (866, &c.) ; this helix is commonly called
the quantity armature^ because the quantity of electricity put
in circuit is proportional to the sectional area of the wire which
constitutes the helix.
Some important practical applications of the magneto-electric
current have been made : one ofthese is to the purpose of electro-
plating by Mr. Woolrich,* in whose machine four powerful com-
pound norse-shoe magnets are placed edgewise in the form of a
cross, their similar poles being in two horizontal planes, between
which a wheel, carrying at its circumference four armatures,
rotates on a vertical axis. By an appropriate arrangement of
commutators for transmitting currents in one direction only
(which it is not necessary to detail), at the moment that each
armature passes the poles of either magnet, the induced current
is directed to the decomposing cell. B^ some of these machines
as much as two and a-half ounces of silver have been deposited
per hour upon articles properl;^ prepared for electro-plating.
Another useful apnbcation is to the development of a sufficient
amount of current lorce to produce the electric light (808) for
illuminating lighthouses. This mode of illumination has been
employed at the South Foreland lighthouse, and may probably
prove one of not the least important contributions of science to
the wel&re of mankind.
908. A very energetic kind of apparatus for ^nerating magneto-
currents has been constructed by Messrs. Siemens and ^dske.
* Mechanics' Mag., toI. zzxriii. p. 146.
:r of honc-ahoe mugnelE, o, <3, Fig. 613, or af bar-macitcla
ted at their further eii(bi with an Iron pUte, and hiiTing
imilar polaa a^acent, are fixed parallel l« each other, but
not in coniAcl, on a ilaud. Tho Hrmutiire e is rotated by a palley
and band f, which paaess over a larger pulley l, driven by a
winch H, to gain velocity. The coil passes leiiKlbnise over Ivo
opposite sides uf the armature e, huiI is encloBed in a caae kept
together by two rinfta shown in the figure One end of the coil
IB connected with tlie axis f, of the armature, which pauea
through the metallic suppnrt, v ; and in thia is a binding-screw,
w, for ono electrode ; the other end is cnnnected with a collar, b,
insulated friim the aita, r, and a projectinn from this comes in
cimtact with one of the springa, s (of which the upper one onlj
ia Hoen in the figure), nt the oionient v/hea each induced current
ia released ; the other electrode is attached to the support of tbeM
springs. It ia evident from the description that the saccesdve
currents will be eignal in intensity, and opposite in direclion, since
tliu induced uuirenti are in cnntrarj directions on the upper and
vniirr udes of the coil, from which tho current proceedi
altemalely.
909. As in tticte cases the electricity evolved bears a ratio lo
the magnetism induced in the iron nucleus of the annatuiea. it
Ibllows, that by increasing the intensity of this mngnetiaro, the
electric cuirent becomes proportion ably increased in energy and
quantity ; and, as by means of a current of electricity of low
potential powerful mi^tnetiam may be excited in an iron bar, the
application of this as the iaducing agent, lias been used !d ibe
construction of these machines: indeed, it was by a contrivuice of
this kind, Ibnt Faraday firat discovered these currents. The
moat powerful electro-macnetic machines are conslructed on this
principle; the following is a description of one of them. Two
ban or verj eoft iron, ■, 8, Pig. 513, ftboul fourteen incboa Ion
«nd an inch in diameter, are connected bj a croaa piece of iroi
Ajfirml; wrerfedlo (hem. These _ ,,„
bare are covered with a coil of
insutattd thick copper vire, abonC
300 feet in leogtb, the «nda of
which are coanecled with the
•crews, D,E. Oier this are wound
aboot 1600 feet of verj thin in-
ndaleil and vamtthed copper
wire, its enda being connected
with the screws, u, u.
Un connecting n, e with a bat-
tery of about ten elements, tbe
iron bars become snlEcieatlj map--
D lift aboQt silt; ponnd
; and, if the copper cj'lin
[i tbe moistened bands, an almost insupportable shock i
ensue, on breaking connexion with the battci7. To oSect this
rupture of contact with fncilitj, a contrivance umilar to that
used bj Mr. M'Oaalcj* will be found veij Dseliil : this consists
of a beam of brasa sup^rted by a horizontal axis at k, having at
one end a ball of soil iron, l, suspended, and at tbe other a forlt
of thick copper wire, so arransed that by its own weight it will
fall into two cupa of mercuiy Sied at k, and thus connect them
with each other. One of these cops is connected by a wire witb
a screw d, whilst tbe other is by a wire z, connected with one
electrode of tbe batterr, the screw e being in communication with
the other electrode. As soon aa these connexions ai« completed,
tbe bar s, becoming mague^c, attracts tbe ball i, which by de-
scending raises the fork h from the cups, thus breaking conlact
with the battery, and prodnci[ig a vivid spark attended n4tb a
loud snap, and combustion of the meri:ury. Tbe ban losing their
magnetism, the fork p falls by its own weight, and re-establishes
connexion witb tbe battery ; l is again attracted, and so ou,
the beam rapidly vibrating amid a complete shower of sparks from
tbe mercury, producing a most brilliant spectacle in a dark room.
910, As a rapid succeasion of powerful alternating currenta tir-
culfttes through the long coil at each rupture of contact, the shock
felt at the screws, o, H, or at the cylinders connected with them,
becomes intensely painful, com pie lely paralysing (he arms of (be
person grsaping the conductors. With these currents evolved at
o, H, the chemical decompoaitions already described (S2I), mny
be performed and other effects produced, as with a voltaic bAttery.
If a piece of charcoal be placed ou a, and a platinum wire connected
with H be drawn ligbtly over it, whilst tbe machine is in action, a
aeries of minute aparka from tbe induced currents will be observed.
512 ELEOTRO-DTKAMIOS.
911. WUde?8 Magneto-eUctrie Machine. — A machine of enor-
mouB and unprecedented power, arising from a happy comUnatioQ
of known facts and principles, has been constnictea dy Mr. Wilde
for the purposes of illumination. In 1838 MM. Moigno and
Baillard* showed that bj using an induced current from a mag-
net, capable of sustaining only a few grammes, to excite an electro-
magnet, the latter could be made to support 600 kilogrammes ;
but it remained for Mr. Wilde to apply an induced current from
this electro-magnet either to purposes of illumination, or to the
excitation of a second electro-ma^et, weighing three tons, the
current induced by which, in a coil weighing 232 pounds, melted
7 feet of No. 16 iron wire, and made 21 feet of the same wire
red-hot: and produced a Kght that, at a distance of a quarter of a
mile, cast a shadow of the gcu-flamei on the adjacent houses-f
It should also be mentioned that a steam-engine of seven-horse
power is requisite for the development of the full power of this
vast machine.
The construction adopted in this machine is precisely that of
Siemens and Halske's inductor (908), except that the arma-
tures are partially enveloped by masses of soft iron bolted to the
poles of tne magnets whether electro- or permanent, and by the
suitable arrangement of a commutator attached to each rotating
spindle, the transmitted currents are all in the Bctme direction.
And further, the magnets are placed vertically, with their poles
and armatures downwards, instead of horizontally, as in Fig. 512,
and the set of permanent magnets, sixteen in number, is placed on
the top of the electro-magnet.
The facts just described present a conspicuous example of the
interehangeability of physical forces : first, heat becomes dyna-
mical force in toe steam-engine ; secondly, this force becomes
electricity in the magneto-inductor ; and thirdly, this latter force
becomes neat and light in ite impeded passage through the di-
electric air.
912. As electric currents are induced by other currents passing
near the conductora in which they are excited, the theory of
Ampere (883), receives considerable support from the facts enu-
merated in this chapter. Granting with him that a magnet is
full of perpetually moving currents of electricity, it induces mag-
netism in a bar of iron, by exciting similar currents, as in the
case already mentioned (883), and then the remarkable fact of
magnets exciting electric currente in wires moved near them,
wilfbe resolved into a similar case of currents exciting currents ;
we are thus enabled to generalise the phenomena of magne-
tism and electro-dynamics, in a very important and satis&ctory
manner.
913. The phenomena of induced rotation produced by rotating
* Hoigno, Tdl^grmphie Eleotriqne. p. 15.
t Quarterly Joam. of Science, October, 18d0.
M AOKETO-INDUCTIYB CAPACITIES OF METALS.
513
a plate of metal ander a suspended magnet, may be referred
to a similar explanation ; the currents in the magnet exciting
Fig. 61^
similar currents in the
revolving plate, which hy
their reaction on the
magnet, cause it to re-
volve. Fig. 614 exhibits
a suitable apparatus for
exhibiting these, and the
converse experiments in
which the rotation of a
suspended disc is induced
hy the rotation of a mag-
net placed beneath it. In
order to make it evident
that the effect is not in
any degree due to mere disturbance of the atmosphere, it is de-
sirable to place a glass diaphragm between the rotating disc and
the magnet.
914. The amount of current force induced by the same magnet
in different metals varies considerably, as may be showu by the
mutual action of the induced and inducing currents. A convenient
apparatus for this purpose was contrived by tbe late Mr. Sturgeon :
this consists of a series of circular discs of different metals of the
same size and weight, capable of being supported on an axis, so
as to rotate between the poles of a horneshoe magnet placed hori*
aontally. If small equal weights be attached to the circumference
of each disc, they will ordinarily oscillate in equal times, if they
receive equal impulses, as by raising the weiffbt in each case to a
level with the centre, and then releasing it : out when thus made
to oscillate between the poles of the magnet, the times of oscilla-
tion will differ consideraoly ; and as the degree in which the oscil-
lations are retarded depends on the force of the induced current,
the amount of retardation will be a measure of the inductive
caracity of the metal.
With a very powerful electro-magnet, such as that used at the
Boyal Institution by Faraday in his experiments on light, and
dia-magnetism, this retardation is so considerable, that the power
of the arm is insuflQcient to draw a piece of thick sheet copper
rapidly between its poles. The sensation produced by this unseAi
resisting force, which increases with the effort made to overcome
it, is very peculiar. The same facts may be illustrated by a
variety of striking experiments, one of which is the following : —
Let a cube of copper, measuring about an inch each way, be sus-
pended between the poles of a powerful electro-magnet, and made
to rotate rapidly by twisting its suspension ; the moment the
magnet is excited, the rotating mass stops dead, but re-commences
its rotation as soon as the circuit is interrupted.
LL
514
CHAPTER XV.
ELECTSO-TELEGRAPHT.
The principles of electrodynamics have met with their most
extensive development in that most important recent contribution
of physical science to the comfort and conveuience of mankind,
the electric telegraph : — a practical application of the principles
of ahstract science, that ought to be received as a conclusive
answer to the cui bono question, with which the trulj philooophic
inquirer is not unfrequentlv assailed.
As time -signalling, whether for the purpose of merely recording
its passaee, as in eUctrie cU>eka, or for the purpose of observing
rerj small intervals, such as that occupied by a projectile in tra-
versing a space of 50 or 100 yards (which is the chief function of
ckronoecopes and chr<mographs\ ap^ars to belong to the category
of electro-telegraphy, some explanation of these mstruments will
be given in the conclusion of this chapter.
915. The earliest notice of the employment of electricity as a
means of telegraphic communication was in the year 1774, when
an electric telegraph was proposed by Lesage of Geneva, conaist-
ine of a bundle of twenty-four insulated wires, connected with
pitn-ball electroscopes, any pair of which might be made to
diverge at will by a char^ from an electrical machine, and thus
to in£cate some conventional signal. Cavallo, in 1795, proposed
to employ the deflagration of readily-combustible substances by
the discbarge of a Leyden battery, as a means of signalling at a
distance.
The earliest electric telegraph actually constructed appears to
be that of Mr. Ronalds in 1816. A disc' carried by the seconds-
arbor of a clock (228) having a radial aperture, revealed succes-
sively the several portions of the surface of a dial, marked each
by a letter, a numlyer, and some other signs. A similar apparatus
^s placed at another station, and the two connected by an in-
sulated wire enclosed in a glass tube, which was surrounded with
pitch and enclosed in a wooden case ; through which a discharge
of franklinic electricity from either station caused a pair of pith
balls to diverge, and the signal in view at the moment was to be
recorded. In all the mom recent and more practicjible forms of
electric telegraph a current of voltaic electricity, or one obtained
by induction, has been employed : but none of those systems,
which, like that of Mr. Ronalds, involve the exact uniformity of
VBEDLB TBLIOBAPH. 515
rate of two clocks situated at a distance from each other, have,
it is believed, been found available in practice.
A detailed description of all the yarieties of mechanism em-
ployed would be beyond the scope of this work, but the principles
both of construction, and mode of action, of the more important
▼arieties, may readiljr be rendered intelligible. An electric tele-
graph consists essentiallj of the following parts ; an eiectromotoTf
by which a current is generated ; a canaueiary by which the cur-
rent is conveyed; a communieatort or transmitting instrument,
by which signals are made ; and an indicaiarj or receiving instru-
ment, on which they are shown at a distant station.
The electromotor which has been found most available consists
of about 40 zinc and copper elements (793), the cells of the trough
beiuj? filled with sand, moistened with dilute sulphuric acid, in the
proportion of one part of acid to 15 of water, and the zinc plates
amalgamated by immersion in a solution of bichloride of mercury,
which renders them more durable.
The conductor consists of a stout "galvanized" iron wire, that
is, wire on which a coating of zinc % has been deposited by the
agency of a voltaic current, or b^ passing the wire cleaned by an
acid through a bath of melted zinc. The telegraph wires are in-
sulated by being supported in the air by a series of tall wooden
posts, with which they are connected either by passing through
porcelain rings, attached to the posts, or by resting in notches in
the top of an inverted cup-shaped vessel of class or porcehtin into
the bottom of the cavity of which the end of an iron hold-fast
is cemented b^ some g^ood insulating material, and the interior of
which ordinarily remains dry, and thus the escape of the current
to the earth, by conduction over a wet surface, is much diminished.
The circuit between two stations is completed by bringing the
ends of the wire into metallic connexion tnrough the medium of
the indicator or communicator with two metallic plates, one of
which is buried in the earth at each station. Although a limited
quantity of earth is found to be a bad conductor of electrici^, yet
when the entire mass is made to form a part of the circuit, its
conductivity is perfect; that is, in other words, the resistance
whieh the mass of the earth offers to the passage of a current is
wholly imperoeptible : it has been ascertained by experiment that
the needles of the galvanometer (856) are deflected precisely to
the same extent, whether the current be sent through a con-
siderable length of wire only, or whether the same wire be carried
out in a straight line, and the current returned through the earth,
by means of the metallic plates above mentioned.
A considerable portion of the current appears to be lost by the
imperfect insulation of the supports; and especially in damp
weather, when the surface of the porcelain cups, as weU as that
of the posts, is covered with moisture. It was ascertained by
some experiments on the wires connecting the l^ine Elms station
ll2
516
of Ibe Soath Weeteni Railway with PortamoDlb, that when tbe
nurent tranBrnitted by one wire was returned hj another, initeid
of by the eartb, tbe escape of the current from one wire to the
other at the numerous points of Bnpport was so oonsiderable, that
the galvanomcler indicated no ditference in the amoant of current
trtnamitted, whether the extremilies of ihe vires were coniwcted,
or whether tboj were iliscaimected, at Fortamonth.
For tbe puipoBea of the submarine tekersph, aerial insalation
is not aiailable, as ou land ; in (bis case the maul ation iacflected
everal jajcra of gvtta percha, or
.-Btion, and the conductor tlius in-
Eulated is called the core; of theae cFTeral are aumelimfs laid
together, and eurroonded with yam Batorated with some miilnre
of tar and grense, or tar and shell-lac, with which tbe interspace
between the cores are likewise filled up, and tbe whole is then
anrrounded with a series of gaWauized iron wires, sometimeB
separately covered with hemp (either tarred or not), wound spirally
in close apposition, so as to form a flexiLle metallic tnhe.
916. TlieNeedU Tdtgraph,iiov so eitenBirely adopted, is in nib-
stance, if not in all its present working details, due to the ingenuity
of Prof Whealsfone. Any two of tiiese instrmnenta, jilaced at a
distance from each other, and connected by a wire on one hand, and
with tbe earth on the other, constitute, with the requisite batterieB,
a closed circuit, and will act as commonicator and indicator re-
ciprocally. The earth-Cora mnnication consists of a metallic plate
buried vertically to a depth of several feet, so as to bo below the
reach of surface-desiccation of tbe soil ; this is called the tarth-
pltUe. 'ile indicator of the double needle telegraph con sials of two
pairs of astatic needles with their coila
IV. 516. ,857,^ p],^ vertically side by side ;
one of these pain of needles, k b, k'e',
is represented in Yie. 616. As an
ener^tio deflexion of the needles ia
reqaisite. to n-hich the middle portion
of the coil contributes tbe least force,
it IB omitted, or in &ct two coils, a, b,
separated by a small interval, are made
a use of, tbe wire being continuous in
direction from one to the other. Tb«
axis of the needles is prolonged, in
order to pota throagh a dial plate, omitted in the drawing, the
needle h a alone appearing in front of the dial, on tbe face of which
are placed two studs, for tbe pnrpoBe of limiting the eicDrcions of
the needle: and the lower end of one of the needles is made rather
Ibe heavier, in order that they may resume the vertical position,
when the current ceasea. c, t>, are two binding; acrewa, by which
917. Tbe
617
aide b; tide beneath the needlei, and Ihs metallic conneiions nre so
armneed, that when either handle ia moved «tdewa;B a bntterv ia
brooghc into the cinniit, and bj meana of a commutator attached
to the ails on which either liondle ia fixed, (he current is sent in
aqch a direction through the coila, a, n, that the needle B a mmj
mave in ihe aame direction aa the handle beneath it. The airange-
ment of the commutator will lie readiij under-
■tood from the diiigmm Fig. 516. Itoouainta Fy.aio.
of a disc of some iuanlaiinK material, a
onitsaiis by theeiteraalhanille, H, inti. . .
circumference of vtbich are inaerted the jiiecea
of braaa, a,b, e,d, t,f, g; of which a, c, am'
/ are connected, also b and e, d and g. Fun
flpringe rest on the circmnferenceof the com
matator, of which E goes tn earth, c and z an
the terminalB of the local butter
the line
Blrument in
handle vertical. In this position the local balteiy ia out of circuit, aa
there ia no metallic connexion between o and x, and tbe circuit from
the distant station is cloaed tbrough L^a— c— /— i, so that aignala
uaj be received from thence. If the handle, h, tie moved to the
rigAi, e comea into contact with z, d with c, and g with k, and a ia
alwaya in contact with i. ; the conrae of the current will then be
0— d— y— e — distant indicator— line -f"— »— l— a— e— ».
If moved to the Uft. then b cnmos into contact with i, o with c,
and E with e ; and the course of the current now ia
c—e — a — L— H — m" — line — distant indicator — b— « — 6— »,
or the reieree way through ihe cortesponding needle coils, and the
defleiions of the needles will therefore correspond with those of
the handle h. The signals consist of movements of either or both
needles, in the same or opposilo directions ; and one, two, or three
tDOremeota are found to afford a sufficient variety of higoala, which
consist of the letters of the alphabet, the numereia, yea, do, wait,
go on, imderstand, not anderstond, and eomo few other conven-
tions. It is, however, to bo regretted that the signals representing
the alphabet were not originally so arranged by Mr. Cooke, that
the letleia of most freqnent occurrence abould invariably be re-
presented by the simplest signals, whereby much nnneeeaaaiT
manual movement might have been saved, as is done in the drdi-
i^aTy arrangement of typea in a compoeiLor^s caae; in which the
letters moet in reqaeat require the least movement of the hand to
tEach them. This inconvenience was felt in tbe earlier daya of
tbe needle-telegraph, and the Morse code of signals (to he de-
scribed presently) ia now ver; mnch employed.
»18. The EUclrk -llonim.— Unle&s the traasmiision of a aigDol
618 BLBOTBO-TELBQBAPHT.
be rendered aodible, as well u Tisible, it would be necessaiy that
the dial of the telegraph should be constantly watched by the eye
of an attendant, by which the smonnt of personal labour would
be greatly augmented. This desirable object is attained by the
electiio alarum connected with each instrument, which, when the
apparatus is unemployed, always forms a part of the circuit, so
that the transmission of any signal is accompanied by the ringing
of a bell ; and the practice at any station is to continue to ring
the bell at any other station at which a signal is required to be
received, until the return of a signal indicates the presence of the
attendant, who fortunately has the powerofsilencing his clamorous
monitor during the transmission of signals. The electric alarum
consists of an ordinary clock train (228) with an anchor escape-
ment (258), a small electro-magnet (877), and a bell. A hammer,-
or clapper, occupies the place of the forked piece, k, Fig. 171, and
strikes the bell at each extremity of its oscillation, consequently
the bell keeps ringing as long as the train continues to run. To
the contrary side of the train a small horse-shoe electro-maen^t is
attached horizontally, and a small flat piece of iron is placea oppo-
site, and very near to the poles of the electro-magnet. This iron
keeper is connected by an arm with a moveable axis placed verti-
cally beneath it, whicn sustains its weight, and consequently en-
ables it to oscillate through a small angle by the application of an
exceedingly small force. Whenever a current passes through the
wire coating of the electro-magnet, the keeper is attracted, and
when the current ceases, it is removed to a small distance by a
light spring. In this latter position of the keeper, a pin rests
against it, which is fixed transversely to the axis of one of the
wueels prolonged, as s, Fig. 171 ; but when the electro-magnet
attracts the keeper, the pm is released, by which the train is
allowed to run, and consequently, the bell to ring.
It has likewise been proposed to make use of tne electric alarum
as a protection against fire, burglary, and other casualties ; for the
first purpose a platinum wire, connected with one electrode of a
battery, is inserted in the bulb of a thennometer, and another in-
troduced into the stem from the top, and descending to a certain
point, is connected with the remainder of the circuit, which in*
eludes the coil of the electro-maffnet belonging to the alarum :
whenever the heat 6f the surrounding atmosphere is sufficient to
raise the column of mercury to this point, the circuit is completed,
an^ the bell rings. The second object is attained by connecting
wires or stringy, attached to the several outlets of a house, with a
lever by the movement of which the circuit will be completed.
919. The letter-showing telegraph is an in^nious device of Prof.
Wheatstone. In this the communicator and indicator are distinct
instruments. The communicator, Fig. 517, consists of a revolv-
ing wheel attached to a fixed support. The circumference of the
wheel is divided into an even number of compartments, which
LETTEB-SHOWnCO TELEOBAPH.
519
Tig. 517.
Tig. 618.
consist aUernately of metal, and wood or ivory : the face of
the wheel is similarly divided, and a letter inscribed on each
compartment; and a row of pins, cor-
responding to the compartments, pro-
ject from the circumference of the wheel.
Two brass springs, each connected with
an adjacent binding screw, rest against
the wheel, one agamst the axis, or any
other part entirely metallic, and the other
against the rim, consisting of alternate
portions of condacting and non-condnct-
mg matter. It is clear that if the binding
screws of this instmment be connected
with the electrodes of a battery, the circuit will be alternately
made and broken, as each letter passes a given fixed point.
The indicator, like the alarum, consists of a cloclc train, with
an anchor escapement and an electro-magnet, the terminals of the
coil of which are attached to two binding screws ; the pro-
longed axis of the scape-wheel carries an index that traverses a
dial. Fig. 518, the circumference of which
is divided into as many compartments as
the communicator, and corresponding let-
ters are inscribed on each. In this instru-
ment the iron keeper is attached to the
axis which carries the crutch or anchor, d^
Fig. 171; and the pallets, a 5, cd^ Fig.
197 ; consequently, one tooth of the wheel
will escape, and the index will advance one
step, at each movement of the keeper : but
this movement takes place as the circuit is
alternately made and broken by the com-
municator; consequently, the movements of the communicator
and indicator will exactly cori'espond, and the letter at which the
rotation of the former ceases, will be shown on the latter. A
somewhat greater degree of certainty in the indication of a signal
may be obtained bv substituting for the index in Fig. 518, a disc,
with letters inscribed round the circumference, which is placed
behind the dial, through an aperture in which the required letter
is shown.
It should here be noticed that the construction of a small
electro-magnet, which could be effectively excited by a current of
moderate potential from a distant station, is entirely due to Prof.
Wheatstone ; although it was first brought into extensive use in.
the Morse-telegraph.
920. The most powerful arrangement, and that best adapted
for the transmission of signals to great distances, on account of
the high potential of ma^eto-currents, is the magneto-electrio
telegraph, which is likewise due to the fertile genius of Prof.
Bmt^tmmm'mm^i^^'^^^^^^^^^r^^^y^f'^^'^^
520 ELBCIBO-TKLEORAPHT.
Wheatstone. In the earlier instramenta of this kind, the elec-
tromotor waa an armature similar to that of the magneto-electric
machine (905), in which a current is set free hy the rapid move-
ment of the armature either towards or from its position of maxi-
mum induction. The magneto-electric telegraph possesses the
advantage of perpetuity of action, without any renewal of a bat-
tery, which is necessary in all other kinds ; this must be looked
npon as a decided advantage. Some of the earliest forms of
electric telegraph were on this principle; as that of MM.
Gauss and Weber, of which an account was first published in
1835; and that of Steinheil, which was in operation in 1837.
Henley's instrument, which has been frequently employed, is
constructed on this principle. A powerful compound magnet is em-
ployed, consisting of a number of fiat bar-magnets firmly bolted
together. An armature, enveloped by two large coils of filne wire,
is placed opposite each end of the compound mi4piet: these
actuate two needles, placed side by side, as in the ordinary needle-
telegraph.
921. An efiective instrument of this kind was many years since
constructed by Prof. Wheatstone. The communicator is a rotatine
disc with projecting spokes, similar to Fig. 617, but placed hon-
zontalhr ; the indicator is a dial, as in Fig. 518. The armature,
instead of rotating, is suddenly withdrawn a small distance from
the permanent magnet, by means of a lever acted on by a series
of cams placed round the circumference of a wheel, or cam-plate
(255), and shaped something like the blunted teeth of a saw; by
means of these the armature is suddenly withdrawn, in onler to
nve full effect to the induced current in the armature, and gra-
attaUy approximated, to diminish the induction of a current in the
contrary direction. The current thus induced is one of very high
potential, and therefore well calculated to overcome the resistance
of a long cirouit : one of these instruments appeared almost to
reouire the intervention of 600 or 800 miles of telegraph-wire, in
order to work satisfactorily ; in fact the distance, through which
signals might be convej^ed, exceeded the existing means of testing
the capabilities of the instrument. The construction of the indi-
cator is the same as in the preceding (919), except that the signals
were only ten in number, the numerals and a cipher. The Ad-
miralty code of numerical signals was intended to be employed in
this instrument.
922. Siemens* Magneto-Teleffraph.— The difficnltjr of starting
and stopping the heavy armature aboye described, with sufficient
facility and rapidity for signalling, rendered the preceding instru-
ments inconvenient in practice : this inconvenience is partially
remedied in Siemens* communicator Fig. 519. In this, as in
the inductor Fig. 512, an armature, e, coiled longitudinally,
rotates between tne poles of a series of parallel horae-shoe mag-
nets, o. Thecnaeortbi
to the ImUer n[ which
one terminal of the
coil is altached. The
coit is driTea by a
wheel, L, gearinff into
Tm); and the wheel ia
atlacbed to the a'lle
bj a cr^la-joinC, in
onler that the rotation
OUT be stopped at the
Tight point, b; de-
pressiog the handle
into a notch in the
•emted rim i beneath
it. One notch cor-
KBponds with each
hall-tuni of the anna-
ture, and coTi«equent-
Ij, with each induced
carrent released; and, as before described
currents are in opposite directions.
•erted into the cap* f, r".
th«
The indicator la n hand moTing round a dial which is fixed on
scape-wheel, driven by a propelment (SOS), which ia
_c .L.. .1 ,_ m .-n i^p scape-
precisely the reverse of that shown in Fig.
wheel. A, is driren by the OBcillnting apnng-
palletA D, E, the moirameQtB of which are limited
by aiMusting-acrews, to prevent "tripping" of
the wheel, as are also the oscillaliona of the
■ofl-iron bar, a, by the aiJjnsting-BcrewB, r, a.
The hiwer end ofs is indnctively magnetised by
the coil c, and according to its polarity, is at-
tncied by one and repelled by the other of the
poles a, B, of two hone-shoe magnets placed on
rither side of it ; and the site mate currents be-
iiiK opposite, the bar a and its attached pallets
wul oscillate and drive the scape wheel and
its index in accordance with (he movements of
the winch, n, Fi^. (519). In thie apparatua
tha coil rotates with the least possible vis viva
it has been frequently employed in practical
teh>gTaphy.
923. irAeii(ftoiw'iifam«to-2WeffrapA.— In the practical apj
plication of the eleotrio telegraph, rapidity of action is a point of
Rg. Ml.
"Ttl
52a
great imponADce ; Rod the ftbuace of thii qoililj, in conaaqnaBca
of the Til viviL of the rotating mui, for aame tinte conatitDled tlie
cbi«f objection to nuigneUvtelegnipha. Thii objoction haa Wea
entirelj obviated in aa eitremel; elegant and portable inatra-
ment, Fiir. 521, coutnded
hj Mr. WheatMone, in wiurk
A ii the commnnicatoT, ud a
the indicator. The eiccItcDce
of thin iaitrament depeoda on
two points of conalnictiiia —
Gnl, the monng parti of the
, indicator am rediiced to ibe
I emaUeat pwsible dinmaMaa,
I bolh as to bnlk and raogc o(
^ molion, and conaeqiwnlly tbe
leant amonni of ^<rk i> re-
quired from tbecnrreDt; asd
aeeondlj, the loft iron keeper eiont rotaU* in the cannDnnicstar,
and rotalea coD^Diiouelj; the induced cnrreota being intemaUr
intercepted, when the^ are not wanted (□ be lent into the circtdt,
— J .1- — I .-. 1... :„ overcoming and restariug the
m. . . .._.7.^ ^j jjj^ ke«p«» ii
' the figure, And I
in^l; iogeniouB contriTance the indnc^d cnireBta ai
alternatelj in opposite direction!.
924. The inducing hon&ahoe n^net h. Fig. 52!, doe* nat cx-
j. ,,_ ceed four iocbea ia bagdi.
^'" In front of the polea are fixed
IfoureqaidiitaDtfoftiroDMna,
tvo to each pole, lamMmdoi
by coili of a cODtiuDOii* win
wound in the lame direction
□□ A and a, and in tbe roa-
trarj direction on c awl a.
It haa been ahovn (BBC) tk«l
the currenta indoced in ap-
proaching and receding Itobi
coDlaot are in Miponte direction!; and in order lotnce tbe direclioaa
of the inducted carranti in thia apparatag, let the keeper ba oppawte
to B Hod D, and let the receding carrentofsbe •*-, theoMDcac n a
similar pole, bnt its coil wonnd oppoiiieli/ to a, tbe appioacbing.
current of O will al» be +, and will conaoqaentl; teinforee tbe
former. The receding-onrrent of o will hence be ~, and aa c aad
D are contrary polei, but their coila are wound tkt «^b«
tmu, the approaching-current of i> will alio be - , and Ibeae again
will co-operate. Similarly the d-a currenla will be both 4-. aad
the A-B, both — ; and as the keeper is a little wider than the m-
SZ3
t«ml b«twMn two aitJBceiit cores, the nmilnr curranU arc in eftch
case oomplelelj' blended into one. Thus it np^an that n current
in one directioD ii diicharged wheDeTsr the keeper is Tertical. and
one in the opporits direction, vheaeTer It is horizontal. The vinble
part ofthecommunicator consiataofadial a, Fig. 521, round which
are placed the letters of the alphabet and aome sigiio, with an
index pointing to them ; then are sairounded bv an equal number
of studs at the ends of radial levers. When no stop is down, the
index ia in gear with the mtating ails, and advances one step for
eitch induced current; but when the index arrives at a stop put
down, it stops, being pat out of gear with the rotating plate,
and while the stop remains down, all currents are intercepted hj
an internal short circuit. When onv lerer is depresBed, it re-
mains down, until another is depressed which raises the former to
its QonnBt position bv a rer; simple and ingenious mechanism.
This consists of an endless chun, Ijing in a circular groove under
the ends of the levers, with just enough of " slock" to allow one
lever only to be depressed. As soon a.s the depresse d ke; ia
raised, the index again travels, and corresponding Gorreals ars
Bent into circuit.
926. The indicator isa dial, B, Fig. 523, sarronnded bj the same
letters and signs, as that of the communicator, the index of which
is actuated bjr the propelment (259), the wheel not being larj^r
than tbe Bcnpe-wheelofasmall Geneva watch. The reciprocating
piece. B, Fig. 198, is governed bj the alternating movemEUts of a
small frame, a. Fig, 523, contwning -. ^^
two curved magnets, not larger than _ "'
knitting-needles, placed sjmmetricall; I
in rcvemed poeitioue. As the centre I
of eravitj of this frame is mode to CO- ■
inctde with the axis of motion, its I
movementi are not in an; position im- 1
peded bf gravity. Tbe pennaneat I
magnets are placed between a pair of I
smul cylindrical electro-magnets, b, I
which are likewise placed in a pHmllei '
but ravened positioD. The magnetism of these is inverted by
each succeeding corrent, consequentlj holh attractions and re-
pulsions are bronght into pla; to produce the movement of
the oscillating magnets, and there is no DeceBsit; for an; spring
or other mechanicBl resistance to be overcome bjthe very minute
electromotiTe force employed ; which is rendered efficient only by
the extreme delicacy of the mechanism. The armature is oon-
524 ELECTBO-TELEORAPBT.
municator in its previous position of rest. Conseqnentlj it is
necessarr to ascertain the correspondence of the indicator and
commnnicator, before the signalling commences. This, in fiust,
is eBsential in every kind of dial-telegraph. This oompendions
instrument has been extensively employed by the Universal
Private TeleCTaph Com[>an^, for private use in the metropolis,
and in several large provincial towns and cities.*
Printing Telegraphs. — Various kinds of mechanism have been
from time to time devised for the purpose of rendering the electric
telegraph automatic; that is, that either letters, or some kind of
conventional signs, should be impressed upon paper by the agency
of the mechanism itself. Some of the earliest successful essavs
in electro-telegraphy were in this direction. Steinheirs tele-
graph, already mentioned, comprised some special mechanism for
this purpose : and that of Prof. Morse merits notice on account of
being still extensively in use in America and on the continent of
Europe. Although the idea was conceived some years earlier,
the actual construction cannot, it appears,t claim an earlier date
than 1837.
926. The Morse Telegraph. — ^The transmitting instrument, or
^ Morse key " as it is commonly called, is a lever a b. Fig. 524,
moving on an axis c, on the under
^•^**' side of which is fixed a stud, d, jost
above another fixed stud b, from
which the lever is raised by a
spring, o, to a distance regelated
by a screw at b. b is connected
with one electrode of a battery, and
F with the earth ; the other elec-
trode being connected by ihe line-
wire with the electro-magnet of the relay at the receiving station.
l*hus it is evident that whenever the handle, a, of the key is de-
pressed, the paper will be marked. A momentarr depression
will produce a " dot,'* and one more prolon^d reqaal to the time
of making two dots) a " dash," and the various letters and nume-
rals are indicated by different arrangements of dots and dashes.
The series of marks that constitute a letter, number, or conven-
tional sign, are separated bv small spaces, consecutive letters by
larger, and separate words by still longer intervals.
927. In the receiving instrument. Fig. 626, a b, is a train of
wheels, wound up by the liandle o. Between the roller p, con-
nected with the train, and another, q, in contact with it, a strip
of paper v m is continually drawn when the machine is in action,
* The writer has made oae of it Jbr Mversl ^een between his reeidenee
in FiUroy Square and the Westminster Hospital, with great oonvenience
and advantage. He has fonnd it perfectly easy to ose, and not Uable to
derangement.
t M. Hoigno, Traitfc de TOtgraphie Eleotriqae, p. 76.
B2&
oDad from s ctnl, n. A lerer i l mareabls «d a fnl-
iBBt theendEapaintedBcrev, that isailjuBtediupreH
OD the surface
per, lehea the nf.BOi.
I. is attracted
t-o-magnot, t.
of the leier i«
a third arm
contact vitb
marked bv
v<l hy tha poini
T into a small
tha roller, q.
t ceases, the marking pmnt is witbdravq from tbe
le action of a BpriDg, s. Ons of th« temiJDals, B, of
is carried to one electrode at a battery, the other ia car-
relay (930) and thence to tha other electrode. Th«
leased, aod the paper drawn over 4, hj tbe oclioti ^
(-acting indicators, u., those vithont a local circuit, the
.' goes to an elecCro-mae»et within the inatrumedt.
le MoTM Alphabet. — -The fuUoning ia the "Morse"
:Dals, in which dne attention has been pnid to hreiit;,
atiog the lettera of the most frequent ck
929. The Morie Priming T«I«^apil,— Tbe suUtitntioaorink-
maTka for the emboBSed miirlu jiul described is nov verj gene-
rally adopted. Several mixlificBtioiis hare been propoaed snd
more or less adopted, the chief vnriation of which conawts in the
mode of dolivetiiig tome kind of oily ink to ■ gmall roller which ii
■ubsti luted for the emboasing poiol. The most coaTcaieat ptui
uems to_ be that of Mr. WhcaUtone, in which a grooFsd roller,
rolla against the marking roller, and feeds it with ink taken np,
hy adhosinn to its edge, from a reBervoir beneath.
930. The Btlay. — Signaia sent throagh long circuits are now
very genevally received by inatrumenls called relays, designed to
transmit currents from a local battery into a freiih circnit, which
may be a aecoud line, or merely' a local circnit, comprising a
recording ot other instrument, which could not be worked directly
liy the ieelle currents received from a instance. The' relay
^. gie_ is entirely due te Mr. fVheat-
stone, but it first came into
general use in America, in con-
neiion with the Horse emboa-
sing telegraphs. A lever rock-
ing on a support i, Fie. 526,
has a spiral spring attached to
its shorter arm, B, the tenaioa
, of which is regulated by a screw,
^ c. Hie longer arm is cicawd
J by the annataro of an «tectn>.
magnet d, and teminates
between two a^usting screws,
B, F, passing through ihe aims
of the standard a, and of which e is insulated from a The
binding screws a, %, connected with the aCandards, o,c, receive
the terminals of the local cireuit, and the electro-magnet, those of
the line circuit. Whenever the electro-magnet is excited, the
armature d is attracted, and the end of the iever coming in contact
with the point of r, completes the local circnit. It is evident that
in this inatniment the line.current has no further work to do, than
to eicite the electro-magnet to attract the armature, and thus to
close the local circuit.
931. Sitmtm' l^larUed Belay. — This appsratos ncaHssea a
magnetic arrangement of great merit, which has been largely em-
ployed in diroct-acdng receiving instruments : and ajtbongfa
specially designed to work with nearly equal reverse cnrrenls, it
can be adjusttd so as to work with -(- or — currents alone of any
strength, as well as with reverse currents of any required relative
Btrenj-th. Fig. 527 represents a plan, and Fig. 628 a vertical sec-
tion of this iaotrument. The electro-magnet, b, b, is composed of
two cores of solt iron, united in the ordinary manner by the cmaa-
bar A. The coils of this electro.magnet terminate in the binding
*r. 627
•erem, I aod 2. Tbe bent pennanent maj^et, h, i, is scnwed
upon the cnMB'b&r, and eommoiiictilea north-polarilj to it, and
to the two coral and poUs of
the electro-mapiet, B, An iron "»■ '^■
tongue, 0, is attached tn the
toath pole, a, of the magnet
voder the bracket a, and this
receives ■onth-polarit}'. This
tongae ia so placed that it ia
free to move eaailj between the
north potea, x and m', of tbe
electro-magnet. Its extent of
motion U regul&ted b; the
Ecrews, D Bnd d'. The point
of the Bcraw, D, is u>ed as the
contact for cloaing the local cir-
cuit, including the printinf; in-
Mnunent, and local battery.
The acrew, d', lisi an a^ale point, and, ^
aguDst it, the local circuit ia broki^ii. p ai
■creva of the local circuit. Hence
it follow! that tbe aoutb-potariied ■
tongue, c, cannot be attracted to P
either of ihe north.polarixed endn I
of the electro-magnet, s, a', whilst I
it is ilationed equidistant from I
both. When this is not the case, ■
however, it will always be at- 1
tracted to the nearer pole.
'When the relay is naet
circuit working with rever
rents, it is so adjusteil that the timgue lies upon that side to which
it waa last impelled. On lines, however, where currents in onl/
one direction are used, the tongue, a, must rest against the point,
l/, which is insulated, when there is nn current in the circuit, i.e.,
while in a state of repose ; it should therefore be more powerfullj
attracted by the pole, k'. This pole, tberefore, performs the fonctiona
of the ^ring in ihe ordinary relay ; and, where in the onlinsTy relay,
the spring is tightened for strong cuirenU, in this inslrument the
ttmgne, c, is simply thrown out of the point of equilibrium between
the poles, K and a', and made to rest on the side «', These
changes are eSecled by means of tbe adjusting screw, k.
932. Bain's electro-chemical telcgnipli dt>es not diOer essen-
tially in its constmctioQ from Moneys ; but, Instead of the style,
which is of steel, being imprassed on llie paper, the latter ia
moistened with a solutiou of cyanide of potassium, and passing
over a metallic cylinder forms part of tbe conducting circuit ; and
528 ELECTKO-TELEQAAPHY.
wherever a current passes^ the salt is decomposed and the paper
Btained by ferro-cyanate of iron, or Prussian blue.
933. Two modifications of printing telegraphs due to the in-
genuity of Prof. \\'heatbtone demand some special notice. One
of these ranks amongst the earjiest important improvements in
electro-telegraphy, although ita -complexity and consequent oosUi-
ness may perhaps have interfered with i t s practical application. In
this apparatus the communicator is the same as that of the
letter-Miowing tele^ph (919) ; and the indicator is also similar,
but with the addition of tne urinting apparatus. In place of the
disc of letters there describea, is a revolving circular plate of thin
sheet brass, which is cut radially into separate strips, and a raised
metal type attached to each ; the types are chaii^ed with printing
ink, by a roller attached to the machine. A shp of paper passes
slowly but continuously over the surface of another roller, very
near to the surface of which the required letter stops, and is then
gently pressed or struck againbt the paper, by an appropriate
action of the machine itself. In order to prevent damage, it is
ingenioufily contrived that the printing action can take place only
when the circle of types is quiescent. By these means any oom-
niunication may be printed in very little more time than is other-
wise required for its transmission.
934. The Automatic Priminq Telegraph. — ^Another form of
printing telegraph has been devised by Prof. Wheatstone, which
appears to possess manv important advantages over its prede-
cessors : it might be not uaptly termed the " Jacquard tele^^ph'*
from its analogy with the loom of that name described m 1^.
The analogy consists in the message being previously prepared by
punching small holes in a narrow strip of paper; this is effected
by a small machine consisting of three nunches placed in a row,
the middle one being smaller than the otner two : these are acta*
ated by three levers having raised studs for the fingers to rest on.
Q'he machine is provided with a ratchet and click (265^, by which
the paper is carried forward a small space after eacn action of
either of the levers : this in machinerv is termed ^feeder. The
alphabet is represented by various combinations of the holes in the
outer rows ; the middle row merelv serving the purpose of carrying
forward the prepared paper in tne communicator, f^g. bt6 re-
presents the appearance of the paper.
fW. A29.
For the purpose of transmitting the message, the end of this
paper is placed in the communicator, and a similar plain strip is
placed in the indicator ; both of these have a feeding action eqnal
AUTOMATIC PBDITDrO TELBORAPH. 529
in amount to tbat of the punching machTne. On torninff a winch
in the communicator, three metallic pina rise against the paper,
one of which will neceaaarily enter a nole, and will close the cir-
cnit by coming in contact with a plate of metal placed oyer the
paper. When the contact takes place through a hole on either
flioe, an electro-ma^et in the indicator becomes actiye, and a dot
of ink is debited m a corresponding position on the strip of paper.
If the circmt be completed tnrongh a central hole, the autograph
in the indicator is merely carried forward one step, without being
marked ; thus a space is secured between contiguous letters, and
bj a larger number of central holes, a longer space between words :
and a printed fac-simile of the outer rows of perforations is ob-
tained. By a more recent arrangement, the indicator prints the
"dots" and "dashes*' of the Morse code (928), now so generally
employed and understood by tele^aph clerks. This apparatus is
not impeded in its action by a motion sufficiently rapid to transmit
500 letters per minute. As the inconyenient delays, experienced
by the public in the use of the electric telegraph, frequently arise
from the length of time necessarily oocupiea in the transmission of
signals on the systems now in general use, it is eyident that much
important time might hereby be sayed, as any required number of
hands might be employed in the preparation of messages. Also
meoaces might at little trouble ana expense be prepared by priyate
indiyiduals, in which any arbitrary signs might be employed.
Another adyantage would be that long and important despatches
might be simultaneously transmitted to different places, as the
prepared paper might pass through seyeral communicators in suc^
cession, all moying at the same rate. It may be remarked that if
the dots above and below the central line be supposed to cor-
respond with the moyements of the needle right or left, the ordi-
nary alphabet of the single-needle teleffranh may be employed.
A double row of dots on each side might be effected by a little
addition to the mechanism, and by employing five punches, which
would pennit the employment of the code of the double-needle
telegraph, which is in yery general use.
935. The copying telegraph, invented by Mr. F. C. Bakewell, is
analogous in its operation to the electro-chemical telegraph already
described (932). The communicator consists of a metallic cylinder,
which is kept uniformly and slowly rotating b^ clockwork, rouna
which is placed the message written on tin-foil with sealing-wax
vamish. A metallic point rests with light pressure against the
sorface of the tin-foil, and is moved gradually forwards by means
of a screw placed parallel to the cylinder, and rotating with it, by
the rolling contact of toothed wheels ; so that when the cylinder
has made one revolution, the point has advanced the distance of
one thread of the screw, and uerefore passes in suocessive parallel
lines over the jpieoe of tin-foil. The cyunder and the resting point
are in connexion with the opposite ends of the battery circuit, and
M If
fiSO HUtOTBO-TILMBArHT.
it 19 eTident tbfti a cnrrsiit will pasa aioopt wtisn the point loti
on onj part oftbe vamiab-letters.
The udicator in thii appantiu is preciwlj nmilar to the com-
muiiicAtor; but in place of the tin-fail ia a piece ot paper nxa*-
tened with a solutioa of F^rroc/anate of potasR, and dilute h^n>-
cblorio acid, the point or at^le retting on vbich ii oTateel. When-
erer a cnrrent masea through the paper, it will be coloared by the
fonnatioD of Pnuaian blue ; and conaeqneatl]', the uncoloured
partioaa of the paper wiU exactly correapond with the writing
on the tin-foil, aa in
'*••*'■ Kg. 680, which i» ■
ifac-aimile of a written
commanicatJon be-
tween London and
Bngbton. It ia eri-
dentJj neceasarr that
the rotatioua d the
two c/lindera ahonld
be Bynchranon* ; this is accompliahed by an electro-msgnelio
regulator, the action of which depends on the force of a aeparala
ourrant, which ia ai^usted by the amount of lurface of the nen-
036. In concluding o
ctical branch of elecl
i« of the difSoultieg that occur in yery long, and especially in
auumarine, circuitg. Oftbeiie the mogt important ia the retention
of the current in the condactor by induction through ita inanlating
envelope on the aarreunding medinm. The conductor in fact be-
comee aaaimilalad to an extremely elongated Leyden jar, in which
the charge ia for a time retained by mutual induclion in tbe ooat-
inga, and exhausted only by degrees. Thug, in the chemical
pnnling telegraph (820), tbe depoait of Pmaaian blue would ter-
minate abniptly on breaking oontact in any moderate aerial ^r-
ciiit; but with a lubmarine circuit the cbeuiicat action aubudei
gradually, and ceaxes only after a considerable interval, thui ren-
dering each signal liable to become blended with the aucceedinr
one. Reversal of the current, the employment of a current of high
potential nbtained from an induction coil (SST), and varions other
expedients, have been reaorted to. In illustration oftbe influence
uf induction in retarding the trannnisvion of currents, a series of
eiperitnents was made on the subteiraneous wires insulated with
giitta percba, and enclosed in metallic tubea, Itid between London
anil Manchester. When the ends of tbeae were alternately oon-
nected. they presented a continuous circnit of more than 1500
miles in length. A period ot two aeoondj was required for an
electric current, entering one end of the circuit, to reach a galva
BUBMABIini CABLB8. 531
nometer placed at the other end ; and the electric wave appeared
to flow on throngh the wire, after the contact was broken. It waa
found practicablci by properly-timed contacts, to deflect the needle
at the proximal end of the circuit by a second galvanic impulse,
at the same instant that the needle at the distal end was deflected
by the first current-wave. These effects are the result of lateral
indueUonf and are necessary consequences of the relations of con-
duction, insulation, and induction as laid down b^ Faraday.
Another important difficulty is, that in consequence ot the loss of
current-force by imperfect insulation, in long circuits the amount
of force transmitted is not sufficient to actuate the mechanism of
the indicator ; this has been obviated by the use of relays (930, 1).
937. Submarine telegraphy dates only from 1851 ; nearly 2500
miles were laid up to the end of the year 1857, and more than
15,000 before the end of 1863 : but the crowning triumph of iup
^nuity, skill, and perseverance was reserved for the year 1866,
m which not only a new cable has been successfully laid across
the Atlantic, but the lost cable of the preceding year has been
recovered and completed. And so perfect is both their conduction
and insulation, that when the ends of the ti*o were recently con-
nected on the shores of Newfoundland, a single element of very
small dimensions sufficed to send a signal from the shores ^
Ireland across the Atlantic and back again.
It would be impossible here to enter into a detail of the various
constructions of cables that have been successfully or unsuccessfully
employed ; but as the last Atlantic cable may be looked upon as a
model, it may not be uninteresting to give a detailed account of
its construction : —
The eonduetorf a strand of seven copper wires of No. 18 gauge
("048 inch in diameter), six being laid round one, imbedded in
Chatterton*s compound (a mixture of shell-lac, and Stocldiolm tar,
or some other similar material^.
The inmiatorf four layers ot gutta percha, alternating with four
thin layers of Chatterton's compound.
The proteeiOTf ten wires of No. 13 gauge ('095 inch in dia-
meter), drawn from Webster and UorsfalT's homogeneous iron, and
galvanised. Five strands of white Manilla yam are laid round
each wire, and hempen yarn round the gutta-percha cere, both
being saturated witn a preservative mixture. The ten covered
wires are then laid spirally round the covered core.
The weight of the cable in air is 31 cwt per nautical mile, and
14} cwt in water, its sp. gr. is therefore r83.
Its breaking strain is 8 tons 2 cwt. ; that of the cables of 1858
and 1865 having been respectively 3 tons 5 cwt., and 7 tons 15 cwt.
938. Siemens andJBalske's Cable. — One other form of telegraph
cable, conspicuous for its lightness and tenacity, may here require
notice. The conductor or core^ a, Fie. 531, consists of a strand
of copper wires, thinly coated with Cnatterton's compound : the
M m2
■LKTBO-TILBOBIPHT.
insulator ooariits of thin Utctb o( Monlcbooe, b, laid kaghadrndy
«od thBir edges nnited by heavy premiue, the sMUns of ■OLti*mi
Uf era being at right angles to etch other j these m
by layer* i» gnlta percba and Chattvrton i oompoinid, e. The
protector coneiBta of two layera of hempen itringa aatnnted will
tar, if and e, laid ipirally in gppoidte ditMtiooa ; and over Af a
■pirallaTer,/, of stripe o( phoaplmietted copper, OTerlaptnug each
other half war, but redaced tea level siiriiK«bf cotuideraUe pna-
(are. TheactualdiRmetarof theseTeral parts is aliown in the fenre.
939. Sooper's Oore. — The iasnlating properties of caont^oDC
being nolonouBly neatly superior to those of gnttapercha, a node
of insulation has been devised by Mr. Hooper, which ia beKand,
and has bitberto proved lo be, free from the liabiii^ to MAMtisK
and consequent loss of insulation, Id which the inaalatCT ia ex-
posed, when vulcanised caoutchouc' is in ooDtact with tlie cener
conductor. This plan ooniists in surroDnding ths condnctar with
a coating of pure caoatchonc, which is sepxrated from another of
the mlcanisabls compound, by a thin layer ofauotber compooBd,
which is impennesble by ihe mlphnr of the ouler layer. "Toe cote
is then exposed for some houn to a lempeiatare of SBO* F. by which
the outer layer is Tulcanised, and the whole modersd oooipact
and inseparable.
The dumbility of this core has been confiimed by Sr C. Bright,
who foonJ that a length tested at T!>' F. in Hay, 1863, bad a n-
NsUnceofeoOB. A. units per knot, and that the sum coil, vhsn
again tested at the same temperatnre in December, 1S6& had
acquired a resistance of over 800 anils. The resistaaoe of Hr.
Hooper's core at a temperature of S12°F. has been faond to he
doable that of the Ferwan Oulf core (insulated with gnttaperdia)
■t 100° F.; and the former sustained no ii^ory by uiat elentioa
of temperalura.
Under a preasare of two tons per square inch (equal t« that of
a column of water two miles deep), the resistance ot^thia iMolalor
was increased one-third in about 31 hours, and two-tUrdi is «ii
days : and it appears from the eiperienoe of tbe Atlantic cable,
tbAt other insaiators are similarly unproied by sabmuinaL
It has been staled by Prof Wheatslone that a roatiag </ CMRt
chouc of a giTen thickness is equiTalent lo ooe of gutte parth* ti
«. MS
the thiokiwBa ; luid tinta tba ciperimenti of Bir W. Thom-
appean that Hr. Hoopar*! innilatoT ha*iag a diamator
) S-76 time* that of the ooadactor, ofien about 10 per oent.
speed in aignalling than a gntta perrha ooveriDg 389
he diameter of the conductor.
A Tery logenioiiB form of differentia] geariag haH been de-
ly Mr. Fleeming Jenkin, for the porpoge of nanling in, or
oat, telegraph cablea : the object of which is to maiDtain
,nired amoimt of coDstant strain on the cable, and at the
ime l« proTJde that the machiae itself Bball give waj,
'er that Hied etnin is exceeded. Thii macbiDe, £1g. d32,
rolled by a fticlion-brenk p.
the mechaoical geniua of , '
le Mr. Appold. In the
7 fnctioa-lnreah the ends of
ak-etrap a, b, are attached
iver moling on a fblcrtun
a them, and Ittim the ehort-
the distance bom the ful-
0 A and ■, it beoomee Tery
1 lo r^nlate the Mction by
a with any degree of nicety.
Appold a break, the fiil-
Appold I
- ,. . _ . -, j, of the breakwheel, and
ight is applied at the end of ■ boriiontal arm, c d. It it
bat the action oftheveightwill be to iDcreaae the distance
ID the points a, b, and e, and therefore to increase the ten-
f the break.atiap, but much mors gradually tbaa by the
ry codttruction. Two interroedialo wheels r, O, are al-
io the break-wheel, and are in gear with an annoUr
■, on the dram, and also with a wheel, B, on the driving
-. It ii evident that so long aa the tension of l is not
r than the friction of the strap akb, the break-wheel will
I at rest, and the drum will be driien by n through r and a ;
e moment the tension at L exceeds the friction, the dram
ther remain at test, or eren pay out, the fiiction wheel only
then driTen round by h.
. Eleetrieal Tati. — The requisite* of a weli-constmcted
,rine telegraph cable are —
jov resistaoce in the conductor ;
fligh resistanoe in the inmlator;
jyn iitduelive aapaeUy, or power of receiving a charge ;
ableo' ' ■ ' "' ■
ise qualities sre capable of admeasurement, by menus of ap-
ate teste ; and other tests are employed in determining —
rbe soundness of spliced joints ;
rbe nalnre and locality of a &nlt, occasioning loss or inter-
n of the current ;
rkeiuteraal reustanoe of the battel? employed.
OK
SSS3C
, S- <.'.^^S
534
ELECtECKTELBQIULPRT.
942. The resistance of the condoctor may he most readily deter-
mined hy means of Wheatstone*s bridge (801). The specific
resistance in B. A. anits at 0" C. of several metals and alloprs
has been carefully determined by Dr. Matthiessen ; as also the in-
creased resistance due to elevation of temperature. The resolts
are given in the following table, in which the first column is the
resistance of a wire one foot long, and weighing one gprain ; the
second the resistance of a wire one foot long '01 inch iu diameter ;
the third, an approximate value of the increase per cent, of re*
aistance for one degree, at the temperature of 20° G.
Silver, annealed . .
0-2214
00994
0*377
o^r :: : :
0-2064
00972
0-388
0-5849
0*1252
0*365
Aluminum ,| . .
0-0682
0-1772
Zinc pressed . . .
0-5710
0-3222
0-365
Platinum, annealed .
3-5360
0-6509
Iron 1,
1*2425
0-5910
Nickel „
10785
0-7578
Tin, pressed . .
1-317
0*8036
0*365
Lead |, . .
3-236
1-1939
0*387
Antimony „ . .
3-324
2-160
0*389
Bismuth „ . .
5054
7-980
0-354
Mercury, liquid . .
18-74
6-000
0*072
ALLOTS.
2 Platinum, 1 Silver
4-243
1*4835
0031
German Silver . .
2-652
1-2732
0*044
2 Gold, 1 Silver . .
2-891
0-6610
0065
It may be remarked that the resistance of metals is reduced by
annealing.
The value of the temperature-co-efficients is nearly the same for
all the solid pure metals, but difiers considerably for mercnrr and
the alloys. It r be the resistance at 0" C, and B that at t*, then if
the respective values of o and & are as follows : —
Pure metals (mean value)
Mercurv
Gold-silver alloy . .
German silver „ . .
Platinum-silver,, . .
a=*003824
„ -000748
„ -000700
„ -000443
„ -000310
5= -h 00000126
„ -00000040
„ -00000006
„ + -0000001 5
fi
00000000
RBSISTAMCB AKD IKDUCTIOV TESTS.
535
943. As copper is aniversally employed in the condact^r, and
german silyer and platinam-Bilver very generally as the measures
of electrical resistance, it may be not undesirable to g^ve a tabular
view of the resistances of these metals at various temperatures,
expressed in B. A. units : —
Temp.
soa
Hard
German
FlalimuB
Copper.
Copper.
Silver.
SJlver.
0*C.
0-2064
0-2106
2-652
4*243
5
0-2102
0-2147
2657
4-249
10
0*2144
0*2188
2*662
4-255
15
0-2186
0-2231
2*666
4*262
20
0*2228
0-2272
2*671
4*269
25
0-2271
0-2317
2*675
4*275
30
0*2313
0-2360
2-680
4-282
35
0*2357
02405
2*685
4-288
40
0-2400
0-2449
2-689
4-295
944. Some minute amount of conductivity remains in all insu-
lators, and in proportion as this is small their resistance is great,
but still a measoreable quantity. The resistance of the insulator
may, like that of the conductor, be measured by means of Wheat-
stone's bridge (801), by immersing a portion of the insulated
conductor in a tank of water (the insulator being carefully closed
over the immersed end of the conductor), and connectm^ the
other end of the conductor with e. Fig. 465, and the water m the
tank with f, and interposing a set of resistance coils at a h. If
the length of the insulator he small, it will be convenient to in-
terpose at K L a resistance bearing a high ratio to that placed at
H o, say 1000 : 1, or 10,000 : 1 ; as in that case the resistance
sought will be 1000 or 10,000 times the resistance indicated : for
if the resistances at e f, o h, k l, n o be a, &, c, </, then
a c J c ,
6=y anda=j6.
945. Induction Tests. — ^The inductive capacity of a body is not»
like its resistance, a fixed and determinate quantity, but depends
on external conditions ; thus a given amount of surface would
have a very different capacity as the lining of a Leyden iar, from
what it would have as the surface of an insulated sphere, sus-
pended in the middle of a large room. A submerged cable may
be looked upon as a long Leyden jar, of which the conductor is
the lining, and the salt water, the outer coating.
In order to estimate the capacity of a cable, it is necessary to
compare it with some known measure of capacity ; this may be
effected by means of a condenser, a df Fig. 533, is a condenser.
536
BLICTRO-TKUeOSAPBr.
of which the pktet e^ d, are put to earth, wf ; and a, h, hmwhkti
from the former bj lajers of gatta pereha, are joined to a key, fh,
FSff, 6SS.
I I I I
by meaDB of which contact maj be made either witli one eleo>
trode of a battery c z, the other electrode of which is pat to earth,
B, or with one terminal of the coil of a nJTanometer o, tlie other
terminal of which is put to earth, tf. The condenser bdsg mow
charged by the potential of the battery, by making ooDtact at b,
is immediately afterwards allowed to discharge itself throi^h the
galvanometer, by making contact at k, and the deflection of the
needle is obsenred.
The cable a b. Fig. 534, is now joined to the key, p h, the (iirther
end of which, k, makes contact with a resistance b, and also vi^
the galyanometer, q ; the further ends of both b and o beinf pot
to earth, tf. The cable is now chaiged by the potentiid of the
batteij (supposed to remain constant from the last ea^rimentX
and discharged through the double circuit o and b ; and b ia ad-
justed by trial, so that the deflection of the needle may be the
same as m the former experiment. Let ^, r, be the resialancee of
G and B respectively, then it is evident that the current now tra-
versing o will be of the whole current, and henoe
capacity of cable a*^ capadty of condenser.
The object of determining the inductive capacity of a oaUe b
its influence on the rapidity of signalling; it naviog been deter-
mined both bjjT theory and experiment that the number of signals
callable of being transmitted in a given time bears an inv«ne
ratio to the inductive capacity of the cable.
The soonduess of a joint may be tested hr the apparatos F%.
533, the electrode z bemg put to one end of the insulated cable,
TBST8 TO DBTECT FAULT8. 537
and R to the irater of a tank in which the joint is immeraecL The
condenser is now chai^ged with the leakage of current (if any) by
maintaining the contact at h for two or three minutes, and then
discharged as before through the galyanometer ; the deflection of
the needle will indicate the amount of leakage.
946. Test8 to Detect Fatdts.—FtLvMe in cables may be thus
classed: —
1. Bnptnre of the conductor within the insulator:
2. Rupture of conductor, ¥nth slight breach of insulator:
3. Rupture with considerable exposure of the conductor :
4. Conductor in contact with iron sheathing :
5. Slight breach of insulator only.
The existence of the first kind of injury is known from the
power of the cable to Tecei?e and retam a chargOi although it
will not transmit a signal. Its distance from either end is ascer-
tained by measuring the total resistance of the insulator, and
dividing the known resistance of one knot by this quantity ; the
quotient will be the distance of the fault in knots.
It is much more difficult to localize the second kind of iigury,
in which there is only a slight exposure of the conductor, for the
resistance at the fault may stilT be considerable, and it is more-
orer subject to great and capricious charges. The resistance-test
(801) in this case merely snows that the fault cannot be beyond
the distance corresponding to the least obseryed resistance : but
if the same test can be applied at both ends, a very probable guess
at the true locality may m made by splitting the difference be-
tween the apparent results.
Tlie third Kind of fault is that which most frequently occurs
when a cable is broken with violence. In this case also there is
a total cessation of the transmission of signals, but the resistance-
test gives uniform results ; and the distance, which is proportional
to the resistance, may be definitely measured, since the resist-
ance of the ocean in the circuit is like that of the earth, absolutely
zero. It was by this test that the operators at Valentia were
able to ascertain that the Atlantic cable of 1865 had met with no
further iiguTT, since its final rupture.
The fourth kind of fanlt is very similar in its results to the
preceding : if the conductor and sheathing are squeezed into con-
tact bv a kink, there will be a frequent succession of feeble cur-
rents, n-om the action of the salt water on the point of contact of
the two metals ; if they be connected, as by a nail or bit of wire
thrust through the insulator, the resistance at the fault will be
slk^t and nearly constant.
The fifth kind of fault may be easily detected ; there will be a
considerable decrease in the insulation-resistance, and some dimi-
nution of the apparent resistance of the conductor, but signals
may still be transmitted, as a portion only of the whole current,
inTenel; proportina*! la tb* reticUuiM of the hnh, MMp
theUnk,orBeL Tbigkind ofr&nltiiia;slK>ba<let«cted I^B
iag the reaistiuice fnm each aod, whoa the otlwr end ii mn
' t mode of comparinft larga r
e 4 in the bridge, Fig. 46Q.
rif. S»i.
eacli coDtuning, m;, 1000 nnita, is iatoipoMd bstwMD e >ad d.
A eecoDd series, r, each of 200 anita nulj bj niiable oooUcti
be Bubetilated for any two of the fonner, and the contw:), i,
msj be made with an; one of thsse. If Rreater socormcj wen
required, a third aeriea of 40-auit coilt mioht be nmilail; anper
posed apon r. B; this Bmngement it ia endBnt that anj injiured
partition of 10,000
inenlstion at soioe particular poM, aa a rnptore of Ihe win woaU
be im mediately Tisibie. He joins Ihe fanltj with a boqkI viteat
tiie distant BlatioD and takes the mealnml ra«i*lai>M of tbe whole,
as the "denominator:" he then puts oM pole of the battoj to
earth, and measures the difiHreooe of the resistances of the proxi-
mal portion of the faulty wire, and its distal portioa plna ths
sound wire (which is evidently double the reaiitaDoe of tin dlMal
portion], for the " numerator.'' Thig Graction of the di«lsiini W
tweeo the stations will be the distuoe of the &alt fim the
further station. The writer was informed that in one inataBce a
clerk was somewhat dislurbed at not being able to And any *>!«
forthe "numerator;" the factheingthattbelaiilt w«b wilhia the
office at the further station.
M». JltiirUmee of a Batttrj. — Aa it ii often n
ascertain the total redstanoe of aoircnit, it ma; he oa
gire here a read; mode of dateimining the internal n
rif. Ufl. P
a batter;. For this purpose let ihe electrodes <^ the battety c^ i.
Fig. &36, be coiuiected with the teiminais of the coil of a gain-
THE ELBCTJUVMAOIIBTIG LOOK. 530
nometer o, tlie resistance of which is n imits, and let a juncfion
A B be made between the terminals, the resistance of which is
one miit : then rth part of the current will traverse the gal-
vanometer, and let the deflection be observed. Next remove a b,
and let a resistance b, Fig. 537, be interposed between c and a,
and be adjusted to produce the same deflection of the needle as
beforehand let m be the indicated resistance, the resistance of the
battery will be — units, the resistances of the connexions a c, b z,
being neglected. If the resistance of a b be a units instead of one,
then battery-resistances— a units.
950. The Electro-moffnetic Loom, — A recent application of elec-
tro-magnetism to the economy of manufactures by M. Bonelli, an
eminent Italian enffineer, may perhaps become too important to
be entirely omitted m even an elementary treatise. This consists
in the substitution of magnetic induction for the ordinary mechan-
ism of the Jacquard loom, in figure-weaving. In order to render
this intelligible to many readers, it will m necessary briefly to
describe the mechanism of the Jacquard loom, by which the pro-
duction of designs in all kinds of textile fabrics has been nniver*
saUy effected for nearly thirty years. The production of patterns
in monochromatic fabncs depends on the alternating predominance
of the loHffUwdinalf^ or warp4hreada, and of the transverBe, or
weft; and this again depends on the selection of warp-threads
under and over which the weft is to pass at each throw of the
shuttle. Passing over antecedent contrivances, it suffices for the
present purpose to sav that in the Jacquard loom, the warp-
threads are respectively connected with a series of vertical wires
arranged in six or eight rows, each terminating above in a hook.
As many rods as rows of hooks, rest in a frame beneath them ;
and the firame, on rising, raises all those threads the corresponding
hooks of which have not been pushed out of its way. The selec-
tion is thus effected : each vertical wire is linked to a horizontal
wire, and the ends of all the horizontal wires present themselves
in a vertical plane, placed equidistantly from each other. In the
action of the loom, a rectangular box, with as many e<}uidistant
holes in it as there are horizontal rods, now presents itself, but
covered by a card in which holes have been punched corresponding
to all those threads under which the shuttle is destined to pass.
The card advancing pushes back the horizontal rods wherever it
is not pierced to transmit them, and with these the corresponding
vertical rods and their hooks ; the selected threads are then raised
by the frame of rods, called the "grifE)'* and the shuttle passes
under them« The displaced horizontal rods are then replaced by
640
■pniigi, another card is presented on the face of the box, and ihtb
same train of actioni is repeated.
A great amount of labour and expense mnsi be bestowed upon
the preparation of the series of cards required for an elaborate
desi^ — a rich pattern for a damask cnrtun, or table-cloth, mar
require from 20,000 to 50,000 cards, the production of which would
occupy four, six, or eight months, at a cost ot, perhaps, 150L, or
more. Nearly all this time, labour, and expense is saved by M.
BonelIi*s ingenious contrivance, which, it may be said, extempo*
rises each successive card from the original desien ; wluch is thus
effected. The desi^ is traced in black varnish on an endless
band of paper of suitable length and width, the suHace of which
is covered with tinfoil. The pattern thus drawn is laid over a
cylinder in the loom, above which stands a row of thin parallel
metallic plates like the teeth of a fine comb, which aie isolated
from each other by non-oonducting matter. The metallic plates
are severally connected with one end of the helices (875) of as
many small bar electro-magnets as there are threads in the waip;
these are arranged horizontally with the extremities in the same
vertical plane, and opposed to the ends of an equal number of soli
iron rods similarly arranged in a frame, which piays the part of the
Jacquard card previously described. The other end of each helix
is connected with one electrode of a small voltaic battery, and the
metallic surface of the pattern with the other electrode. The rods
in the Jacquard box have each a small enlargement or button at
the further end, which is opposed to one end of the rods with which
the threads of the warn are connected as in the Jacquard loom.
The several parts being tnus arranged, the series of plates desoeady
and their pomts rest on tlie surfaoe of the pattern cylinder. By
each plate that rests on the metallic surface the circuit is
Dieted; its corresponding electro-magnet becomes active, and this
Dv its induced magnetism withdraws its corresponding rod into
the Jacquard box. By the plates that rest on the varnisn the cir-
cuit is not completed, and their corresponding electro-magnets
remain inactive. A sUght vertical movement m the front <m the
box now secures tbe buttons of the rods not withdrawn by the
electro-magnets, and prevents their being pushed into the box.
This now advances affainst the series of rods connected with the
warp-threads, and fulfils the office of the " card" in tiie selection
of the threads. After the throw of the shuttle, the comb is nused,
and all the circuits beine thus opened, the electro-macnets cease
to act, the rods in the box are replaced by springs, the cylinder
moves on through a space equivalent to one thread of the weft,
and the above series <a actions is repeated. It is manifest that
by this ingenious contrivance a large portion both of time and
expense may be saved in the production of ornamental designs.
The same system is applicable to weaving in a variety of colours :
for this purpose the requisite patches of tinfoil are isolated from
THB BLBCTUC CLOCK. 541
the general surface, by a bit of paper ixiterveningi and independent
circuitB are comj^leted by as many isolated strips of tinfou at the
ed^ of the design as there are separate colours, each marginal
Btnp being in metallic connexion with all patches corresponding
to the same colour by means of slips of tinfoil at the back of tbe
pattern, the eods of which pass through the paper.
951. The Electric Clock. — This term has been applied to two
different kinds of apparatus ; one of these is merely an indicator
identical in its construction with that of the dial telegraph (815),
and derives its movements from a clock at a distance, possessing
some mechanism analogous to that of the communicator, by which
the circuit is periodically completed and interrupted.
952. The apparatus to which the term electnc clock more pro-
perly applies, is one in which the motor power is derived from an
electric current : several varieties of mecmuiism have been devised
for this purpose. In Bain's electric clock the weight or bob of the
pendulum consists of a hollow cylindrical coil of insulated wire,
one end of which is connected with the pendulum rod, and the
other passes up by the side of it. The axis of the cylinder is
horiasontal, and towards either extreme of the oscillation of the
pendulum, the coil passes over the pole of a bar magnet, two of
which are fixed in a suitable position, their contrary poles being
placed opposite each other. Tne current is made to pass through
the coil in such a direction that it may be attracted by each magnet
during its approach, and repelled during its recession ; the current
being reversed at each extremity of an oscillation by means of a
small sliding piece moved by the pendulum itself. In these clocks
the ordinary action is reversed, tne pallets driring, and the scape-
wheel following (228, 259). The amount of impulse communi-
cated to the pendulum by this arrangement, and the consequent
time of an oscillation, depend on the mutual action of the magnet
and coil on each other, but as both these are variable, the former
by changes of temperature (637), and the latter by the force of
the transmitted current, time cannot be accurately kept by a clock
of this construction.
953. Shepherd's electric clock will probably be in the recollec-
tion of many of our readers, as having occupied a conspicuous
place in the centre of the south transept at the Great Exhibition of
1851. ^ The scape- wheel of this is furnished with a ratchet wheel
and click (265), to ensure its progressive movement, and to prevent
recoil ; and the impulse is given to the pendulum by a remontotre,
that is, the prime mover is not directly employed in impelling the
pendulum, but in periodically raising a loaded lever, which by its
descent impels the pendulum with a constant force.
954. Henry's apparatus, Fig. 538,* exhibits a reciprocating
motion, produced by magnetic attraction and repulsion, which might
* BHott BrothanP Ostalogue of Slectro-magiMftlo Inatrimienta, p. 12,
fig. 99.
HLEOTB0-TII.EOlU.rBr.
b« emplojed to drive e elock'trEiD ; bat it n
open to the sEme objection u BeId'e clock
(952), DEDielr. that tlie impabe oo the
todatum would not be necnuritjt nnifom.
conuBts of two electro-mEgnetE Ettack«d
I a horiiontEl beam poiaed upon k cenlial
[IB, with (wo permEneDt magneta placsd
irticallj one under each pole. The elec-
odea of ibo battery tennin'" '
t the n
eutnl w
port, in which the ends of the mill an
immerved allemEtaly, so ■■ to reTeree the polea of tko elrctro-
mEgnets at the ellremity of each aacillatian of the bean.
S6d. Id all the better constnicled iottnimenta, aa in Sbepberd'a
clock, a conitant inipnlM iagiTea to the peudnlum by mcEna of a
■prinK or weighted lever released only by an electn»-map>et, aad
therefore independent of the Toriatioiia uf the carrent. It will
Buffice for the objects of tbii treatiae to deacribe one plan of cea-
■truction, deTimd by M. Froment, and mmswhat rnDdified hj M.
HanJj.
The pendulum, p, fig, 639, n Buapended by a thin luruBa of
B with
of the ctnl irf an electm-niagnel
at (he end of an arm r atlached to the
pendulum cornea in contact, mt tbe ei-
'.remity of its oacillatioDj witli a a|
tremity of its oacillEtion, witli E apring
H, the filed end of *liich,.BDd the
DDper end of the coil a, are maiMcIei
with the electrodes of a battetj. IV
end of the apring a reata on s atad D
on ■ vertical rod « ■>, which ia jainlml
at A to the end of a Oat piece of kA
iron, placed near the end of the elsclrv-
magnet, and raised from it bj a ahort
spnng B, the force of which ia rnnlaled
ijy a acrew Egainst the point of whick
it rests ; and A c being thus raised, raises the end of th« apring
K. When the arm f cumes in contact with a the ciimit is i tiaiil.
and the electro-magnet B, by attracting c depreaaea a ■>, irtirk
thus releases a aoii allowa it to are h^ lu elasticity tha ra^lAai
impulae to the pendulum, (he rod A n is kept in a Tertical pariila
by ila lower end paasing through a Gied hole at a. Tba naMtf
o( impulse on the pendulum evidently depends on tbe atraagAsf
the spring, s, and not on the strength of the current. Many oditf
ennlnvBni.-e9 Era much more complicated, without being kbit
effectual.
956. Ii
ices E series of electric docks, diatribated ii
are all actuated by a single ragnlBtor, by
:( effecled at each beat of tl
and coDTnnicnt mods of dmng ibii hu been derited bj
cault. The pendalnm, p,
is connected bj» hoiiwwitJ ^- '*'■
Ih another link B eqoal aod
1 the upper part of the pen-
ad the joncticm of these ia
by a tbird link c with the
igbt spring, ■, ■ Mnall itnd
is tiTDUubt into coQiKt irith
a similar spring t beneath
moment that the pendalnm
I, and the links B and c, an
iilo a etraighl line. A< these
; nn the principle of the
30) the contact i> efected
I pull of the pendulum, and
l^heaUlinu'i ChriMaKopt. —
a exceedingly minut
t the
. of its
lentn designed to
I H mm vKiy luiuuLo initrraia oi Lime have been termed
iprt; vhile those that record the inlerial by means of a
DiDt on paper or other malerisl ire called thTimographt.
e important function of theie insTmments is that of re-
the velocity of projectiles during any required portion of
ht. A toIihIc current was first apjilied to this purpnee
^healstono in ibejear l)MO, but the instrDnient then de-
I liim has since been conaidemMy modilied and improved,
tent rr-cordin;! instrument cnnsisis <if a smidl clock-ttain
- a weight. The teeth at the scape-wheel are released
nd of a libraling free-reod (oB2, b), which makes 600
B, and releases lOOO teeth in a second. The precise
.f vibnitinna may be readily determined by the pilch rif the
need. The train is held by a detent, which is withdrawn
.'ciro-magnet the instant that a vcittaic circuit in closed,
min runs until the detent is released and allowed lo act,
:Teraal of the current, b, Fig. Ml , is the battery, c the
^pe,BDdaa small pair of organ-bellows, worked by a winch
tree-lhmw crank (80), which act by exhaustion, like an
), and draw in sir through the aperturs in which the reed
. in Older that the eicapement may not ba disturbed by
■ny outward current, irith which a fr«e ret-d is niually acliuted.
Two targets, each crosaed by many fulds of string an placed at
BDY required intenal. A spring contact-kej is hM back bj the
stnng at tha fint target, d, and contact a made the iiulaDt that
Itrinf^ is broken, and the train ii relesHd. At the wcnnd target
the wires I, f, UFO connectad with an inverBor (854) mored by a
similar spring, wblch is held by tbe string; and when this is
broken, thn current is reToraed, and the train stops. Very great
accuracy of measuremeat may be attained by this appantui.
956. Among the raried results of the vast amount of hnmaii
iogennit^ that has been employed in mudifying tbe applicatimu
of electnc force, there is none more striking than that proposed
by Prof. Whsatst^ine in 1S41, and subsei^uentl^ applied V Prof.
Bond, of the United States, to the registration of the precdae
epoch of astronomical pheDomeua. The clock of this apparatus
communicates nnifonn rotation tc a drum or cylinder oorered
with paper, and alao to an axis parallel to that of the cjlioder, on
which a screw is cut. A small electro-magnet is to attached as
to be carried slowly forwards by the rotation of tbe screw, and a
pencil attached to the iron keeper of the electro-mapiM
^' ' rests on the surfaue of tbe paper, and would, if undit-
pleted, and a small deviation of the pencil ocean, a*
In tig. 642. The connexions of the circuit ai« so
arranged, that an ohserrer stationed at either of the
instruments can by merely praasing a stnd or key with
his finger, complete the circuit, and cause a similar
moTemeDt of the pencil, as seen at a or a, at the pteciM
IJ moment when a star, or other beaTonly body, to be ob-
serred, is on the wire of his telescope. On appljiaga
■cale of inches, divided into 100 parts, to these lines, there is no
difficalty in estimating, to half a divisiDn of the scale, the podtioo
of * or B between two contiguous marks ; and conKguently, the
epoch of tbe obseryation, to the ^tb part of a second 1 6u:h is
the wondeHiil accuracy of obaerratian to which modem acienoe
has attained. In order that tbe morement of the cylindar may
be uniform, it is evident that the motion of the clock-train mnat
likewise be nnifonn, and not intermittent, like tbe movement at
an ordinary clock : for this pnrpoee the clock is furnished with a
conical pendulum (338), which performs one revolution in a aBoond.
959. In the registering apparatus now in uBe at the Boyal Ob-
servatory, Greenwich, tbe impression of a steel-point npon the
paper is employed instead of the displacement of a pendl-line ;
this from its minntenofis affords a more exact indication of the
epoch dengned to be recorded. The clock was oonstructed by
Mr, Shepherd, and is supplied with a very ingeniously contrivAd
SUGTBIC TDfE-BALL. 545
water regnlator, bj which the time of rotation of the pendalum
is controlled, and rendered extremely uniform. This consists of
a small radial plate dipping edgewise into an annular troagh
of water. Increased speed of rotation depresses this plate, and
bj thus increasing the resistance, retards the motion. Dy the use
of this apparatus, the '* personal equation, '^ as it is called, that is
the estimated time to be allowed for perception and action in each
individual obserrer, is considerably mminished.
960. The Oreenwich TimeMl.—A krge ball, through the
centre of which passes the support of the vane on the top of the
Bojal Observatory at Greenwich, is a conspicuous object from the
adjacent part of the Thames, and surrounding country ; this ball
is daily raised half-way up the post by a winch, at five minutes
before one, p.m., as a preparatory signal, and being subsequently
raised to the top, is released by the movement of a detent (265),
or trigger, and commences descending, precisely at one o'clock :
thus giving an exact epoch, by which the chronometers of our
commercial navy may be regulated.
In order to preyent the concussion that would result from the
unimpeded descent of the ball, the rod that supports it terminates
at the bottom in a piston, which works in a cylinder filled with
air, and nearly closed at the bottom, a small aperture being left
to permit the gradual escape of the compressed air. Formerly the
ball was released by the hand of an assistant, who watched the
time by a clock : it is now released by an electro-magnet actuated
at the proper moment by the regulator. A similar time-ball was
subsequently erected at Deal, and both were simultaneously and
automatically released by means of electro-magnets ; the circuits
being duly completed by the mechanism of tne clock at Green-
wich : and by similar means, time-signals are now daily conveyed
to distant parts of the country, as it is of groat importance for
the prevention of accidents, that exact uniformity of time should
be maintained at the various railway stations. It may also be
stated that time-signals have been exchanged between observers
seated at their transit telescopes in the observatories of Greenwich
and Cambridge for the determination of longitude, and the same
has been effected between Greenwich and Paris.
961. An ingenious contrivance has been devised for setting the
clocks right at distant points, as, for example, at the several
stations of a railway, by a self-acting mecbamsm. This consists
in attaching to the minute-hand arbor a circular plate with an
angular notch in it. A wedge at the end of a lever is daily
pressed into the bottom of this notch, at a fixed time, by an
electro-magnet, and the position of the minute-hand, whether it
be fast or slow, is thus corrected.
The applications of the electric current to useful purposes are
in the present day almost endless, but it is hoped that none of its
more important applications have been altogetner unnoticed.
M N
646
CHAPTER XVI.
THEBirO-ELBCTBIOITT.
962. Whbk two different metals, as copper and bismuth, are
soldered together, and connected hy wires with a ealvanometer
(855), an electric cnrreot is aeyeloped on
lig, 648. heating or cooling the point of juncture of the
ft 1^^<:='\>^ ^^^ metals. The most convenient form of
J^^^^^f^ apparatus for exhibiting a thermoelectric
A^^^u^l \r cnrrent is that of Pouillet, Pi^. 643, in which
(\\\ ^ ft bar of bismuth, bent at right an^es to-
1 I wards each end, is supported on a stand.
vL/ Thick copper wires, c, c, are soldered to the
ends of tne bar, and also to the feet of two
binding-screws b, b, on the top of the stand.
One of the points of junction of the copper
and bismuth maj be heated by a spirit-lamp;
and the current will be considerably anf>
mented,if the otherjunction be simultaneous
immersed in a freezing mixture, or in pounded ice.
If the ffalvanometer he sufiSciently delicate, the deviation of the
needle wfll occur when the point of connexion of botib metals is
grasped in the hand ; a very slight elevation of temperature b«ing
sufficient to produce this effect. In general, the most powerral
currents are evolved by heating or cooling the more crystalline
metals, as bismuth and antimony ; and they increase within cer-
tain limits with the change of temperature. The following table
by Prof. Gumming contains the names of several metals, any two
of which being employed as a source of electricity, by heating
them at their point of iunction, currents are developed in such a
manner that each metal becomes positive to all below, and negative
to all above it, in the series ; ana the reverse order is observed, if
the point of junction be cooled :
+
Bismuth
Mercury
Nickel
Platinum
963. This mode of developing electricity was discovered in
Palladium
Rhodium
Cadmium
Cobalt
Gold
Iron
Manganese
Copper
Arsenic
Tin
Silver
Antimony
liCad
Zino
_
I8!l, br Pmf. Seebeck, of Berlin, and bu bwu atixlied with
mcceu bv Prof. CTimniing, of Cainbridga, Mr. Sturgeon, and mmj
other philoeophera. In examining tbewt correnti, u they ore of
too loir iateaaitj to forc« tbeir v^y through ver; long conducting
wires, the galruiometer should be cooglructed in the manner u-
iwuij explained (856), but the coil ahonld consist of » small number
of tania, and be composed of thick and soft copper wire, so as to
oSer M little resistance as possible to the passage of the current.
964- It is by no meoni Decesaary to employ twodifierecl metoU,
forif two pieces of cOTiper wire be twisted togelher, a current of elec-
tricity is emilYOd On bifldingaspirit-lamp on one side of iho juncture.
When a homogeneone bar of metal ig bested at one end, the cold aor-
tioii oaaamee a oegalive, andthe hot apoaitive electrical state. Tbie
effect Duj be augmented by
repetition, and rendered en-
Kf-U*.
deotby a gakaoometer, if ft ^ n /
St >, be connected with it. On heating one nde of jtg eu
the end d with a apirit-lomp, a positive current
paesea horn d to A, and caoses the gBlTUiometer
needle lo donate from its position oFreet.
966. A ready mode of demonstrating the eicita-
tioD of electric cnrreDta by h«at, by meana of their
electro-dynamio eSWcta, is met with in the little
apparatue contrived by Prof. Cummine. A piece
ot thin ailver wire ia bent into the figure ess,
Fig. 545, and mapended by a filament of silk
from any support ; the lower arm of the rectangle,
F, being composed of platinum. If the flame of a
■pirit-latnp be applied to one of the junctiona of
ttkeae wires, and a horae-aboe magnet be held near
one of the vertical anus, attraction or repulsion wilt "
ensue according to the direction of the current, and *
the poaitian of the polee of the magnet (867). C
966. Tbt phenomena of electro-macnetic rotation |1
may be readily produced by means of thermo-electric *
eorrents; for this purpose twist round eacii end of
a bar of bismuth, an in<:b In len^h, n thick copper
wire, and haviae unalgtunated tbe other ends, im-
merse them in toe circular trough, A or B, Fig. 494,
the bar resting on a point at its centre. On heating
one end of tbe bar, a current of electricity will pass
throDgh the apparatus from the copper lo the
bismuth, and the conducting wiree will revolve with
967. Another cotivenient form of apporatos ia d
•ihibited in Fig. HA, in which a, a, an two light ^
irira frames, or caeca, each coniisdng of a boriiODtalriiig of pla-
tinum wire, to which are soldered four vertical copper wires, beat
Orer and united together at the upper part, and having a slael
point dvpendiug from the point of uuioo, on which the;r majr rotate
on a little cavity in the uitremitiet of the polei of a vertical horwe-
ihoe magnet. A spirit' lamp, placed uudemealh the frames, will
beat the points of junction of tho two metals, and (he currents
thui generated will traveree the copper wirei, and cause them to
rotate in opposite directions round Ibe pol<B of the magnet.
968. The intensity of these correnia U incnaaed by combining
a series of alternations of too metals, as copper and pUtinnin, or
bismuth and sntimoaj, as in the
t^, U7. ordinary electric pile. Fig. MT
I representfi Watkins's massive ther-
moelectric piles, consisting of an
I alternate aeries of square pTatea of
antimony and bismuth, having
p their upper and lower edges alter'
I nalely soldered logelher, so aa to
I farm a composite battery, and
packed into a frame aith non-
conducting matter, so a* t« leav*
the upper and lower surface! of
junction eiponed. When the upper
surface of the pile ia cooled DJ
filling the npper projecting portion of the frame with poanded ice,
and the lower surface heated b/ radiation (rum a nctaogiilar
piece of red-hot iron, placed
stand (seen in tl
figure) beneath, a thcrnio-electric
current is t'enerated sutBciently io-
tenee to I'ifaibit the ordinal^ voltaic
eflei'ts of ii^hl, heat, electro-mag-
netism with lis rolBtioDS, and induc-
tion : a Wright of 9X pouuda has beno
austained by an electro-magnet thai
excited. The electrodes of the tnle,
ODH of which is seen at K, have a
mercurj cup, a bindiug-screw, and a
grooved surisce ; by drawing an*
pointed piece of metal, connected
with the other eleclrodi., over this
BUrface, vivid Bparka »™ produced.
969. By ueii.g slender bars of
bismuth and antimony, baring their
alternate endssoldertd tngelber, and
packing a series of 36 into a rect-
angular bundle, l''ig. 546, an ar-
efiected in which a
THEBMO-BLVGTBIO HrOROUETEH. 549
current is generated by the sligLtest alteration of tempe-
ratare of either end of the bundle. If one of the faces of the
bundle be blackened, the mere approach of the hand is sufficient
to excite a ver^ perceptible electric current ; to detect which a
galvanometer with a thick, well-annealed copper- wire should be
employed. This instrument then becomes a most sensitive ther-
moscope, greatly exceeding all forms of thermometers in indicating
small alterations of temperature ; and in the hands of Melloni and
Forbes has led to the beautiful discoveries of diathermancy, and
the polarization of heat.
970. Peltier's Thermo-eUetricSygromeUrfViQ. 549, is another
example of a composite battery of slender bars of
antimony and bismuth, arranged alternately in a Fig.b4ti.
coronary form, and united in pairs with solder;
and the extreme bars are connected, by two
pieces of thick copper wire, with binding screws
attached to the stem of the support. A platinum
capsule, containing distilled water, rests on the upper
projecting points of the combination, the surfaces of
contact bemg as large as possible. An electric
current is developed by the reduction of temperature
owing to the evaporation of water in the capsule;
and the deflexion of the galvanometer needlea by
the current may therefore be taken as a measure of
the rapidity of evaporation, and hence of the hygro-
metric state of the atmosphere.
971. M. E. Becquerel has proposed a thermo-electric combina-
tion, which is said to possess considerable energy. This consists of
metallic copper, and sulphide of copper cast at a temperature a
little above its point of fusion. This, like the succeeding com-
bination, has the advantage of sustaining a very much nigher
temperature than the antimony-bismuth element.
A very energetic thermo-electric combination has been devised
by M. Marcus, consisting of two alloyq.
The positive metal consists of
10 parts of copper, 6 of zinc, 6 of nickel,
the addition to which of one part of cobalt is said to increase the
electromotive force.
The negative metal consists of
12 parts of antimony, 5 of zinc, 1 of bismuth,
which is improved in quality by repeated fusion.
The positive metal melts at about 1200" C, and the negative at
600° C. : the bars are not soldered, but screwed together. They
are alternately united in an extended layer like the lines of a W^,
and one series of junctions is heated by gas jets. As the heating
of the positive metal chiefly influences the development of electri-
city, tus is placed underneath, and the negative metal heated by
550 TEBBMO-ELBOTEXCITT.
condaction only, oonseijueiitly the latter may be heated nearly ii|i
to the melting point, dj means of this large amount of heat, a
current of considerable potential is produced. \^th 125 elemente,
25 cubic centimetres of gas were eroWed per minute, and a plati-
num wire *02 inch in thickness was fused ; and 30 elements gave
an electro-magnetic lifting force of 150 lbs.
972. An interesting illustration of the conversion of heat into
electricity is presented in the fact that water which is used in
cooling the contrary series of contacte becomes rapidly heated
when the circuit is open, but slowly, when it is closed. The
writer has recently obserred a similar fact in a single thenno-
electrio element of antimony and bismuth. It appears (975), that
if a current be sent through this element from antimony to bia*
muth, heat is developed at the point of contact, and on the contiaiy,
oold, if the current pass from bismuth to antimony. It wodd,
therefore, be in accoraance with the dynamical theory of electridty,
that there should be a gain of current when there is a loss of hes^
and vice versd : and it was so— for on duly balancing this thenno-
element in a Wheatstone's bridge (or bakmce as it is sometimes
called). Fig. 465, the deflection of the galvanometer needle fol-
lowed the direction of the current, indicating a ffoin of corrent
in the one case, and a lo$s in the other.
973. Thermo-electric currents are generally of too low potential
to effect chemical decomposition : it has, however, be<ai stated
that by employing a lai]^ number of alternations of platinum and
iron, M. Botto, of Turin, succeeded in decomposing water, and
various saline solutions. Prof. Wheatotone* obtained a spark by
induction (887\ from a small pile with a coil of insulated copper
ribbon 50 leet long ; and the late Mr. Watkinsf subsequently ob-
tained the same result from a single combination of bismuth and
antimony, weighing only ten grains.
974. Dr. Andrews, of Belfast,^ has discovered that platinum
wires connected with a multiplier, and plunged into fused salts,
are traversed by an electric current. On fusing a little borax in
a loop of platinum wire, by means of a blowpipe, and quickly in-
serting the previously heated end of a second wire also connected
with the multiplier, mto the fused bead, the needles flew to the
extreme of the scale, from the development of a powerful current.
By means of these curious thermo-electric currents, Dr. Andrews
succeeded in obteining distinct evidence of chemical decompositian :
and the same results were obtained when other fused salta, aa
carbonate of potass, chlorides of potassium and strontium, iodide
of potassium, sulphate of soda, and even boracic acid, were used.
975. Eledro-tnermio £lffects.% — ^The converse of the resulte now
• Phfl. ICag. joL z. p. 414. f Ibid. toL xi. pp. 90^ 988.
f Ibid. Tol. X. p. 433.
§ A definite uid intdligible meeaing may be ffiven to these oompodte
wovdia if the flnt oomponent ilwsyi represent ue aetimg eMwr, ana ttM
BLBCT&O-THBBaaC BFFECT8.
551
described is foond to be eqnallj true, as has been already men-
tioned (817), nainelj, tiiat the passage of a Yoltaic carrent pro-
daces cnange of temperature at the point of jonction of two dis-
similar metals : and that change is proportional to the remoteness
of the metals from each other m the thermo-electric series. The
electi'O-thermic are the converse of the thermo-electric phenomena,
namely, the passage of a carrent from a thermo-positive to a thermo-
Fig,660,
.z:x
negative metal produces depression of tempera-
ture, and the transmission of a current in the
contrary direction, elevation of temperature, at
thepoint of junction.
Tnese phenomena may be conveniently ex-
hibited by Peltier's apparatus. Fig. 550, which
consists of a compound bar of antimony and
bismuth, ▲ b, passmg through the centre of a
glass globe, to which a long tube is attached,
which passes through the cork or stopper of a
^laes vessel, c, half filled with coloured water,
in which the end of the tube is immersed. A.
divided scale is attached to the tube, constituting,
in fact, an air-thermometer, in which the expan-
sion or condensation of the air in the upper ^lobe,
produced bv an^ change of temperature in the
compound bar, is indicated by a corresponding
depression or elevation of the fluid in the tube.
It ma;^ be remarked, that a single voltaic ele-
ment 18 sufficient for producing these results,
which will, however, be proportional to the inherent electromotive
force of the oombinadon employedi
Moood tlM remiUinff ^(utt that, a thenno-eleotrio smwaratot would mean
one in which heat developfl elaotnoity, and an eleotro-tnermic, one in which
thapaaaageofaneleetrio current prodaoes change of temperature. Similar^,
deotro-magnetio induction would mean the development of magnetism by a
cfomnty and magneto-eleotrio indnotion, that of a current by magnetism.
552
CHAPTER XVII.
OBOAHIO KLECTBIOITT.
976. Cbbtaih fishes have irom remote antiquity* been known
to possess the property of communicating a benumbing sensation
to persons who have incautiously grasped them. This remarkable
effect, of which the intensity is sometimes so great as to amount
to a severe shock, has been'most satisfactorily traced to electricity;
and no real difierence exists between the electric current thus
secretedf or excited by these animals, and any of the other modi*
fications of that curious form of dynamic force already described.
The fishes hitherto met with, which possess this extraordinaiy
faculty, are but few : of these the torpedo occelata, and manno-
rata, are alone met with in Europe. The others, including the
gymnotus, tetraodon, silurus, rhinobates, and trichiuruselec^cns,
are confined to the tropics. The torpedo, gymnotus, and silurus
have been submitted to veir careful investigation : the firsL
chiefly by Hunter,^ Dr. John Davy,' Gky-Lussac,* Colladon/ and
Matteucci;' the second, by Rudolphi,' Walsh,' Ingenhonsa,*
Humboldt, Bonpland, and Faraday :* and the last by Rudolphi,^
and Miiller.^^
977. The electric or^ns of the toipedo lie on each side of the
head and branchiie ; being made up of numerous five^r-six-sided
prisms, placed in such a manner as to present their bases to one
surface of the fish, and their summits to the other. Hunter
counted 1182 of these in a single organ. They are divided hori-
zontally, by numerous septa, the interspaces being filled up with
a gelatinous fluid. These organs are copiously suppliea with
nerves, which are chiefly brancnes of the par vagum, or pnenmo-
• Aristotle, Hist. Anim.Jib. ii cap. 13, and iz. csp. 37. Pfiny, Hkt.
Nat., lib. xzxii. o. 1. .AliaD, de animal, natura, lib. i. cap. 36, Ac.
I Phil. Trani., 1773. • Ibid., 1838 and 1834.
• Ann. da Chim., Ixr.p. 15, joint paper with Humboldt.
« Sfanoea de I'Aead. de Sdenoes, Ootob, 1836. > Ibid.
• Abhaad. der Aoad. t. Berlin, 1820, 1821. 7 phil. Trans., 1774
• Vermischte Sohriften, p. 272. Yienna, 1788.
• Phil. Trans., 1839. lo Abhand. Aoad., Berlin, 1884.
1^ Handbuch der Physiologie des Menschens, i. p. 66, Cobleoai 1837] or
Bailej's trandation, London. 1837.
The Tetraodon is described br Paterson in Phil. Trans., 1766, p. 388. The
Triohinms is flffored by Willonghbj, in his Ichthyolofu ; Appendix, t. 3, ilf . 3,
and described bj Nienhof in " Zee on Lant Beise door Wast en Ost-IadicB,"
p. 870, Amsterdam, 1682.
THE QTICHOTUS AHD 8ILUBUB. 553
gastric neryjes. The power of communicatiDg the shock depends
upon the integrity of the nerres, for the heart may be cut oat,
and the animal flayed, without its losing this faculty; but as soon
as the nerves are divided, it vanishes entirely. The intensity of
the shocks is increased by irritating the oripn of the electric
nerves with the point of a knife. The electric discharge is directed
from one surface of the fish to the other, the dorsal surface beine
positiye, and the ventral, negative ; and no shock is experienced^
unless direct or indirect communication be made between the
belly and the back of the animal. A complete polarization of the
two sur&ces does not occur, as that portion of the animal nearer
the electric organs is positive, or negative, according to the par-
ticular surface, with respect to other parts nearer the tail. Dr.
Davy socceeded in decomposing acidulated water, and iodide of
potassium, as well as in heating but not igniting platinum wire,
and in magnetising needles placed in a spiral coil of wire, by
means of currents from the torpedo.
978. In the gymnotus, the electric organs are on each side,
double, and extend from the head to the taiL They are each
formed of longitudinal membranous structures, placed at a
short distance from each other, provided with numerous tran»>
verse septa, and filled, as in the torpedo, with a ^latinous fluid.
These organs are supplied by spinal nerves, in which respect this
differs from the last described fish ; these consist of 224 pairs of
intercostal nerves. The gymnotus resembles an eel in appear-
ance, and is often four or five feet in length ; its shock is ex-
tremely powerful, and capable of paralysing horses and mules.
Walsh and Ingenhouss, in 1776, observed a spark to pass between
two pieces of tinfoil through which the discharge of this fish was
transmitted. This was doubted until, in 1836, the power pos-
sessed by electric fishes of yielding a spark was again assertea by
linari ; and in 1839 this statement was placed beyond a doubt by
the researches of Faraday, who, availing himself of the electric
eel publicly exhibited at the Adelaide dallery, succeeded in ob-
taining a current of sparks, and by the aid of an induction-coil
(887), and once even by the direct current between the surfaces
of two pieces of leaf-gold.
979. Faraday obtained the electricity from the gymnotus whilst
immersed in water, by means of collectors formed of sheet copper
bent into a saddle shape, so as to grasp gently the sides of the
animal. The backs of these collectors were covered with sheet
caoutchouc, so as to insulate them from the water. Conducting
wires, also insulated by being covered with caoutchouc, were sol-
dered to each collector. The shock was best obtained by placing
one of the hands near the head and the other near the tail of the
fish ; it was conveyed with facility to the moistened hands by the
condnctors. "When the conducting wires were connected with a
galvanometer, deflection of the needle to 30** or 40° took place.
554 OIOAHIO BLBCTSICITT.
And was in sacb a direction as indicated a current firom the an-
terior to the posterior extremity of the fish. When the current
was allowed to trayerse a short helix, a steel needle placed within
it became magnetic. In like manner, when the conductors wen
furnished with platinum terminations, and allowed to rest upon
paper moistenea with a solution of iodide of potassium, polar de-
composition ensued, iodine being evoWed at the end of th« wire
connected with the anterior part of the fish.
980. On whatever part of the animal the collectors were placed,
the current of electricity was alwap found to pass from that
nearer the head to that nearer the tail. So that if three collectors
were placed on the animal, one near the head, the other on the
middle, and the third near the tail, the first was found to be pon-
tive with regard to the second ; which, although negative with
regard to the first, was positive in relation to the third. It appears
that the moment the gymnotus wills the shock, the lines or force
dart ofi^ diverging from him in the water, and whatever is in their
course receives the shock. Hence, if a person immerses one hand
only in the water near the fish, when it wills a shock, he expe-
riences its effects, although not so powerfully as when in contact
with the animaL
981. The silurus is still less known than the gymnotns ; its
electric oreans are double, and are separated by a tough aponeu-
rotic membrane : the most external of these organs lies imme-
diately under the skin, the deeper one beins imbedded in the
muscles. They are both divided inte cells ; their nerves are, it is
remarkable, the same as those of both the torpedo and gymnotus,
one of the organs being supplied by the pneumogastric, the other
by the intercostal nerves.
982. Among invertebrate animals, a few have been stated to
have claims to be considered as electric, but this is extremely
doubtful. Molina* relates that a certain Chilian spider possesses
the property of benumbing the hand of the person who touches it.
Kirby and Spencef mention a species of cimex, the redwnnt
ierraius, as having the power of conununicating what have been
regarded as electnc shocks. An account is on record sAso^ of one
of the great marine annelidsB, loonice ^anteo,^ giving a powerful
shock to the person who touched it.
983. Prof. Galvani, of Bologna, in 1791, published a com-
mentary " de Yiribus Electricitatis in Motu Musculari," aod aa-
Bounced those facts which laid the foundation of that scieiioe
which bears his name. He then stated that a particular form c£
electricity, denominated by him animal eUdrtcUy^ existed in all
animals : but he believed that he merely excited and rendered
sensible this electricity by coating a nerve and muscle with
* Natorgeaohiohttf tob Chili, p. 175.
t Introduotion to Entomology, i. p. 110.
^ SiUimui'i Joomal, xr. 867.
EZFEBDIIHTB OF OALYAIII JLHD TOLTA. 555
netaU, and failed to regard the latter as the real source of tht
exciting current.
This celebrated experiment, although well known, is one of
xeally so marvellous and remarkable a character that, repeat it as
often as we may, it can never be looked at without a feeling of
wonder and delight. Prepare the legs of a frog by denuding them
ef their skin, and removing them from the body, together with the
portion of the spine from which the lumbar nerves arise ; and
having laid the preparation
on a glass plate, place a <^* Ml*
piece of zinc, z, Fig. 551, ' *
in contact with the nerves,
and allow the feet to rest
on a thin slip of silver, s.
The limbs will remain at
rest, and appear dead and powerless ; but there exists a power
which can be caJled into action, capable of endowing these appa-
rently dead muscles with vital power. The only spell required to
er^ka this power is a piece of wire, w, one end of which must
toQch the zinc, and the other the silver plate ; instantly the legs
-violentlv contract, and kick away the silver plate. It has been
stated by Prof. Matteucci, that this curious observation was not
original with Galvani, but was made some time before by the
celebrated Swammerdam : and that the experiment was exhibited
by him in the presence of the Ghrand Duke of Tuscany.
984. Shortly after the announcement of this discovery. Prof.
Volta, of Pa via, in repeating this and other analogous experiments,
arrived at a different conclusion ; and he showed that the elec-
tricity was reallv excited by the metals, and the contraction of
the muscles of the frog was only an index of its existence, fle,
however, supposed that the electricity was excited by the mere
contact of the metals (762), as the necessaiy agency of chemical
action was not then recognised. It is now almost universally
admitted that in this experiment the zinc is acted upon by the
chloride of sodium or other salts existing in the fluids with which
the tissues of the frog are moistened. Although these and other
discoveries of that great man obscured for a time the views and
researches of the ifiustrious Galvani, attention was again drawn
to them by the experiments of bis talented nephew, Prof. Aldini,
of Bologna. He was inspired with so much zeal in defence of his
uncle's theoiy, that he travelled through France and England for
the purpose of demonstratiog the truth of his views ; and, in the
presence of the medical officers and pupils of Guy's Hospital, he,
in the year 1803, supported and defended a series of propositions
00 satisfactory and conclusive, that he was presented by his audi-
tors with a gold medal commemorative of his labours.
985. Aldini's propositions and conclusions are so important and
of such high interest, that a brief reference must now be made to
U6 ora.
«oma oTtheto, aa they appear to deiDOTiBtnite,iii»ni<MtMta'ifMiOTf
tDHnoer, the eiiBteuee of e1e<:tricity in imimals ; and, u vill >p.
pear tn all coDTemtnt witb this branch of phyaiologj, moat r»-
markabl; anlicipate the later resaarchei of hii countrj'man, FmC,
Matteucci.
Pbuf. 1 . — MiiBciiIar contractiona are excited bj the dereli^
ment of electric potential in the animal machiiie, vhich la tranv
mitted from the nervea to the mowlea without the concurrence or
agency of metals.*
EiF. A. In proof of this statement, Aldini procared the head
„ „, of a recenttv killed oi, Fig- 532. Wilh
^'- "^ the one huid he held the denuded lea
of ■ fro?, eo that the portioni of tCe
spine alill connected irith its lumbar
nerres toucbsd the tip of the tongue,
vhich had been previously drawn out
_of the mouth of the oi. The circuit
WHS completed bj grasping wilh the
other hand, well moistened wilb salt
J and water, one of the e»i» : the frog's
I legs instantly contracted ; the contrac-
n tions ceasing the instant the circuit
rl vas broken by removing the hand fitun
M the ear.
The intennt; oT these cnatT«ction(
was much incressed by combining two
~ or three heads, so as to fonn a sort of
batteiy ; just as Matlencci forty years later ibond to he the cae
witb bis pigeon and rabbit batterv.
Gip. a. Aldini, having aoaked one of bia hands in ealt and
water, held a frog's leg by ita loe, and, allowing the ischistio
nerves to be pendulous, ne brought them in contact with the tip
of his own tongue. Contractions instantly ensued from a current
of electricity traversing the frog'sleg in its route from tbeeitemml
or cutaneous to the internal or mucous covering of the body. By
this very interesting experiment Aldini demonstni led the eiiBlence
of the mtiscuto.cutaneous current, and completely anticipated ill
reJiacovery bj Donn6 some five-and-tbirty yeare afterwards.
-, ... EiF. C. The proper electiicity
'^■™' of the frog was found by Aldini
to bo competent to the production
of contractions. For ibia pnrpoae
he prepared the lower ertremitiaa
of a vigorous frog-and by bending
up the Was in Fig. 553, brought
the muscles of the thigh in conUct
wilh the lumbar nerves, when con-
* Aldini: AcoDimtoftiifllatflliDpiDVeiiintolaQaiTaoiiiii.Ma. Loodoi^lBOS.
THE0BIE8 OP OALYAHI AKD YALLI. 557
tractions immediately ensued. This experiment is now a familiar
one, and has been repeated and modified by Miiller and others.
Exp. D. a ligature was loosely placed round the middle of the
crural nerves, and one of the nerves applied to a corresponding
muscle : contractions ensued ; but on tigntening the ligature, this
convulsions ceased.
d86. This last statement is very important, as upon its accuracy
or error depends what has been regarded as one ot the tests of the
identity or diversity of the electric and nervous agencies. It wa0
repeated soon after Aldini's announcement of the fact by an
Italian pb^ician of celebrity, Signor Valli, who commenced his
researches in 1792,* only a year alter the publication of Galvani's
discovery ; and he found if the ligature were applied nearer tfie
fttuade it did not allow the contraction to occur ^ but if nearer the
9pine^ it did not prevent it; and this was afterwards corroborated
by Humboldt : but it has been since found by Matteucci, that if
care be taken to insulate the nerve, a ligature applied to it will
arrest the contraction, as well as the passage of a very weak
artificial electric current. «
987. It must not here be omitted to notice the neuro-
electric theory of Galvani. He assumed that all animals are en-
dowed with an inherent electricity appropriate to their economy,
which electricity, secreted by the bram, resides especially in the
nerves, by which it is communicated to every part of the body.
The principal reservoirs of this electricity he considered to be the
fibres of muscles, each of which he regarded to have two sides in
opposite electric conditions. He believed that when a limb was
willed to move, the nerves, aided by the braiu, drew from the in-
terior of the muscles some electricity ; discharging it upon their
surface, they thus contracted and produced the required change of
position. This theory was adopted and defended by Aldini.
988. Valli, whoseexperimeots nave beenreferredto(986), believed
the neuro-electric "fluid '* to be secreted by the capillary arteries
supplying the nerves, bv which it was conveyed to the muscles ;
these he uelicved to be always in an electric condition, the interior
being negative, the exterior positive. He also noticed the curious
fact, that ill experiments on frogs, the nerves lose their irritability
to the stimulus of electricity at their origin first, retaining it
longest at their extremities ; and on this hazarded an opinion
that probably the distal extremities are really the origin of
these structures. Both these statements are of deep interest;
the former from its bearing on the later researches of Matteucci,
the latter fri)m its curious connexion with some views of Dr.
Marshall Hall, regarding the peripheral origin of incident, or sen-
sory nerves.
989. It may now be asked, what proof do we possess that the
action on muscular fibre here alluded to, where no metals are em-
* Willdiuon's QalT«iU8m. London, 18M. Vol. i. page 40.
558 OBOAHIC SLEOTBXCITT.
ployed, 18 really prodnoed by electric ourrents? One gretX evi-
dence in favour of this opinion is at once found in the tact, tliat
contractions produced in frogs can only be excited wben connexion
is made between a nerve and a muscle by a conductor of eleo-
tricity, all insulators interfering with the production of this phe-
nomenon. The only experiment amounting to positive proof
before the researches of Matteucci is that of Valli, in whicn he
formed a sort of battery of fourteen prepared frogs, and by the
electricity thus accumulated succeeded in producing the pheno-
mena of divergence in a delicate electroscope. It is to be regretted
that no accurate account of this experiment has been left on
record ; for if true, it must be regarded as most satisfactory in
grovine the identity of the electricity of the fipog with that obtained
t>m ouier sources.
990. The researches of Prof. Mattencci,* of Pisa, hare, how-
ever, completely set this matter at rest. He has incontestably
proved that currents of electricity are always circnlating in the
animal frame, and not limited merely to cold-blooded reptileo^
but are common to fishes, birds, and mammals. From the re-
searches of this philosopher it appears that the interior is always
electrically positive to the extenor of a muscle; and that al-
though the potential developed is exceedingly small, yet that
by arranging a series of muscles having their exterior and inte-
rior surfaces connected, he developed sufficient electricity to pro-
duce enei^tic effects. By thus arranging a series of natf-
_j thighs of frogs, Fip.
^" 654, he succeeded in
decomposiog iodide of
potassium, in deflecting
the needle of the ga£
vanometer to 90°, and
by the aid of a conden-
ser, caused the gold leaves of an electroscope to diverge. When
more delicate tests of the electric current were made use of, its
existence was demonstrated in the muscles of all animals, and
even of man himself. Dr. Wilkinsonf calculated that the irri-
table muscles of a frog^s leg were no leps than 56,000 times mors
delicate as a test of electricity than the most sensitive con-
densing electroscope. I>r. Wilkinson found that two pieces of
zinc and silver, each presenting a superficial area of yW inch,
produced violent contractions in the leg of a prepared frog ; whilst
two laiige circular plates of zinc and copper reonired to be broneht
twenty times in contact with the condenser, before any sensible
divergence of the gold leaves of an electroscope was produced.
By comparing the area of these plates, multiplied by the number
of contacts, with the superficial area of the minute pieces of zinc
* Fhilosophioal TranMctioBS, 1845, p. S88.
t BlemcnU of OslTmnism. 1846. 8to. Y ol. ii. p. S16.
DIBSOnOH OF OUBBBVTB IH MUBCLBB. 559
tfid BilTereioployed to affoct the frog*B leg, lie smved at the eon^
elusion here stated.
991. Mattencci availed Mmaelf of this circumstanoe in his
contrivaDco of theyro^rAeoseo^. This is made
by skinning the hmd kg of a nog, and separating ^< (^*
it from the trank, taking care to leaye as long a
piece of sciatic nerve projecting as possible. The
teg is then placed in a glass tube, the nerve bang-
ing over, Fig. 555. In using this contrivance all
that is necessary is to let the piece of nerve touch
simultaneoDsI J in twoplaces tne part of which the
electric condition is 'CSbe examined. If a current
exist, the muscles of the leg will become convulsed
at the moment of contact. In this way Mattencci
detected a current in man ; by making a clean in-
eirion into the muscles of a recently amputated
limb, and bringing the nerve of a frog rheoscope
in contact at once with the two lips of the wound,
contraction instantly occurred.
992. In pigeons and fowls, as well as in eels and frogs, cur-
rents were readily demonstrable ; indeed, by alternating a series
of the former by approximating their sides, the ratr surface of
the muscles of which had been exposed by a quickly made cut,
Matteucci formed a sort of battery resembling that made of the
thighs of frogs. The result of this experiment thus proved that
energetic currents existed in hot as well as cold-blooded amimals :
more intense, indeed, but very soon disappearing on the death of
the animaL
. 993. By means of the frt)g rheoscope (991), not only the exist-
ence, but the direction of a current may be discovered ; for if the
leg be kept for a short time before using it, so as to a little
diminish its sensibility, the muscles will contract on making con-
tact with the bodv under examination, if the electricity pass
from the nerve to the leg, whilst it will contract on brecJeing con-
tact, if the current move in the opposite direction. Using this
delicate test of an electric current, Matteucci discovered that the
intensity of such currents rises in proportion to the rank occupied
by the animal in the scale of being, their duration after death
being in the inverse ratio. He found that when a mass of muscle
belonging to a living animal, or to one recently dead, was placed
in contact with a piece of wire so that one end of it touched the
tendon, and the other the body of the muscle, a current could
always be detected circulating in the mass in the direction from
the tendon to the external surface of the structure. He further
demonstrated* the very important fact that everything which de-
creases the vis vitcg of the animal diminishes the evidence of
electricity immediately after death. Thus, when frogs were killed
by asphyxia, aither by immersion in sulphuretted nydrogen, or
MO OBOAVXC ELECTSICITT.
in water freed from air, the electricity detected in their femoral
mascles sunk to a minimum ; and the thighs of frogs whose hearts
had been previously removed, gave less evidence of the existence of
this important agent, than those which had not been thus injured.
994. It has been shown that certain fishes (976) possess a
peculiar apparatus by which they are enabled to accumulate the
electricity developed in their structui'es, and thus to produce the
recoenised effects of potential, as shown in the Denumbing
shocK felt on grasping the torpedo, or silurus. This endowment
is, however, peculiar to very few creatures, and all the electricity
developed in other organisms is only to ^be detected by com-
paratively delicate tests. It is, however, very remarkable that in
the batrachians generally, especially the frog, an electric current,
denominated by Matteucci tne proper current, possessing some
approach to tension, and capable of deflecting the needle of a
galvanometer to 5**, can readily be detected; its direction is
always definite from the feet towards the head. This remai^abk
fsicX was probably first pointed out by Nobili, but accurately
studied by the Pisan philosopher, whose researches have so often
been referred to.
995. We are indebted to M. du Bois-Reymond for very oon-
siderable additions to our knowledge of the existence and direc-
tion of electric currents in the muscles and nerves of animals.
Aided by the very delicate galvanometers already described (858),
and by the peculiar arrangement of the electrodes employed, he
succeeded in demonstrating the existence of electric currents in
mere fragments of muscular and nervous tissue. We must refer
the rea^r to the published account* of these very interesting
researches, as they demand the most careful study : the author of
which has taken extreme care to remove all possible sources of
fallacy, and his mode of carrying on his investigations is very in-
genious. The electrodes employed in these investigations con-
sisted of two cushions of lint, dipping into, and overlapping the
edges of two glass vessels filled with a saturated solution of salt.
Slips of platinum of equal size, and carefully cleansed, are sup-
Sorted by clamps, and immersed to equal depths in tne saline
uid. The clamps are in metallic connexion with the terminals
of the coil. Without the most scrupulous attention to the uni-
formity of condition of the two electrodes, currents will inevitably
be developed, so as entirely to vitiate these delicate experiments.
The electrodes proposed by M. J . Begnault, consisting of plates
of amalgamated zinc, immersed in a saturated solution of sul-
phate of zinc, have more recently been preferred to the preceding.
The more important results arrived at are the following : —
A. The muscular and nervous structures are, while living, en-
dowed with electromotive power.
• Untersacbnngen fiber Tbierischa Elektrieitiit. Berlin, IMS-S.
Animal JSlcctricity, by Dr. Bence Jones. London, 1861.
RELATION OF EFFECTS TO BTBEKGTH OF CUBREHT. 561
B. The cnrrent is always developed in the same direction, in
both nerve and muscle ; everj longitudinal surface being positive,
and every tranwene section negative. And every fragment of
muscle or nerve, however minute, obeys this general law.
G. As a necessary result of this law, the exterior of every en-
tire muscle will transmit a cnrrent through the coil of the galva-
nometer to a clear transverse section.
D. These currents, discovered in muscles and nerves, must be
regarded as derived portions of greatly more intense currents cir*
cuiating in their interior around their ultimate particles.
B. In the contractile tissues, the electromotive power is always
in proportion to their mechanical power.
M. du Bois-Reymond has demonstrated the existence of the
nerve-current in every species of nerve, as well as in the brain,
spinal cord, aod other great nervous centres : he has also traced
toe muscular current previously detected by Matteucci from the
entire muscle, to the single fasciculus, and shown its existence
in almost every department of the animal kingdom ; — in man,
rabbits, guinea-pigs, and mice; in pigeons and sparrows; in
lizards, snakes, toads and salamanders ; in tench, in fresh-water
crabs, and in earth-worms. To him we are also indebted for the
investigation of the changes occurring in the muscular and nerve-
currents, in the interval between the death of the aanimalj and
that of the tiatue; an iaterval the duration of which is generally
in an inverse ratio to the normal activity of the circulation : — as
well as of the changes occurring during ordinary muscular action,
and nnder the influence of continuous voltaic currents, — changes
which are of fundamental importance in clearing up much that is
obscure in the physiology of muscular motion.*
996. Much tuat was unintelligible in the action of both con*
tinuous and momentary currents, without the supposed existence
of an " electro-tonic state" of the muscle, or some such arbitrary
hypothesis, has been elucidated by the researches of M. Chau-
▼eao, of Lyons.f He has shown that if a momentary current
(as from a coil machine) be passed through the body from one
hand to the other, if powerful, it affects all that portion of the
frame through which it passes ; if much weakened, it is felt in the
hands only, and if sufficiently feeble, the shock is felt in the hand
towards the negative terminal only; moreover if several persons
joinine hands receive this feeble shock, it will be felt by each in the
negative hand only. Also if the electrodes be placed on the facial
nerves of a horse, a feeble current produces spasm on that side
only the nerve of which is under the negative electrode : and if
severed hor:C8 be similarly placed in circuit, the sides under the
influence of the negative electrodes will alone be convulsed. It
* Baddiffe, Bpileptie and other ConvaUiTe AiFeotioiis of the Iferroog
System. Churchill. 1861.
t BrowiiP^i^uard, Joumal de la Phynologie, July 1859 to Jolj 1800.
O O
562 osaAviG blbgtricitt.
bAs likewise been obeenred that the negative pole ramAining on the
nenrey if the poatiTe pole be placed ei^er above or below it on the
same nerre, or anywhere elsei the aame result enmiee ; ihe only
neeeuary condition being that the nerve be under the influenoe of
the iM^of ioe electrode.
A precisely similar result ensues, if a sufficientlv feeble current
be applied to a separated muscle, or to a senes of Bep«rate
muscles, connected by metallic conductors: — contraction is visible
only in the fibres a<iUacent to the negative pole, or poles as the
case may be. With a stronger current, contraction appears around
both poles; and with beyond a certain strength of current^ the
whole muscle is thrown into spasm.
A discharge of franklinic electricity from hand to hand through
the body is found to produce precisely^ similar results : aocoiding
to its strength, it is felt throughout, in the hands only, or in Uie
negative hand abne. And if any of the experiments, previooslj
stated as resulting from an induced or momentary current, be re-
peated with a duly regulated franklinic discharge, an identical
result is obtained.
997. The pb^iological effects iucidental to the passage of a
continuous voltaic current are of a more complex character ; they
are universallv acknowledged to be manifested on/y at the moments
of closing ana opening the circuit, but are modifiea by the influence
of the current, during^ its continuous passage, on the portion of
nerve-tissue included in the circuit.
In order to interpret correctly all the observed physiological
eflects, it becomes necessary to consider carefully the dynamical
consequences of the passage of a continuous succession of electric
waves, here assumed to constitute a voltaic current ; and it will ap»
pear that the physiological results are remarkably in accordance
with the dynamical theory of electricity. A voltaie current, then,
must comprise three distinct phases : nrst, the momentary passage
of the molecules of the conductor from a state of rect to a state of
motion ; secondly, the indefinite continuance of molecular motion ;
and thirdly, the return of the moving molecules to a state of resL
The first phsse will be accompanied Jby a rush, or sudden im-
pulse of increased potential in the direction of the current, because
there must be a condensation or accumulation of motion, in con-
sequence of the inertia of the molecules at re$t opposing the
transmission of the wave : this may be termed the inittal currenL
During the second phase there will be (supposing the potential of
the current to remain constant for the time) continuous and uni-
form wave-motion. During the third phase, the excitation of mo-
tion ceasing, the vU viva of the movmg particles will accumu-
late motion towards the termination of thd conductor, whence
a reflex motion may be expected to be propagated (as in the
case of a wave travelling from the hand along a stretched cord,
which reaching the fixed end, is reflected back again), giving rise to
DTVAUGAL BBLATIOH8 Of CUBBXim. 563
a momentafy terminal current, inferior in intensity, and oppodte
in direction to the mUud current.
Tliese initial and terminal carrents, or imptdteSf as they might
more appropriately be termed, have long since been recognised at
the extra^urrentt of Faraday ; but their relative direction and
intensity have been assumed to coincide with that of the induced
secondary currents, and therefore to be the reverse of what dynami-
cal considerations nave suggested ; it has, however, been demon*
strated by an experiment ofM. Chau veau, that the dynamical view
is the correct one. It has been incontestably proved by the experi-
ments previously mentioned, in connexion with many others, that
physiological effects are produced, and produced only by a sudden
efflux of electricity from a nerve or muscle into a negative elec-
trode, whether the active agent be a discharge of franklinic elec-
tricity, or either of the induced, or of the extrsrcurrents, already
mentioned. Starting firom this as an admitted fact, the experiments
of M. Chauveau appeal to the most sensitive of all tests of the
direction of an electric impulse (using that term as synonymoua
with "shock" or "momentary current")— a living nerve. He
places the electrodes of an electromotor over the opposite fiDtcial
nerves of a horse, and having^ duly a^usted the strength of the
current, he finds that on closing the cirooit, that side of the face
onl^ is convulsed (by the initial extrchcurrent)^ the nerve ot
which lies under the negative electrode, and on opening the cireuit,
the contrary side is Uee strongly convolsed (by the terminal extrth
currenJt\ the nerve of which lies under the positive electrode. M.
Chauveau also found that with a still further reduced current, con-
▼alsion occurred in relation with the negative electrode only, the
contrary or terminal extra-current being then too feeble to affect
the nerve.
The same &ct has been observed by M. Claude Bernard* in a
prepared frog's limb, in which the vitality of the nerve is unim-
paired ; with a sufficiently reduced current convulsion occurs on
closing the circuit, and only then, whether the current be direct
or inverse ; because the terminal extra«urrent is then inoperative.
998. It may here be remarked that the well-known relative
direction and intensity of the initial and terminal secondary or
induced currents are the necessary dynamical consequence of the
above assumed conditions of the extra-currents. The initial extra-
current will excite a similar impulsive motion in the secondary
coil, just as one stretched chord will excite another capable of
vibrating in unison with it (for electro-dynamic induction is pro-
bably analogous to the reciprocation of sound [500]) ; and the
recoil of this impulse (the initial induced current) will be weak-
ened in opposing the continaous motion induced by the continuous
primary current. Again, the recoil of the impulse induced in the
* Lecons vox 1» Physiologie et la Paihologie da Systtaie Nerveax. Puis»
1858. YoLi.p. 163.
O O 2
564^ OROANIO BLEOTRICITT.
secondary coil by the terminal extrarcnrrent in the primary, (the
terminal induced eurrent)j will likewise be in a direction contraiy
to that^of the inducing impulse, but its potential will remain undi-
minished, as it is unopposed by any continuous induction ; it will'
therefore possess greater force than the initial induced current
But it maybe asked by those who find a difficulty in dispossess-
ing their mmds of lone-established and time-honoured notions,
Is not all this about " inertia" and " vis viva" and " impulses"
sheer hypothesis ? — ^not so ; — ^the writer has long since observed
and recorded a precisely similar phenomenon (apparently an exact
analogae of the initial extra-current) in an unquestioned case of
wave-motion; — an experiment on the interference of sound-
waves, due in common with so many others to the genius of ProC
Wheatstone, which has been mentioned in " Acoustics," (554).
It can scarcely be doubted that the impulsive recommencement of
the resonance, after its interruption by interference, is entirely due
to the dynamical cause here assigned to the initial extra-current.
M. Chauveau (no doubt correctly) ascribes the physiological
effect of an electric impulse to the dynamical molecular disturb-
ance which it produces.
999. It has oeen conclusively shown by M. Chauveau that the
excitation of functional activity in a nerve is due to the impulsive
influence of the negative electrode only, and not to the direction
of the continuous current, as has been generally supposed. For
this purpose the hinder limbs of a frog are prepared in the manner
of Galvani, that is, they remain connected with a portion of the
spine by the lumbar nerves only, and are placed astride the edge
of a piece of elass. The electrodes being placed upon the lumbar
nerves, a feiS>le voltaic current is transmitted ; passing up one
nerve, across the spine, and dmon the other nerve. As in the
former experiment on the horse, the limb in relation with the
negative electrode is convulsed on closing the circuit, and that in
relation with the positive electrode, on opening it. In the former
case, the direction of the current in the nerve was " direct*' or
" centrifugal," in accordance with the old theory ; but the fallacy
of this hypothesis is shown by the progress of the experiment.
If both the electrodes be now placea on the same nerve, convul-
sion ensues on closing the circuit, indifferently whether the course
of the continuous current be up or down the nerve.
If now the sciatic nerve be isolated, and the lower part of the
femur removed, lea vine the leg connected with the thigh by the
sciatic nerve only, and the positive electrode be placed on the
lumbar, and the negative on the sciatic nerve, the leg only is con-
vulsed on closing the circuit, the muscles of the thigh giving do
response to the " direct'' current through their nerves. But if
the electrodes be reversed, both parts of the limb are convulsed
on closing the circuit (by the influence of the negative electrode) in
spite of the " inverse" current through the motor nerves of the thigh.
^■nai^nH
EFFECTS OF COHTIHUOUB CUBEEKTB. 565
Bat another experiment placed the facts in a still clearer light.
For this purpose tne sciatic nerve of an uninjured limb is isolated
and raised from the limb on a wire, and another ^ire is laid on
the muscles beneath the former. If the wire supporting the nerve
be now made the positive electrode of a very weaK current, and that
resting on the muscle, the negative, on closing the circuit the leg
ou^ht according to the " direction" theory to be convulsed, but
neither on closing nor on opening the circuit, does any such effect
take place. If on the contrary the nerve rest on the negative
electrode, then notwithstanding the inverse or " centripet'il"
direction of the current, spasm ensues on closing the circuit, and
then only. In this crucial experiment the potential of the current
muBt be reduced below the point at which the inverse terminal
impulse is capable of exciting any functional energy.
1000. The change of action that ensues when a musculo-motor
nerve has been for some time traversed by a continuous voltaic
current, may be conveniently observed by means of the rheoscopic
limb (991). It appears that the effect of the continuous current
is to exhaust the vitality of the nerve, from its free extremity
towards the limb with which it is connected. While its vitalitv
is unimpaired, the nerve responds to both exit-impulses at both
points of contact ; when the nerve at the further point of contact
18 dead, it responds to the two exit-impulses at the nearer point
of contact only, viz., those attending the closing of the direct, and
the opening of the inverse circuit. When the vitality at this
nearer point is impaired but not extinguished, the nerve ceases to
obey the latter or weaker of these two impulses, but when quite
dead it obeys neither. The existence, suspension, or subsidence of
functional energy in the nerves must therefore in all experiments
be taken into account, as an important element, and this has pro-
bably been a fertile source of erroneous physiological conclusions.
The following experiment of M. du Bois-Reymond is also very
instructive in showing the relative influence of the positive and
negative electrodes on the nerve-current. He removes the whole
length of the sciatic nerve, and placing its extremities in position
on two galvanometers (i,e. with the transverse section resting on
«ne terminal, and the outer surface on the other), he finds that
nearly the same amount of nerve-current ia indicated in both.
The electrodes of a small element are now placed on the inter-
mediate portion of the nerve, and the result is, that the nerve-
current is augmented under the influence of the positive electrode,
and diminished under that of the negative, as shown by the
altered positions of the needles in the two instruments.
1001. It is the result of observation that the subsidence of the
muscular and nerve currents is coincident with the subsidence of
functional activity, and that the direction of these currents is
liable to a change, or to a succession of changes, when they are
verging on the point of extinction. Moreover, when thes^ cur-
566 OBOJLHIG SLBCTBlClTr.
rents have entirely disappeared, functional activity is also extinct,
and riffor mortiSt the state of universal muscular contraction,
speedily supervenes. It is, moreover, a noteworthy fact, that
causes inducing an extensive depression of the vititl powers, such
as excessive haemorrhage, or strychnine, induce also a convulsive
action, consisting of irregular and involuntary contractions of the
muscles.
An experiment of M. du Bois-Reymond* demonstrates that a
diminution of electric potential always accompanies muscular
contraction. To show this, the gastrocnemius muscle is removed
from the limh of a frog, with a long portion of its nerve remaining
attached. The muscle is now placed on the terminals of the
galvanometer (858), the transverse section heing in contact with
one terminal, and the external surface with the other ; and the de-
flexion of the needle, due to the muscular current, is noted. A portion
of the nerve is then laid across the terminals of an induction-coil
(896), and the muscle throvm into a state of spasm by the rapid
succession of shocks': the galvanometer-needle will instantly
swing back, and subsequently take up a position of equilibrium
much nearer to the zero of the instrument than before, thus indi-
cating a considerable abatement of the muscular current. The
same observer has also found a similar abatement of the nerve-
current, whenever the muscle is thrown into a state of spasm ;
this is best seen by placing the isolated ischiatic nerve in position
(like the muscle), on the terminals of the galvanometer, and then
poisoning the animal by the introduction of a few drops of solution
of strychnia under the skin : as soon as the spasms are developed,
the needle shows a diminished potential of the nerve-current^
which may probably be the immediate cause of the spasms.
These, and many other similar experiments, tend to establish
the conclusion that muscular contraction is always accompanied
by, or attendant upon a diminution of electric potential ; and this
appears to favour the views of the nature of muscular motion en-
tertained by Dr. Badcliffe ; f namely, that contraction is due, not to
the direct influence of any vital or other stimulus, but simply to
the negation or abatement of that electrical condition, on the
due maintenance of which the continuance of the state of relaxa-
tion depends.
1002. Numerous experiments have been made by Prof. Elckhard
of Qiessen,:!: in which the nerve of a rheoscopic limb was suh-
mitted simultaneously to the action, at different points, of a con-
tinuous current, ana of a succession of shocks ; firom these it
appears that the susceptibility of the nerves to the influence of
electric impulses is considerably modified by the action of con-
tinuous currents. In other experiments the chemical stimulus of
a strong solution of salt was substituted for the shocks. A rerj
* UntennohvogeD, &o., rol. ii. pp. 60, 89.
t Bpileptio and other ConvnlriTe Aiffeetioiis, &o. CharehUl, 168U
X Beitr&ge lar Anatomi* und Phytiologie. Gieaaen, 185B.
JIXTB OP TIUV81IIB8IOK OF IMPBBSSIOKS. 567
instructiTe experiment was loug since made by Bitter,* in which
a pair of rheoscopic limbs were immersed in the water-electrodes
of an electromotor, when it was funnd that the functional enerf;y
of the nerves is weakened bj the continuous action of a direct
current, but that it maj even be subsequentlj restored by the
action of an inverse or centripetal current. These facts show that
the power of a motor nerve to evoke the contraction of the muscle
is suspended, or entirely annulled by the influence of a centrifugal or
direct) and may be restored by that of an inverse, continuous cur-
rent*: a result directly opposed to the theory hitherto generally
received.
lliat this is so may be farther inferred from the fact that the
oonvulaions resulting from poisoning by strychnine may be. and
those occurring in spontaneous tetanus are stated to have oeen,
entirely arrested by the maintenance of a continuous voltaic cur-
rent down the spinal cord.
1003. Bate of TraTumiaion through the Nerves. — ^Many elabo-
rate series of experiments have recentlv been conducted for the
purpose of ascertaining the rate at which the impressions of sen-
sation and volition are transmitted through the brain and nervous
system. The rate of transmission through the sciatic nerve of a
irog has been determined by the myoffraphion of Ptof. Helmholtz.
For this purpose the gastrocnemius muscle is removed from the
leg with a small bit of the thigh-bone, and the whole length of
the nerve attached to it : the bone is fixed in a clamp, and the
tendon connected by a hook with a light lever, carrying at its
-extremity a traciue pointwhich rests on the surface of a small
plate of smoked glass. This plate is capable of being moved
rapidly by a spring in a plane parallel to the plane of motion
of the lever, and m a direction at right angles to that of the
tracing point ; and it may, at will, be set in motion by the in-
duced current that, acting on the nerve, induces the contraction
of the muscle. By first causing^ the muscle to contract only, a
vertical base line ▲ b, Fig. 556, is drawn by the tracing point on
the plate at rest; secondly, by allowing the plate only to move, a
horizontal base line a o is traced. If the terminals of the coil be
now pjlaced on the muscle itself, and the plate be started by the
electric impulse, the curve d will be traced, and a d will represent
the time occupied by the muscular fibres in obeying the impulse.
If the terminals be placed on two points of the nerve, near the
* Beveis dsM «m telbtUtiiidifer OalTMiinniit, Ac. Weimar, 1786.
570 OBOASIC BLBCrrBICTTT.
these mvat, like the carbon, by this Tery act become soaroes of
electricity. Bat a more important sonrce of electric excitation is
found in the series of decompositions which take place in the bodr,
daring the action of the yarioas vital processes. It is impossible
that any two elements can be rent asnnder without setting free a
current of electricity, which insignificant as it might theoretically
appear, is nevertheless competent to the prodoction of many
important phenomena. As one amon^ many examples, the case
of common salt may be cited, which pMys so important a part as
an article of food, and for which perhaps alone, of all condiments,
an universal appetite exists. In addition to the proportion of
this substance which enters the blood unchanged, and becomes an
element of all the secretions, a part is decomposed, and one ele-
ment in unison with hydrogen appears as hydrochloric acid in the
stomach ; another, in union witn oxygen, constitutes, as soda, an
important element of the bile. What, it ma^ be inquired, can be
the influence of these apparently infinitesimal developments of
electricity, evolved thus from the resolution of a few grains of salt
and water into its elements ? But it is easv to produce a mass of
evidence to show that these small quantities of electricity are
more so in appearance than in reality. A reference to the powerful
electrolytic infiuence of weak currents (832, 3) will afford sufficient
proof of this.
1009. It is a remarkable fact, that when an acid and an alkaline
solution are so placed, that their union may be effected through
the substance of an animal membrane, or indeed of any other porous
diaphragm, a current of electricity is evolved, the causes of which
have already been investigated (844). Now, with the exoeptioit
of the stomach and ccecum, the whole extent of the mucona mem-
brane is, in the human sulject, bathed with an alkaline mucous fluid,
and the external covering of the body, the skin, is as constMitly
exhaling an acid fluid, except in the axillary, and perhaps pnbic
regions. The mass of the animal frame is thus placed between
two great envelopes, the one alkaline, and the other acid, meeting
only at the external outlets. This arrangement has been shown by
Donn^* to be quite competent to the evolution of electricity, and
accordingly he found that if a platinum plate connected with the
galvanometer be held in the mouth, whilst a second be pressed
against the moist perspiring surface of the body, the needles will
instantly traverse, as tney did in the experiment just shown with
an acid and an alkali. The current thus detected by Donn^
at once explains the cause and confirms the accuracy of the oe]e>
brated experiment of Aldini, in which he excited convnlsionB in
a fro^ by holding its foot in the moistened hand, and allowing the
sciatic nerve to touch the tongue. His curious experiment with
the head of an ox (985) admits of a similar explanation.
• BeoqnareL Traits de TElectxioit^, voL iii.
UBBIO'S BB8EARCHS8. 571
1010. The remits of some researches of Liebig* have rendered it
very probable that a large proportion of the electricity of mascnlar
structures isowingto the mutual reaction of an acid and alkaline fluid.
The blood, in^ a healthy state, exerts a decided and weU-marked
alkaline reaction on test-paper : now it is remarkable that although
a piece of muscular flesh contains so larse a proportion of alkaline
blood, still that when chopped up, and digested in water, the in-
fusion thus obtained is actually acid to litmus paper. This curious
circnmsti^noe is explained by the fact aunounced by Liebig, that
although the blooa in the vessels of the muscle is alkaline from
the tribasic pho^hate of soda, yet the pro^r fluid or secretion of
the tissues exterior to the capillaries is acid from the presence of
free phosphoric and lactic acids. Thus in every mass of muscle
we iiave myriads of electric currents arising from the mutual
reaction of an acid fluid exterior to the vessels on their alkaline
contents. Whatever may be the ultimate destination of this large
quantitT of electricitv, it is at least remarkable that a mus<3e
Bnould be really an eiectro-genic apparatus. We have thus two
flources of the electricity of muscles— tlie efiEbcts of themetamorphofliB
of e£fete fibres on the one hand, and on the other the mutual
reaction of two fluids in different chemical conditions. It is cer-
tainly curious thus to find a muscle, an organ long regarded as the
mere motor apparatus of the bony levers of our frame, invested
with new and important properties.
In the course of twenty-four hours, a considerable proportion of
watery vapour is exhaled from the surface of the body : this has
been variaoly estimated, and in all probability is liable to great
Tariation, but from thirty to forty-eignt ounces of water may thus
be got rid of from the system. It is more than probable that the
evaporation of this amount of fluid is sufficient to disturb the
electric equilibrium of the body, and to evolve electricity of much
higher potential than that set free by chemical action. Evapora-
tion may thus probably account for the traces of free electricity
generalfy to be detected in the body by merely insulating a person,
and plaong him in contact with a condensing electroscope. The
causes of the variations in the character of the electric con-
dition of the body admit of ready explanation in the varying
composition of the perspired fluid. For if it contain, as it gene-
rally does, some free acid, by its evaporation the body woiud be
left positively electric (753) ; whilst, if it merely contain neutral
salt^ an opposite condition would be induced.
1011. iaectroscopio indications of animid potential have been
detected by several observers ; by Qardini and Hemmer, about
the time of Galvani's great discovery; by Ahrens, in 1817;
by Nasse, in 1834 ; and recently by Dr. Kadclifie. Pfaff and
Ahrens generally found the electricity of the body thus ex-
amined to be positive, especially when the circulation had
* Oomptes Bendua de I'Aoad^mie, Jan. 18 and Feb. 8, 1817.
572 otOJ
been excited bj partnVing of alcDboHc aldmiiUnts. HeBmer,
ftnother abierrer, found that in 2422 expenment« on hiina*lf| ki<
bodj' wBB positiielj electric in 1252, negatiire in 771, aod neutnJ
in 399. B/meane of [hepairofpositiTeandnfgktiTc electnanijiei
(663), Dr. RsdcliSe h&a frequeatl; obUbed trmna »f potaitul.
•ometimog panitire and Borne Limei negative, in his ovn body, afid
in that of othera. Ue bas alio found traces in fresh-dnwn blooj
fram the shambles, generally negatire, somelimei very ■ligbtly
Seitlve, but these indicatinns alwaj* diaitppearing In an hinr oc
K. The Bpiua] cords of oieu mere rather ■trongly poBtiir,
the cerebellum of a sheep neutml, and tba brain of a donkey
□egaliTe. I^eces of muscular tissue gBTs more nncerluii iodiea-
tioDS ofpotentitd.
1012. The potential quality of animal electricity vai demoo-
■trated by a remarkablo experiment of Humboldt. He prepared
the leg of a very ligorous frog, and arranged it as aliowa in
J, ^ Fig. 557. The leg ia laid ot a
clean dry plate of glaas, aod tbe
' nerre being divided, tbe cni
ends are laid at a imall diataocv
from each other, and iLe pniii-
I mal end of the nerrc on a amall
plate of meta). The interval
betneen tbe ends of tke nerri
being about 2"~, when c«>-
neiion vaa made between the body of the moacle and ihe pitir
of metal by means of a pair of conipassea, contraction ensued,
ahowirg that the potential of the nerve was then laScicnt w
act by induction through that interval. In the cooiae of
about ten minutes, however, contraction did not ensue, onlna
the dividad portions of the nerve were brought inio actual con-
tact, the vital energy of the nerve being even in that short time
thns far impaired.
1013. Independently of combustion, chemical action, or evapo-
ration, the mere contact of heterogeneous erg&nie matter* is cun-
petent to produce indications of electricity. ThuB a pile orallemair
slices of muscular tissue and brain, with pieces of wel I»tbet in-
terposed, has been found by Lagrave to evolve a current: s>d
Dr. Baconio, of Milan, has shown that a few alternations of Blk-va
of beet-root and wood of the walnnt-tree were capable ofgrneratinE
sufficient eltwtricity to eicite convuUinns in a (rog when couven^
to ita muscles by nieaus of a conductor formed of a leaf of ecorrV'
grass. Matteucci has ibrown out the soggeslion, that the oij!»-
nixation of a muscle is possibly Bucb as thus by heterogeneity of
■trmclure to account for the development of electricitjr ; be cod-
■iden the analogy between Ihe voltaic arrangements and the mi-
■titution of muscle to be complete, if we conceive the poaiiive
plate u> be represented by tbe tme 6bra, the negative plate bj
ths sarcolemma, and the exciting Said by the blood.
HEBBOHEL^B HYTOTHBSiS. 573
. 1014. Secretion and nervons agency have always been the
favoarite phenomena which electncity has been called in to ex-
plain, and with some considerable appearance of probability. Dr.
WoUaston, nearly forty years ago, first suggested from the resolu-
tion of salts into their elements under the influence of feeble
currents, that secretion depended essentially upon the electric
state of the secreting glands ; he thus regarded the kidneys as
constituting the positive, and the liver the negative electrodes of
the electric apparatus of the body. A curious anecdote is related
of Napoleon, who is said by Chaptal to have remarked, on seeing
the voltaic battery of the French Academy in action, " VoiU,
doctenr, Pimage de la vie ; la colonne vertebrale est le pile, la
yessie le pole positif, et le foie le pole negatif."
1015. There is, in connexion with this hypothesis, a most inte-
resting and important observation of Matteucci, to whose
ingenuity and perseverance we are so largely indebted: this
philosopher introduced a plate of platinum into the stomach uf a
living rabbit, placed another on the liver, and connected both
with a galvanometer ; the needle instantly traversed an arc of
20**, proviDg the existence of a powerful current between the liver
and stomach. This, it maybe observed, shows the existence of a
currenty but does not prove whether it is to be regarded as an
effect or cause of the chemical changes alluded to, for it has been
already shown, that when an acid and alkaline fluid are separated
bjr permeable structures, they actually develope a current of elec-
tricity : and as the stomach contains an acid, and the liver an
alkaline secretion, this might afford an explanation of the current
observed by Matteucci ; and had the experiment ended hero, this
plausible objection would have been a fatal one. But the nerves
and vessels passing into the abdomen were divided above the
diaphraem, and in an instant the needle of the galvanometer
was deflected to 3"* instead of 20** ; and on cutting off the head of
the rabbit by a sudden blow, even this little deviation almost
entirely vanished. Nothing could be more conclusive than
this experiment in proving that the electric current was the
cause, not the effect, of the chemical metamorphosis of the saline
ingeata, the decomposition of which furnished acid to the stomach,
and alkali to the liver. How this current is excited is unknown,
although it can hardly be doubted that one of the causes which
we have already examined is competent for this purpose ; but
then there remains the difficulty of pointing out the route taken
by the current to reach respectively the liver and stomach, for the
pneumogastric nerves, at least in man, cannot, from their anato-
mical distribution, explain this.
1016. Sir John Herschel has beautifully expressed the possible
relation between galvanic electricity and the vis nervosOj and
hinta at the brain being either the organ of secretion, or at least
of the application of this agent ; adducing in illustration the dry
piles, as they are termed, of De Luc and Zamboni (818), and
574 OROAvio xLBOTBicrrr.
remarks, that " if the brain be an electric pile conatantlj in action,
it may be conceived to discbarge itself at regular interTals, wben
tbe tension of the electricitj reaches a certain point, along the
nerves which communicate with the heart, and thus to excite the
pulsation of that organ.'* Bj the " diy pile " a ball maj be kept
m motion for many years, without any obvious waste of power,
and some analogous arrangement would constitute the most con>
stant and economic mrimum mobile of a moving oijnn which the
resources of limited numan reason can suggest. Dr. Amott has
also hinted at some such cause being the active asent which
keeps up the regular pulsations of the heart. Aocoraing to Dr.
Raaclifl^'s views, the periodic inaction of the heart is the intelli-
gible and necessary result of tbe periodic stimulus of the supply
of blood through the branches of the coronary arteries.
1017. The exercise of the vital functions of vegetables appears
to be frequently attended by the excitation of electricity, sufficient
to evolve even sparks, at least if we are to believe reports on this
sul^ct. Pottillet has satisfactorily proved that electricity is
evolved during germination, and Dr. Donne has shown that car-
rents may be detected by means of a delicate galvanometer, in all
ripe firuits, passing between their bases and apices.
From a few observations made by the late Author on this sub-
ject,* he arrived at the following conclusions : —
1. The great improbability of vegetables, on account of their
feeble insuution, ever becoming so charged with electricity as to
i^ord a spark ; and the probability of those luminous phenomena
said to be exhibited bjr some plants, depending on other soaroes
than on electric potential.
2. That verv feeble electric currents are always circulating in,
and exerting their influence up<m, vegetable tissues in every stage
of their development.
3. That electric currents are developed during germination, and
assist in producing the important chemical changes proper to that
process ; and that by causing the seed to assume an oppositely
electric state, its development is retarded or checked.
Bbferences.
On the subjects treated of in this chapter, the student should
refer to Becquerel, Traits, vol. iv. ; and to the first volume of
Miiller*8 Physiolc^. The second volume of the Traits complei
de Physiologie, ofTiedemann, contains some interesting informa-
tion on this subject. The writings of MM. Matteucci, Brown-
S^ouard, Du Bois-Beymond, Claude Bernard, Eckhard, Chauveau,
ana Dr. Badclifie. contain much more valuable information than
could possibly be mtroduced into this brief abstract.
• MagMine Vat. Hist, i 198.
675
CHAPTER XVin.
XJOBT; CATOFTBICtt AHD DIOFTUCfli.
1018. Soke nncertainty still remains as to the actual nature of
light, notwithstanding the innumerable ohsenrations that ha^e
been made upon it, and the various hjrpotheses that have from
time to time been based upon them : passing over the theories, or
rather vague ideas of the ancients, there have been two dififerent
hypotheses that have, in modem times, attracted most notice.
The first, and, until within a few years, almost universallv adopted,
was that of Newton; according to whom, light consists of an
emanation of indefinitely minute particles of matter, thrown off
fitmi the sun and other self-luminous bodies, with an enormous
velocity, and capable of bein^ again thrown on by reflection from
bodies upon which they impinge, and hy which such bodies are
rendered visible. The sagacious mind of Newton, however, could
not fail to perceive the difficulties of his own hypothesis, and its
inadequacy to expjlain many observed phenomena, especially those
of dififraction and interference (Ch. XIX.), which are nothing mors
than necessary dynamical consequences of the wave theoiy. To
explain these, he had recourse to the ver^ strained hvpothesis of
his light-molecules being subject to penodical "fits of easier
transmission and easier reflection ; that the molecules might be
egg-shaped, and making perpetual somersaults in their onward
progress, if they tumbled endwise against a refracting medium
they entered it, but if they fell sideways, they rebounded off
again. Another conspicuous difficulty of the corpuscular theory
is that of conserving a motion of translation in matter, at such an
enormous velocity as that of light, but the transference of motion
onJ^may readily oe imagined. Again, the ascribing to any kind or
form of matter "imponderability" or an exemption from the uni-
versal law of gravitation, is an nypothesis entirely opposed to all
reason and experience: in the corpuscular theory it was indispen-
sable, but it will presently appear that in the dynamical theory
it is by no means necessary.
The second theory, being that toward which philosophers of the
present day almost universaliv incline, is a modification of one
proposed by Descartes, and adopted by Huygens, Euler, our late
talented countijman, Dr. Young, and all other eminent physicists.
This hypothesis regards light to be the result of undulatory or
oscillatory movements, in a subtile and highly elastic medium called
576 Lia^.
ether^ pervading all space. This undulatorj or dynamical tbeory,
as it IB termed, is capable of affording a ready explanation of
the causes of many phenomena, to which the Newtonian hypo-
thesis of emission is to a great extent inapplicable ; and, on that
account, has received the support of most philosophers of the
present day. A third theory, proposed by Oersted, regards light
as the result of a series of electric sparks : this has met with
very little support, and may be at once dispensed with.
1019. According to the dynamical theory, as hitherto received,
the evolution of light is supposed to be produced by the oscillations
of the universal ethereal medium, existing in the interspaces
between the molecules of every material substance, and extending
beyond the confines of our atmosphere into infinite space, in the
same manner as sound is produced by the vibrations of the denser
medium, air, which constitutes our atmosphere. The movements
thus excited in the eminently subtle ana elastic medium, ether,
are readily communicated to what is ordinarily termed a vacuum,
but which we must suppose to be pervaded by this imponderable
matter, as well as to transparent bodies, by causing, in all proba-
bility, their particles, as well as those of the interstitial ether, to
assume an oscillatory movement. The ethereal medium contained
within the interstitial spaces of transparent bodies is less elastic
than that contained in vacuo, and this elastici^ appears to
diminish with the increase of the refractive power of the substance.
The remarks already made on the vibrations of solids (369 — 378),
and on the undnlatorv or wave-like motions of elastic, and non-
elastic fluids (462 — 471),will tend to facilitate our conceptions of
the nature of analogous movements in the eminently elastic
medium called ether. Indeed, it is necessary to add but little to
the description already given of the wave-like motions assumed by
air under certain circumstances, remembering of course that the
excessive elasticity and tenuity of this ether permits it to assume
the peculiar movements under consideration with almost incon-
ceivable facility and rapidity.
1020. That some material medium pervades infinite space, as the
means of transmission of the light- and heat-waves (as the case
may be) of the heavenly bodies, is indispensable to the dynamical
theoTj ; but is it equally, or indeed at all, necessaiy to imagine that
the portion of space within the confines of our atmosphere, which
is occupied by ordinary matter, amenable to our senses as well as
to the universal law of gravitation, must be occupied by a strange
and anomalous form of matter amenable to neither? Probabilities
appear to negative this question. But it will be immediately asked,
how can ether be imagined to avoid pervading our atmosphere
and all other kinds and forms of matter ? — simply by ascribing to
it a property of nan-mUcibility with our atmosphere, like oil with
water — a quality not entirely adverse to experience, nor repagnant
to reason. The question then naturally arises — what becomes of
VATUBE OF ETHEB. 577
the wJEives of heat and light, when they reach onr atmoaphere?-^
and 18 ordinary matter sufficient and effectaal for their trans-
iniraion? This question can be answered only from analogy,
which appears to infer an affirmative.
That sound-waves are transmitted by air, and not by inter-
stitial ether, is unquestionable ; and if air be capable of trans-
mitting 25,000 vibrations in one second, it would probably be
difficult to assign anv valid reason why the same medium is in-
capable of transmitting the far more rapid waves of heat and
lignt ; and if odpable, then where lies the necessity for assuming
the presence of another medium ?
Again, the refraction of sound, as demonstrated by the experi-
ments of Hi\jech and Sondhaus (563, 4), is in exact accordance
with the laws hitherto assigned to the refraction of light and heat.
But the phenomena of the refraction of light reouire a very forced
addendum to the interstitial-ether bypotnesis-^namely, that the
elasticity of the ether is dependent upon that of tlm medium which
it pervadei — ^an unprecedented influence of one kind of matter on
other merely contiguous matter. And it appears that the velocity
of sound in solids and liquids is much greater than in air (545) ; in
water it is nearly 5000 feet, and in iron nearly 17,000 feet in one
second: is there, then, any known fact whatever that tends to
assign a limit to the possible velocity of transmission of wave-
motion throueh these and other material media ? — ^if not, then the
presence of ether, as generally assumed, cannot be deemed essential
to the transmission of li^ht; and if not essential, why should the
old hypothesis bo entertained ?
** Neo Deaa iaterait, nisi dignos vindice nodas
Inddarit."
Moreover, Prof. Tyndsdl, to whom the progress of Dynamical
Physics is indebted for many laborious and important researches,
has observed that in various kinds of wood there is a remarkable
harmony between their respective conductivities for sound and heat
in three mutually perpendicular directions, namely, longitudinal,
transverse-radial, and transverse-tangential (546) : now although
there is certainly no direct analogy between tne conduction of
heat, and the radiation of light, beyond that of their common
dynamical origin, a much closer analogy may nevertheless be
traced through the phenomena of phospborescence, fluorescence,
and calorescunce. Is it, in fact, generally believed that the
transmission of heat-motion is effected by interstitial ether, and
not by the molecules of the medium itself? If not, why should a
hypothetical medium be assumed for lieht-motion which is not
required for that of heat, since the rapidity of the undulations,
and their velocity of transmisbion, is so much the same in
both? While at the same time the converse permeabilities to
light and heat of crystals of alum, and those of dark smoky
quartz, present striking examples of the existing yet unknown
p p
678 LIGHT. •
differences of physical constitution, which are met with in the
various kinds of matter.
It may further he remarked that the dynamical theory of elec-
tricity, if tenable, presents addiiioual readons for denying the
necessity of the presence of ether in ordinary matter. For if the
molecules of a rod of copper can transmit an electric wave at
the rate of at least 250,000 miles in a second, why cannot those
of a rod of class transmit a wave of light at about three-fourths
of that velocity ?
If, then, these premises be established, it must he granted that
the presence of ether is not essential to the transmission of any
known kind of wave-motion ; and if so, is it not more in accordance
with the true spirit of philosophy, in default of positive knowledge,
to abandon uunesitatingly tne mora violent nypothesis, and to
adopt provisionally the more reasonable one above suggested?
1021. The necessity will subsequently appear of making a
farther hypothesis with regard to tne vibratory movements con-
stituting light ; namely, that the oscillation of each particle is in
a plane perpendicular to the direction in which the wave is mov-
ing, or, in other words, that the undulations of light are trans-
verse, and never longitudinal (376), as is generally the case with
sonorous vibrations. When a series of undulations travels over
the surface of water (473), the motion of all the moving pixrlicWa
is in the same direction, namely, in a plane perpendicular to
the horizontal surface of the fluid ; but if we communicate by
means of the hand a series of undulations to a rope (371), thosj
undulations may be made to take place in any plane pastdnsr
through the rope that we please, but in all cases the movement of
each portion of the rope will be perpendicular to its length.
And, furthermore, if, instead of moving the band simply back-
wards and forwards, we move it in a circle or an oval, we shall
communicate a sort of spiral or corkscrew undulation to the rope.
Now the rope may be considered as a row of pHrticlei<, and we
may conceive a ray of light to be made up of an indefinite num-
ber of parallel ro/7e« of particles uiiduliting in all possible direc-
tions, and in all the various ways above mentionea ; the motion
of each individual particle, be it remembered, being always in a
plane perpendicular to the path of the ray. •
1022. The waves of light, like those of sound (531), are thus
transmitted in every direction, extending on every side of the
luminous body. As sonorous vibrations are conveyed to the ear
through the atmot>phere, by the particles of air assuming undu
latory movements, so any self-luminous body, as the sun, or a
lamp, excites analogous undulatory movements in a surrounding
medium,which, being conveyed by contiguous particles, eventually
reach the eye, communicating the sensation of light to th it
organ, in the same manner as scmorous vibrations convey tha
sensation of sound to the ear : and the cessation of undulations.
TELOCITY OF LIOHT. 579
or the repose of the transmitting medinm, resnlts in darkness, as
the absence of similar movements in the air o(!casions silence.
It has been objected to the undnlatory theory, that if true, light
ought to bend round opaque objects, in the same manner as the
waves of water (467} are propagated round fixed obstacles, and
to be communicated through curved tubes, like sound, and con-
sequently that no true shadow ought to exist. These objections,
however, are more apparent than real ; for, taking the case of sono-
rous vibrations, we hnd that they do not bend round obstacles with
facility, and that an acoustic shadow does really exist : thus the
sound of a rapidly moving carriage becomes less distinct as it
turns the corner of a street ; and sounds passing through water
are still more readily obstructed (565). The existence of an
acoustic shadow may be better shown oy a vibrating tuning fork,
held about six inches from the ear; on suddenly interposing a
piece of card between the latter and the sounding.body, instantly
the tone will disappear, and on withdrawing the card, it will again
become audible. In the case of curved tubes, we know that whilst
sonorous undulations are readily transmitted through them, those
of light are completely obstructed ; for no one can see through a
bent brass pipe. But, in this case, it must be recollected, that
the sides of the tube, whilst they are sufficiently smooth to reflect
sound, are infinitely too rou^h to reflect the undulations of light.
There is no difficulty in seeing objects through a tube bent four
times at right angles, provided __ ^
suitable means be employed, as ^.oa ,
in the well-known optical toy,
Fig. 558, by which we are appa-
rently made to see througn a
book, or other opaque object ; but
ordinarily, objects are not visible
through bent tubes, because the substances of which they are
composed absorb or disperse any luminous undulations (1019) that
may enter them. Lastly, whilst sonorous undulations have been
shown to pass round projecting obstacles, more or less freely ac-
cording to the nature of the medium, so those of light are capa-
ble off to a certain extent, passing round the edges of opaque
bodies, and entering their shadow, as shown in the phenomena of
inflection or diffraction (1120).
1023. Luminous undulations (or, in other words, light) are pro-
pagated from the sun through space, and to the surface of our globe,
with an enormous velocity, at the rate of about 191,515, or in
round numbers, 192,000 miles per second ; and this motion is the
same for light evolved from tne most distant fixed star, as for
that from the nearest self-luminous body. This rate of propaga-
tion of light was first diKcovered by Olof Boemer, a Danish astro-
nomer, in the year 1676 when observing the occultation and
emersion of the {<atellites of Jupiter: he found that when the
p p 3
580 UOHT.
CArth was directly receding in its orbit from that planet^ as from
A to B, Fig. 559, the emersion of its first moon, m, from its
shadow at m, occurred 15 seconds later than the calculated time.
To make this clear, let us suppose that an observer on the earth
^^.660.
at A, watches the immersion of the satellite m into Jupiter's
shadow, then it is known from the period of its entire reroJution
that it ought to emem at m in 42 hours, 28 minutes, 35 seconds ;
but if at the end of that time, the obserrer again looks, he will
have to wait 15 seconds later before he will observe the emergence
of the satellite at m. The reason of this is that in 42^ 28"" 35',
the earth will have moved in its orbit from ▲ to b, a distance
of 2,880,000 miles, and the 15 seconds were occupied by the
light of the emerging moon in traversing the space between a.
and B. In like manner, when in the opposite side of its elliptic
path, the earth advances towards the planet, the emergence of its
moons will appear to take place proportionably earlier. The light
of the sun consequently requires 8™ 13* to reach the earth, whilst
that of the planet Herschel occupies 2*^^ 40™ in trayelling to us ; and
at least six years are required for the light of the nearest fixed star
to reach the earth. The experiment of M. Fizeau, in which
light, passing through a notch in a revolving serrated disc at
Montmartre, was reflected from a plane mirror on the Observatonr,
at the opposite side of Paris, and that of M. Foucault with light
I'eflected from a rotating mirror, tend to show that the velocity in
air of light is about 185,000 miles in a second : this is not incon-
sistent with its greater velocity in ether, as previously determined.
1024. All bodies may be divided into those which are $elf-lumi-
nou8, LCf capable of exciting luminous undulations of themselves,
as the sun, or a lighted lamp ; and those which become luminous
only in the presence of the former ; thus the moon and planets are
luminous only in consequence of the presence of the sun about
which theyrevolve. A great number of bodies possess the pn>-
perty of intercepting the passage of light, and thus produce a
shadow by obscuring the source from which the liuninous undu-
lations proceed : these shadows in general present the same figure
as the outlines of the intercepting bodies. Such bodies as permit
light to pass frccl;^^ through tnem are termed transpareiU^ in oppo-
sition to those which intercept it, constituting opaque substances;
bodies that transmit light imperfectly ai*e termed trantlueent.
LAW OF nrraxsiTT; photoxetzbs. 581
1025. Non-luminoTiB bodies become Inmiooiu in the presence of
8 sonrce of ligbt, either by reflecting the nndnlatoiy movements,
or if phoephorescent, by having vibrations excited in the molecules
of the boaj itself, which are thence communicated to the sarround-
ing medinm. Thus, then, bodies are not rendered visible by any
matter given off from a luminous source, and impinging upon
them, but by the undulatory movements successively com-
municated to contiguous particles, which reaching the illuminated
body are dispeTsed in all directions from its irregular surface ;
and thus the bodjr becomes itself a source of divergence of fresh
luminous undulations.
1026. The intensity of the illumination of any body in the pre-
sence of a given source of light depends upon its distance from Uiat
source, and obe^s the general law of radiant forces, as attractions
(31), the intennty of the light varying inveraely aa the Bquare of
the dUtcmee of the^ luminous bodyjrom the source of light. Thus,
if a single candle illuminate a boay to a certain extent at a dis-
tance or a foot, it would recjuire the light of four candles at a dis-
tance of two feet, and of nine at three feet, to produce equal illu-
mination. This will be more readily understooa from the following
experiment: — ^Having ruled a sheet of paper or pasteboard in
squares of one inch, place a piece of card one inch square with
a slender support, as a piece of wire, two feet from a screen with
a small hole m it, and place a lamp or candle as close as possible
behind the screen. If the ruled card be now held in the shadow
of the small square, it will be found that at four feet from the
screen, the shadow will occupy four squares, and at six feet, nine
squares : and as the light received upon a surface of one square
inch is thus shown to occupy four incnes in the first case, and nine
in the second, it follows that each inch at the distance two,
received only one-fourth of the li^ht, and at the distance three,
one-ninth ; or, generally, the intensity of the light at any point is in-
versely as the square of the distance of that point from its source.
1027. PhoUymeters. — ^It is often important to be able to compare
the intensity of two sources of light, and for this purpose instru-
ments termed photometers have been contrived. The instrument
most frequently employed for this purpose is the photometer of Les-
lie ; this consists of a rectangular treueh from six to ten feet long,
at the open ends of which the two lights to be compared are
placed. Two mirrors meeting at an edge, and each inclined at
45** to the axis of the trough, reflect tne two lights on to the
a^fftcent portions of a translucent screen fas of thm white paper
or ground glass), placed level with the surtace of the trough . The
mirrors are moved along the trough, until die two portions of the
screen are equally illuminated, when the ratio of the intensities of
the two lights wul be found from that of the squares of their dis-
tances from the middle of the screen. Another veiy convenient
instrument is that contrived by Frof. Wheatstone, consisting of a
582 LIGHT.
bead of Bilvered glass rapidly moving l>ackwards and forwards in
a straight line, by means of a simpie and ingenious mechanical
contrivance, — ^a wheel revolving within a fixed annular wheel (176)
containing twice as manj teeth (184), and having a bright bead
attached to its circumference : on the moving bead the two lights
to be compared appear bj reflection as two luminous parallel lines.
Then by altering the relative distances of the lights until the
luminous lines appear to be of eoual intensity, and squaring these
distances from the photometer, tne relative illuminating powers of
the two sources of light may be readily discovered. Some
approach to a comparative measurement may be obtained by
ascertaining the distances at which any two sonrces of lights a-s
two candles, require to be placed, to cast upon a wall shadows of
a rod of wood or metal of equal intensity ; the squares of these
numbers will be to each other in the ratio of the intensities of the
light evolved from the two candles. The illuminating power of
any source of light will of course not only depend upon tne inten-
sity of its light, and its distance, but upon the extent or area of
its luminous surface, thus, according to Dr. Wollaston, it would
require 200,000 millions of such stars as Sinus, or 5563 wax
candles at the distance of a foot^ to produce a light equal to that
of the sun.
1028. If the Bur&ce or internal structure of a substance be snch
as to be influenced alike by all the luminous undulations emanat*
ing from a source of light, it will communicate to the eye the
sensation of white light ; but if it be so constructed as to absorb
all the luminous undulations which impinge upon it, it cannot
become the source of a fresh set of analogous movements, and is
said to be Hack. We know that in the .£olian harp the strings
assume different states of vibration, and evolve corresponding
sounds, when acted upon by a current of air, according to the
diameter and tension of the chords (576); the more tense or
thinner string evolving the higher, and less tense or thicker, the
grayer note. In an analogous manner are the undulations arising
from any source of light supposed to be affected by the physiau
structures of bodies, by which some assume undulatory movements
analogous to the tightly- stretched oord in the u£olian harp, and
these communicate to the eye the sensation of violet or purple
light ; whilst the particles of other substances under sinular in-
fluence, oscillate with a less degree of velocity, and these vibra-
tions convey the idea of retly on reaching the eye. The rapidity
of the undulatory movements assumed and propa^ted by colonred
bodies immensely exceeds that of sonorous vibrations; thus,
whilst to evolve red light, it has been calculated that a body must
communicate about 45i3 millions of millions, and to evolve violet,
not less than 699 millions of millions of undulations in a second,
the middle C (570) is produced by only 522 vibrations in a second
of time. It has also oeen calculated that if a string of the proper
LUM1K0U8 CNDULiLTlONd. . 583
length to produce a sound when 'vibrating, corresponding to this
miridle C, wero bisected 40 times, it would, supposing it were
possible to make it vibrate, evolve not a sound, but a yeUotOMh
green light : the vibrations of a chord increasing in rapiditj in
proportion to the diminution of its length (576). Colours conse-
quently are no more innate or abstract properties of bodies than
any particular sounds or notes which they emit; the latter varying
with the teusion, length, and thickness of the subiitances, and the
former with certain, possibly analogous, modifications of physical
structure.
1029. Undulations radiate in all directions from every portion of
a luminous body (1022), but vary in their rapidity with the colour,
of the substance. If a small hole be made, or, still better, if a
convex lens of suitable focus be fixed in one end of a wooden box,
blackened internally, and it be presented towards any object or
landscape, an inverted image will be painted upon a piece of white
paper, placed at the opposite end, and presenting the very same
hues as the object of which it is the image.
1030. A ray of light on the undulatory hypothesis, is a wave
propagated in a ri^ht line from the luminous body, and the undu-
lations producing it are transverse to the course of the ray, which
therefore must be considered as merely expressing the direction of
an effect, namely, the motion producing the impression of light.
When a ray of light falls upon the surface of any substance, it
may undergo one or more of the following modifications : —
A, it may be reflected back into the medium in which it was
moving (1033) ;
B, it may pass into the substance, and herefraeted (1052), still
retaining ita original characters ; or
C, it may be divided into two portions, each possessing distinct
physical properties (Ch. XXI.) ;
D, a ray may become coloured by having some of the undula-
tions producing it absorbed; or
£, it may excite a fresh set of undulations, and consequent ra} s,
in the substance, thus rendering it visible (1113);
F, it may also, by meeting with a second ray, have its intensity
modified by their mutual interference (1116) ; or
G, it may during its refraction, or reflection, or partial absorp-
tion, acquire new properiies, characteristic of polarized light
(Ch. XXI.); and lastly,
H, it may have the rapidity of the undulations producing it so
affected as to give rise to the various phenomena of fluorescence.
1031. When luminous rays proceed from a very distant body, as
the sun, they may be regarded as parallel; when they are given
off from a point, extending as tney proceed, they are termed
divergent; and when they gradually approach each other, as when
acted upon by a concave mirror, or convex lens, they are said to
be convergent.
584
OATOFTRIGS.
4»
O
A
ff
-y'
f
^
y 1
ir
1
s
R
1082. Wlien parallel rays fall upon a plane Borface, tbe illami-
nation Tarics with tbe angle at which thej meet the surface, being
greatest when they fall perpendicularly upon it : let tbe paralM
rays, o, 6, c, d^, e,/, Fig. 660, fall perpenoicularly upon a surface,
o p, then it is obvious tbey will
all be effectual in illuminating
it; but if the surface be in>
clined, as pb, fewer rays will
impinge upon it, and it will be
proportionably less illuminated.
Draw B 8 perpendicular to /p,
then, since the illumination of the sur&ce is proportional to the
number of rays falling on it,
Illumination of p b : that of op:: B8:(ops)pr:: cos p b s : 1.
The angle p bs is evidently equal to tbe angle of incidence (296)
of the rays : hence for a given intensity of light,
lUumination a cos angle of incidence;
and it follows from this, and from 1026, that generally
JU *«/rf • dbeolute intensity x cot angle of incidence
(distance of source)*
1033. Whenever a ray of light falls upon a polished sur&oe
capable of reflecting it, it obeys the same law as that of the
oblique impact of a perfectly elastic body (296), the angles of inci-
dence and reflection being equal. Thus, let a b. Fig. 561, be the
jiy^, Ml. surface of a plane mirror, and d c
the direction of a ray incidient upon
it at the point c ; draw the normal
to that point, c p, and o b, forming
the angle p c s, equal to the angle
PCD, then will the ray be reflected
in the direction ce: pcd being
the angle of incidence, and p c ■
that of reflection. If, instead of the
ray being incident on a plane, it
had encountered a curved surface, it would have obeyed tne same
law, being reflected from a point in the surface, as from the plane,
which is a tangent to the curve at that point. Thus, if the nj
D c were incident upon tbe concave surface, a 6, or the convex one,
A.' b', it would still DO reflected from c in the same manner as if
it were incident upon a c b, a tangent to either curve at c. The
lines, D c, p c, and e c, or the directions of the incident and re-
flected rays, and the normal to the point of incidence, will always
be in the same plane.
1034. A considerable proportion of the luminous undulations are
absorbed or transmitted on impinging upon the reflecting sur&ces
of transparent bodies, consequently the intensity of the reflected
is never equal to that of the incident ray ; this loss increases as
LAWS OF SEFLBCTIOH.
585
the obliquity of incidence is diminished. M. Bou^r has pven
the following table of the number of rays reflected at difierent
anzles from the surfaces of water, and of glass, the number of
incident rajrs being supposed to be 1000 : —
•
Incidence.
85°
80
75
Water.
Glau .
Incidence.
Water.
Gkn.
501
333
211
549
412
299
40°
20
10
22
18
18
34
25
25
Even when reflected from the surfaces of the most perfectly
polished metallic mirrorSj much light is lost ; thus, from the surface
of mercury at an angle of incidence of 78" 5', only 754 rays out
of 1000 are reflecte£ When the reflector is diaphanous, as a
elass plate, more li^ht is reflected from the second than from the
first surface, and this proportion is increased by coating the back
with some resinous cement, or still better, with metallic amalgam ;
the vividity of the reflection from the second surface then com-
pletely echpses that frt>m the first: thus, in the common looking-
glass, the bright images seen in it are reflections from the second
or coated sunace.
1035. Any substance pomessing some regular form, and suffi-
ciently polished to reflect light, is termed a tpecvlum or mirror.
These are made of various materials, as of polished metal, or of
glass, covered at the back with an amalgam of tin, or with silver
precipitated in a metallic state from a solution. Mirrors are made
in various forms, of which, the plane consists of a level reflecting
surface ; the concave presents a hollow surface like the inside,
and the convex, a projecting superficies like the exterior of a
watch-glass. Besides these, mirrors have been constructed in the
form of the curves called conic sections, the parabola, ellipse, and
hyperbola.
1036. Bays of light incident upon the surface of a plane, mirror,
as a looking-glass, always retain their relative rectilinear direc-
tions after reflection. Let ab, Fig. 562, be the surface of a plane
polished mirror, and
cxj i>y, be parallel
rays incident upon its
snrface, thev will be re-
flected in tne direction
y df accord
Fig. 682.
xc, or
ing to the law already
mentioned (1033). Di-
verging rays proceed-
ing from E will, after
incidence, continue to diverge in the directions e, e, 6, and converg-
ing rays, as f, f, f, will continue to converge after being reflected
Tram A B towanla ihs point a : the pninta orronverp>ncii orAirr-
gence of tlie incidcDt ind reOecleil ravs lieiiig at equal and oft-
]>oait« distancea frum the mirror. In all thcM caae*. u obJKU
itpp^ar to ihe eye Ui be aitualo in the ilireotiMi of (lie tavi wUch
ovedtiially reach that organ, to npeclaton placed ai e o^ e < e, atid
O, the rajTB C n, i, and err, vill appear lo haie come frooi bt-
hind the mirror, ad, in the direction of the ilotled linea, c', d'; %' ;
f", r', f".
tU37. Ab all bodies became, nndrr certain cin:uiiiitaiic«c, lbs
BOurce of luminouB tindulatioua proci'eiling from erer/ pnint of
llieir Burfftce, it follows, that anj object [Isced at en. Fig. 3^3,
will appear to a spectator at ed, to be in the direction etf, df',
as the ra^B eTolred from the object will, afler reSectioD from a b.
proceed in the iJireclione xe, yd, and conwtjuentl^ appear to tbe
obnerror to haro been given otT from some object aitnatcd at c*, r>',
us far behind A a ai c ii is before it. ThiB repnaeDtalion of ihr
object BO vividly prpsenled to the eje ia tenued an imapr, and
precisely reiembleB the real object, to which it owea ila origia.
103S. When two plane miiron are placed parallel to each other.
and any object ia utuated between them, a long aeriea of imagei
n-lll be Been in each mirror, from the object and ita image in one
being reflected by the other, and so on, nntil then figurea appear
BO remote aB to become inrisible. If the two reflecting maifaeri
be inclined towards eacb other at any angle, the imagea of an oK
ject placed between them will appear to lie in the cireiimfeniKv
of a circle of which the mirrors reproent the radii. Tlia i* tlie
S-inciple of the well-known kaleiduscope inrented by Sir DaTid
rewster: in Ibis elegiknt iostrumsnt, the imagei of Ike otgcctt
placed between the reBectura ara seen most beautifully airanf^
when the latter form an angle which is an aliquot pait et 180' ;
the number of images formed, including the object, will tben be
equal to 360° divided by that angle. Thiu, if tlie angle betwere
the mirrors be 60°, the images of the object will appear arrangrd
in a circle, and a heiagonal figure will be produced.
1039. Wheu an assemblage of rayi called a pencil of licht hit*
j^^ B83_ on the surface of a mrred airrcr,
_ each lay of ibe pencil ia relet ltd
I from the point vf the miner ea
I I whichitfallB,predaeIjaaitwnaU
I have been from a tuig«nt pUnr.
I or flat sni&ce looching tb*
I mirror at that point (913)1 Ltl
" "' diTcrging fhiBi
563, he incidFEt
pberical anibn
, A B, of which o i* tW
t c^ the centre of the carratBra of
■, which ia called Uie axit of the
t pencil of rays diTcrging fhiai
■be point p. Fig. 563, he ini' -
in the concave apberical ai
FOCUS OF A REFLECTED PENCIL. 587
pencil. The line o c joining the centre of the mirror and its
centre of curvature is called the cucis of the mirror ; and when,
a8 in this case, the axis of the incident pencil coincides with
the axis of the mirror, the incident is said to he direct^ other-
wise it is called oblique. Join a p, a c, and draw a q such, that
the angle caq^scap; then, since ao is perpendicular to the
mim)r at a, a q is the reflected ray corresponding to that incident
on A from p. Take any other point, d near to o, and find as before
the reflected ray d/; also take o p = } o c. Wherever d may be,
between a and o, / is always nearer to c than q, and when d is
indefinitely near to o, then / is called the GeometriccU focus of
the pencil.
Let p o=v, co=r, and/o«v; then, because the angles p a o,
G A Q are equal (End., vi. 3), p a : a q : : p c : c q ;
and, ultimately, when d moves up to o, p o : o/: : p c : o/,
or u:t>: :»— r:r— 1?, whence tt(r—»)«»» (« — r) ;
dividing this equation by ut7r, we obtain
= , whence — + - = — ; [a]
from which v may be found, when u and r are given.
The same expression will be obtained if / be the focus of inci-
dent, and p the geometrical focus of the reflected rays, conse<^uently
the points p,/, are reciprocal, and are called conjugate foci.
1040. When the point p is removed to an infinite distance from
o, or in other words, when the incident rays are parallel, then
— =0, and [a] becomes — = — , or t? = — - :
in this case, the point /'becomes the principal focus^ r ; and o f,
which may be called /, is the focal ^ ^^
length of the mirror. Thus a pencil of ^- *^
parallel rays, d, «,/ ^, A, Fig. 664, will,
after reflection, converge to the prin-
cipal focus, f, midway between o and
■ : and, conversely, a diverging pencil
incident on the mirror, from f, will,
after reflection, form a parallel pencil
d efg h,
12..
Since -t= — , substituting this value in [a] we obtain
U V f
that 18, ike 9um of the reciprocals of any pair of conmgaie focal
distances is equal to the reciproad of the focal length of a
mirror.
It is obyious that as the luminous undulations constituting the
T*^ d, t,f, g, \, will be raflected lowtrds r, Bad, kniriBS si tkat
point will c&nae >d accmniiUtian of motion correspoodiiiK to tbe
united force of bU the nndnlationi propagated from tbe Rflectne
■nrface. On tbii Bocount bII the light uid heat belonging to Ibf
incident rsji will become concenlnlcd at p, and InmiaoeB and
oalorific eSecla of oorreapoDding intenntj will be eiched ea bb*
bod; placed at that ptnnt ; thii point wbb hence oTiginaDr tuiaid
the foeui, at fatflaee of Ibe minor, a e b, for the puBlkl aoUr
raye.
IMI. If the radiBOt point be placed neanr the m ' — -"■ ^
orincipai fonu, the raja will be reflected, not pua
feni, Bs tbough thej were eiolred from Kune point pUoe4 be-
ind the mirror : and, oonveraely, ' ' '
principal fonu, the raja will be reflected, not pualU bat ^m-
s thoUL' ■' _ . . - . ^ ■--_.. ...
sraelj, when oDtnvTRing ts^ii ■
cident on a concave minor, Ihej will be reBccted to a
(Wirer tbe mirror than the pnnoipal fbcni, r; the rerwae
eequentlj of divetsing nj«. These raja mnat be aaiiiinii
converge towards aoms point aitoated behind the mb
tbej an acted open in a manner oppoaite to that in which tkej
were bj a codcbts reOectiiiR Bsr-
'*■'•*- face ; for whilit a coocave rdtector
•i^r leMsoi the diTergencj, and iu-
/ I' creBee* tbe conTeigencj, of bH
ji y „-^ incident reje, a convei ooe in-
.■■\i ~^ " cTOBMi their dirergencj. Bad di-
" A tniniahea their connrgencj. TliBa,
"slreyi, a,i,e,d, a, be iB-
— e if parallel n
_rf cident on th
^^
of which c ii the eentie of tnm-
tnre, the; will be reOected accrad-
ingto the general law (1033), in
the direction! r a" r f , r ^, r c',
•« if tfaej had proceeded from s point, r, placed behind tfae mimr.
which thni becomee the wrtaal, or m^w focai of the icBected
_ nm. Tbe/oeoldutneee,/, Rr
'^■'^ p^mOlel IB j» U ooe half of 4e
and r ii alwaji ntnatad b^iad
fledor, p ia'belrae it (1039V la tk
caee of diTerging nj*, tbe focal
distance will be leaa, and far c»-
Terging rare greater, thu/.
1043. When a pencil of raj* i>
incident npon a cnmd rcAectiv.
thej, after reflection, mntnallj interaeet each other, and tbiK
pointa of interaectiDn constitute a cnrred line, tetnted b tmmtic.
FORMATION OF I1CA6B8.
689
^.667.
Let the rap p a, p b, &c., be incident from p on the mirror a o, and
let A a, B 0, &c.| be the corresponding reflected rays ; it is evident
to the eve that the reflected rays are accumulated in the neigh-
bourhooo of the curved line, or caustic, in which their socces-
sive intersections take place, which locality will therefore be
more highly illuminated than the ad-
jacent parts. To exhibit this caustic
curve by reflection, nearly fill a glass
tumbler with milk, or fit . a circular
piece of card into it about half an inch
from the top, and, exposing the con-
cavitT of the glaiis to the sun or a
candle, then a brilliant double curve,
represented in Fig. 567, wiU be seen
on the surface of the milk, or paper.
A rim about an inch wide, cut from
a cvlindrical glass shade six or eight
incnes in diameter, silvered on the outside and laid on a sheet of
white paper, answers exceedingly well for this experiment
1044. Images are formed by spherical mirrors in the same man-
ner as by plane ones (1037), but difler from those produced by the
latter instruments, in being of a different size Irom the object.
Thus, if rays be supposed to emanate from a distant body, they
will, on being incident on the concave mirror, a b, Fig. 668, of
which c is the centre of curvature,
be reflected to a focus at f, a little
beyond the principal focus (1040),
and there produce an image of the
object E D, diminished in size, and
inverted in position, because the
axis of the pencil of rays from e
being above the axis of the mirror,
the axis of the reflected pencil
will lie below it, and vice versa
with regard to the rays from d. The image p will be extremely
Tjvid from its being illuminated by all ihe luminous rays in-
cident from this object on the mirror. The magnitude of the
image p will be found to bear the same relation toED as the dis-
tance of p from the mirror does to that of the object from it. If
an object be placed at p, its image will be depicted on a screen
placed at e d, diflused over a large space, and consequently mag-
nified.
1045. In the case of convex mirrors, the images are in an erect
position, much diminished in size, and behind the reflecting sur-
face as in the plane mirrors. For if an object d b. Fig. 669, bo
phiced before a convex mirror a b, of which the negative focus
18 at F, the luminous rays will, after incidence on a b, be reflected
diverging ; and being seen by a spectator at h, they will appear
Fiff. 668.
inwpnKAe^gfram>nobi«et, dc,behiDd the mirror, and ooc-
'ablysiiullertbuiDB,of whicbit IB me:«]jft dimiaiHlml ima^.
The ini«ge !n spheric*! mimn u
^- ^- always distorted ; that of a itnii^i boe
beiDg one of the conic Kctiona. Jepead-
■ iog on iti distance from the mimr.*
1046. Tbea6erra(io«oratiyt«aect(d
is the d
tncal foctu of tlie pcDcil, and the piiiit
wbere the reflected ray interaecta the
aiisi thus in Fig. 563, q f ik the aber-
ratiim of Ibe reSecled lay, a q : and the
o&crraltoH of a pencQ <k light U the
aberration of Ihe eitreme raj in aoT
■ecljon of Ihe penri! paasing throng
'' ' " B, be the eitieme poinli
of the
r, then
e Uarar aperltire, and ibe
ntiglc siihtcnded at c bv a b, the angtdar aperture of the mimv.
It we proceed, ns in 1039, to find the actual value ofqo, K-e oUain
nn expression similar to [a\. but with the additioa of a term io-
volving the aqnars of the aperture, which expreans the diAtcBcr
belnean o r and o Q, or the aberration of the mirror ^ hence it
appears that the aberration of a ipheriad wurror it proportiimal
to the ttpiare of the aperttire,
104?. Let o F, Fig. 670, be the v
«y.ITO.
%the e
I two rays in the hhw plane.
meeting Ihe aiii iita: pni.
duce A r to cut a ■ La t, and
draw t n perpc odicnlar (» o F.
At the poiDl K Tecedea ftecD
o, the point of intentctiu m'
II with Ar pmdoecd. wiU
adyance from r towards t, until 0K = 4olli bejond which the in-
tersection will graduallr recede to r; aa therefora kr •sd ■« an
the extreme rajs, t, their paint of iiiteraectioa is the unaraat poiDl
lo O P, within which both paes, and a circle of which the radma u
til will be the soiBllest area through which all the tays paa: it
ix hence called the Uatt eirde of cAtrratum. The Taioe of the
radius is found to I'C
* Coddinglon'iOpt'ri, pj
t aciBa-a OjHia, pp. j^».
h>ie hitherto contidered oolj direct reflection (1039),
^flection be ohllqoe, the axis of the pencil not coin-
that of the mirror, the condition of the reflection ar«
ifferaot from those alrevir deturmined; there is no
cat point, bnt, in place of it, tva/o«oI iinet separated
iterral, and at tight aogles to each other, to which
rajB cODTerge.
> eipUin the fonnation oT these Tocal IIdsb, letoQ,
the axia of a pencil iacidrnt directly on a spherical
face. If this pencil be made ap of n series oi^ conical
aja having 04 as their commDn axis, sioce all the
1 surCice vill be similarlj reflected about OQ, the
f the re- ^
i-ill rorm a
Dical ■
7,a,ii
uca the
1:0 (1043).' If, instead of taking the whole pencil, we
. portion ofit on]]' which is incident on an aanulos of
Z sat-fBce that would be generated h; the revolntioD
o Q, we must have, cortesponding to this, an annulot
ic snKace, tlirough >ame point of which each ray of
hell ofllie reflected rafs passeii. Ifuc lie small, this
la oauatio surface may be considered a circle, q^ t, in
«ndiciilar tooq, and to the plane of the paper.
if the entire conical shell of light, if we now consider
portioD of it, adjacent to H >, ne arrive at the case
liqoje reflected pencil, of which the axis is a o, g,; and
ra; will pass through SDme point in a Email circular
. to q,, which maj be considered approximately a
perpendicnlar to the plane of the paper ; this lice is
imary focal iine, am) the paint j, the primary foeKt.
:tion of the reflected pencil by a phme tbroiigh g,
plane touching the SDriace of the mirror at x, though
:ry elongaled li^nre of 6, resembling a. Fig, 6T1, may
ai a straight line, and is called the lecondary focal
le point, q^ in which the axis of the reSecU^d peneil
alkd the leconJiiru fociu : also the plane <)oa, or the
Faper, is calle.l the primari) plane. Hence a small
after reflection conTergea lo, or diTorgas Iroiu, two
al lines, one of which lies in the primary plane, and
perpendicular to it.
593
Whan tho abeimtion of ddErect pi ._ , .
potitivt, the primir; tocat of a small olitique pancil, froni the ume
origEn, ii neaivr io tha iiarfiice than iha Bocondarj focus, as in
Fig. 571 ; bnt the contrary, vrhen from the suriaca, or lugativt.
1049. If a aectioD of the reflected psDcil be mnd« b; a plan*
parallel to thn tangent plane to the reflecting sorface at a, that
■action haa been explained to be a straight line perpendicular to
the primary plana, when the plana paaaea through q^. If lbs
CDe be new Boppoeed to move parallel to itnlf from q, to q^ tbs
adth or the Heclion gradually increases in the primary plane,
and decrvaaea in the perpeodicular direction, until at g, the sec-
tion becomes a straight Ime in tho primary plane : at aome point,
therefore, between q, and q„ the two ditnenBions of the section are
eqnal, and the section very nearly circular ; this particular section
of the reflected pencil ii called tho circU of leiat eon/uuxi, and
exhibits tho amsUent spare through which all the rays of the
pencil pass. It is fo called, because, when an im^e of an object
IS formed by reflection from a spherical surftce (1IM4, 6), the
reSscled pencils from two adjacent points of the object will at
that point urerlap each other aa little aa poaaible, and will them-
fore create the least posaible amount ofcoufuaion in the image.
1050. A glance at Fip. 566 will readily point ontthat there is a
very largo amount of aberration in concave spherical mimra of
large angular aperture; hence, when the amount of light required
to be reflected is considerable, and consequently a lai^ apertara
IS indispensable, it becomes necessary to alter the eamture, or
figure, as it is termed, of the mirror, in order to diminiah the
aberration. It is a wellknown property o( the conic acctiDns,
that lines ioining the foci and any point of the curre, make eqnal
angles with the tangent at that point: consequently, by the law
of reflection ^1033^, if the surraca of a mirror be farmed by tha
reTolution ofa conic section about its axis, a pencil of raya inci-
dent upon its surface from either focus, will, after reflection, ooo-
verge to, or diverge from, the other focus, aa the case may be.
It follows from (lie ordinary pruperlics of ihe curve, that if tba
form of tho mirror be a parabola, as p. Fig. 572, a pencil of parallel
rays, a, b, e, &c., will converge to the focua y, aud coiktarsely, a
OOBBEOnOV OF SPHERICAL 0UBYB8. 593
pencil incident from p will be reflected parallel. Mirrors of this
form are made use of in reflecting telescopes, and in lightlionses,
in the former the incident^ and in the latter the reflected, is the
parallel pencil.
If a large pencil of diverging rays be required to converee accn-
rately to a focus, an elliptic mirror, b, is requisite, in which all
the rays of a pencil incident from either focus on the mirror will
be reflected to the other focus. Elliptic mirrors have been made
nse of by Prof. Amici and others, in the construction of compound
microscopes, and by the writer in his self-registering magnetic
apparatus (624).
If a^ hyperbolic figure, H, be given to the mirror, a pencil of
rays, diverging from f, and incident upon the conyez surface of
the branch a, or upon the concave surmce of 6, will, after reflec-
tion, divei^e from Fy A concave hyperbolic mirror is usefiil for
accumulating light m a divergent pencil, for the pencil diverging
after reflection from 6, evidently contains the light due to a much
larger angle, than if it had proceeded direct^ from the same
source placed at f^ ; always excepting the amount of light lost by
reflection n034).
1051. It either of these curves be compared with the circle of
curvature at the centre of the surface, o, it will be found that the
curve lies without the circle, except just at the point of contact ;
hence a spherical mirror requires to oe flattened out towards its
periphery, or else to be deepened towards its centre, in order to
dimmish its aberration. This, in the construction of specula for
optical instruments, is in practice accomplished bj a method
purely tentative : namely, in grinding a speculum, which is usually
composed of an exceedingly hard, brittle, and colourless alloy of
copper and tin in atomic proportions, a lateral and circular motion
of the hand are so combined as to abrade most at the centre, or
at the peripbeiy, at the will of the operator. In grinding and
polishine the specula for telescopes oi large size, such as those
of the Larl of Kosse, Mr. Lasseli, and others, these movements
are mechanically accomplished by a due acyustment of circular
and eccentric motions. In order to giye an idea of the extreme
accuracy requisite in the curvature of these specula, it may be
stated that m a speculum large enough for a five or six feet tele-
scope, the definition of the image may be sensibly altered by the
polisher in a few minutes, the quantity ot metal removed from the
whole surface during that time not Ming an appreciable fraction
of a grain. Accormng to Sir J. Herschel the utmost variation
of a speculum, of four feet focus, from a spherical curve is less
than -00005 inch.
1 052. A few years ago it was proposed by M. L. Foucault to con-
struct the specula for telescopes of glass, and to render the curved
surface reflective, bj^ depositing on it from a solution a coatinff^ of
metallic silver ; which, when perfecUy dry^ obtains a high polish
594 DIOPTRICS.
by a little gentle friction with clean wash-leather and finely levi-
gated rouge. If the reflecting surface be tarnished or otherwise
damaged, it may very easily be renewed without any xisk of
altering the curvature of any part of the mirror.
1053. The process of silvenng ^lass specula is as follows : —
P^pare three solutions, A, B, and G ;
A. Ciystals of nitrate of silver ... 90 grains,
4 fluid ounces ;
1 ounce,
25 fluid ounces ;
4 an ounce,
5 fluid ounces.
Distilled water
B. Potassa, very pure . .
Distilled water . . .
G. Sugar of milk (powdered)
Distilled water . . .
The solution G must slways be fresh.
To prepare a quantity sufficient for sUyering an 8-indi
speculum : —
Four 2 ounces of A into a ^lass vessel capable of holding 36
ounces. Add drop by drop (stirring constantly with a glaas rod)
as much liquid ammonia as is just sufficient to dissolve the grej
precipitate first thrown down : then add 4 ounces of B. The
orown-black precipitate now formed must be just re-dissolved
by the gradual addition, as before, of liauid ammonia. Add dis-
tilled water to make up 15 ounces, and tnen drop by drop a little
of A, until a grey precipitate, which is not rehdissolved by
stirring for three minutes, is obtained ; then add 15 ounces move
of distilled water, and allow the precipitate to subside.
When the mirror is ready for immersion, add to the above pre-
pared solution 2 ounces of G, and mix them thoroughly wiui a
glass rod.
The glass speculum must be cemented by pitch to a drcular
block of wood attached to a ring by tiiree eaual strinss. The
adhesion of the ^lass to the warmed surface of the pitcn will be
promoted by moistening the glass with oil of tuipentine. The
speculum may^ then be suspended face downwards in the fluid.
Before immersion the surface should be thoroughly cleansed wiUi
strong nitric acid, by means of a smiJl pad of cotton wool tightly
inserted in the end of a bit of glass tube ; it should then be
rinsed in common water, and finally with distilled water, and
then placed in a shallow vessel of alcohol, while the silvering
solution is being prepared.
The process of silvering will be completed in from fifty to seventy
minutes according to the temperature of the atmosphere. When
the speculum is removed from the solution, it must be immediaUijf
rinsed in plenty of water (as by letting a tap run over it), then
with a little distilled water; and lastly, placed in an oblique
position to drain off the water, and left to dry. When, dr^, it
may be polished by a piece of very soft wash-leather and a httle
fine rouge, with a circular motion progressing spirally firom the
circumference to the centre.
THB LAW OP SINES.
595
1054. So long aa a ray of light traverses a imiform medium, it
continues its rectilinear path, which it also preserves when it is
incident on a homogeneous diaphanous substance in a direction
perpendicular to its surface. But if the ray be incident in an
oblique direction, it is then more or less bent, or refracted^ out of
its original course : this bendine, or refracticm, not being the
same in eveiy substance, as is the direction of reflection (1033),
but varyine considerably in different kinds of matter. Let a b,
Fi^. 573, DO the surface of a
reflecting medium, as water, of Jfj^.BTO.
greater density than another
medium, as air, above it ; draw
c B D peipendicular to a b, and
let PE be a ray incident on ab
at b; a certain portion will be re-
flected, the remainder will enter
the medium, but instead of fol-
lowing its original direction,
B o, will be^ refracted or bent to-
wards ed in the direction bk.
The line pb will, therefore,
represent the incident, and ex,
the refracted ray ; f e o will be
the angle ofincidencef and d b k
the angle of refraction. Take Bf=Khj and draw the lines /c,
h d, perpendicular to o d ; the former will be the sine of the
angle p b c, and the latter that of the angle d e k ; the law of
retraction, or, as it is usually called, the law of HneSy is that the
sines of the angles of inddmce and refraction loiU always he to
each other in a constant ratio for each refracting stthstance : and
it is^ another law of ordinary refraction (bo called, in contradis-
tinction to extraordinary remiction, which will be subsequently
considered), that the refracted ray always lies in the same plane
with the incident ray, and a normal to the common surface of the
two media, at the point of incidence : the incident and refracted
rays are moreover always on the opposite sides of the normal c d,
and of the sur&ce a b.
When^ a ray is incident on a refracting surface, bounded by
curved lines, the same law obtains as when incident on a plane.
For if A B were replaced by a concave or convex surface, as a 6,
or of b'f the ray p b will follow the same course as if it impinged
on the plane which is a tangent to the curve at the point of
incidence.
1055. As the visibility of any two points is mutual, it follows
that a ray of light, x e, passing from a denser refracting medium,
ADB, as water, will, on reaching the surface, ab, of a rarer
medium, as air, be refracted in the direction e p. In this case, as
s B is the incident, and f b the refracted ray, the line/c, which is
QQ 2
596
now tk« lina of Tefraction, ii ipeater thin the line dk,or troe of
incidence, the reverse of the former om ; alee the rsj ■ r is r»-
frtciedfrom tha normal or pet^ndiculat c c, in place of torarda
it, aa in the fbrmer case.
1066. The relative poBitions of the incident and refracted rsyi
may bo convenientlj iCustraled bv means of a moveaUe diagram.
On a board. Fig. 574, take any three points, a, b, b, in the sane
strught line, and to tbewpointt
"S- O'*- Mladi moteably the eqoal rodi
I A a, b/, ■ p, ■ ■, uid let /b be
prolonged to b, ao that /a : b6
may be the repaired ratio of
the sioea of incidence and re-
fraction. Connect a K, rf,hj
links (154) respectively eqiul
li) 1 K, E B, and connect a b, by
a link, when a a and n h are
both vertical. Then it is evi-
dent from the cons true tioa, that the distances of b and / from a
vertical throngh b, are in the given ratio, and are in opposite
directiors ; bnt fe is parallel to /b, and ei to ao, and the di>-
lancei of a and b fmm verticals through A and B are approxi-
mately equal, coowqucntly re and Kt, the distances of w and i
from a vertical through b, will bo veiy nearly in the required mtio^
that is, in all positions, rx, be, will represent corresponding in-
cident and refracted rays. Let the hoara be now covered with a
sheet of paper, on which draw LorJEontal and vertical lines thronah
the point E, and a circle with centre b and rsdios e f. Let the
pap^becut ihrougUin the vertical line, CD, so that F/and a%
may past through the slil, and EP,Fe, eh, tk, may appear in
front of the paper, the remainder of the links being unseen, as
ivpresanted by [he dotted lines.
1057. Let the my p b be lupposed to pass tlirongh a highly
attenuated attnoapheiv, and let the ratio of /e: iJit, Fig. 57^ b«
represented by fi, that is,
fc-.d
■■3 H a called tl
then the quantity fi |g ciUled tUv injtx of refraetiim, or rtfraetiee
potetr, of the medium in which e e is the direction of the cor-
responding refracted ray. The index of refraction varies oon-
siderably in difierent media, being for chromate of lead 2 974, and
for air 1000294, between which limits varions intennediale de-
groosof refraction exist. It was ascertained by Newton, that in-
flammable bodies in general possessed a higher refractive power
than other substances ; on which account he made the bold aog-
gestion, that the diamond, of which the re&sclive index is aboat
2-439, consisted of a combustible substance {" qni ut probabile
est, substantia est unctuoaa, coagulata ;"*) a statement of which
* KcwtoD. Optin, liT* da i«ll«ioii^i, te.. Inns, III). 11. pan S. Lat.
red. S. darlia, LosdoD, 1718.
INDICES OF BEFBACTIOH.
597
the correctness has heen amifly demonstrated bj the diBOOTery of
the true chemical nature of the diamond. As a general law, the
greater the specific gravity of a body, the more it refracts light
passing through it ; to which the chief exception is found in
the case of inflammable bodies pointed out by Newton : and if
the ratio of /t* — 1 to the specific gravity of the body be taken to
renresent its ab»olute refractive power^^ this class of substances
will be found to possess a greater ahiolfUe refracting power than
any other bodies. In the following table the indices of refrac-
tion of severed substances, when a ray is incident upon them
from a highly rarified medium, are contrasted with their absolute
refractive powers : —
Sotetaaoes.
Taenum . .
Hjdrogen . .
Oxjieii ...
Common air .
Kiirogen . .
Ammonia . .
OarbonioMid .
Chlorine . .
Tibasheer . .
IC9 • • • •
Wat«r . . .
Bthor (anlphc.)
Alcohol . . .
Hjdrochlorio )
aoid . . /
Nitrle acid . .
Sulphariti acid
Flaoripar . .
Alum. . . .
Olive oU . .
Index
AbBolnte'
of
refract. >
reir.
pow*r.
1-000000
0-0000
1000138
8-0063
1-00027S
0*3790
1-000294
0-4628
I'CiOOSOO
0^4734
1*000386
0*4734
1*000140
0-4537
1000772
0*4813
1*111
?
1309
P
1*336
0-7846
1*358
2-56IH)
1*872
1*0121
1*410
0*6614
1*410
0*0240
1*434
0*6124
1*434
0*3414
1*4«7
0-6&70
1*470
1-2607
Index
Absolute
Sabitancea.
of
refract.
refr.
power.
Oil tarpentine
1-476
1*3510
Cantor oil . .
1*490
1*1480
Oil of doves .
1-536
l-3('90
Crown glaas .
to .
1-526)
1-534 f
0-6260
Plate glaas . .
1*614)
1*632 f
p
to .
Amber • . .
1*547
1*3664
QnarU . . .
Flint glaas . .
to .
1*646
0-6416
1*585)
i*e«oj
0*7086
Oilofoaaaia .
1*641
1-7634
Bisulphide ofl
carl>on . j
1*768
1*4200
Sapphire . .
1-794
0*5666
Garnet . . .
1*816
0-6423
Zircon . . .
1*061
0*0064
Sulphur . . .
2*143
2*20(10
Phosphorus
2*224
2-8867
Diamond . .
2-430
1*4566
On looking at this table, it will be found that the absolute re-
fractive power of hydroeen exceeds that of all other bodies, in
consequence of its Terr low specific gravity. These absolute re-
fractive powers are calculatea on the supposition of the ultimate
particles of all bodies being equally heavy.
1058. When the refractive power of any medium, on any
ray entering it from a very attenuated medium, is required,
it will be found from the above table; but when the di-
rection of a ray passing from one medium to another is sought
for, it may be found by dividing the index of refraction of die
second medium by that of the first, and the quotient will give
the ratio of the sine of incidence in the first medium to that of
refraction in the second, or the index of refraction of a ray pais-
* Ibid., prop. 10.
598 DIOPTRICS.
ing from the former medium to the latter. Thus, if the index of
re&action for a ray passing from water into plate-glass were re-
quired, the index of reiraction of the former being 1*336, and of
tue latter 1*642, it is found bj dividiu^ the latter by the former
number, or Ulii = ri54, the required index of refraction.
It appears from the experiments of the Rev. T. P. Dale and
Dr. Gladslone* that the refractive index is in all substances
found to be diminished with increase of temperature.
1059. Luminous undulations are propagated through media with
a velocity varying with their refractive power ; the higher the
refractive power of the medium, the slower the ray of light moves
through it, the velocities through any two media bein^ in the in-
verse ratio of the sines of refniction ; consequently, if during a
given time a series of luminous undulations are propagated
through a tube filled with air, of 100 feet iu length, a similar
series in the same period of time will traverse but 75 feet, when
the tube contains water. ^
1060. From an inspection of the moveable diagram fie. 574,
▲ Bc being a rarer, and abd a denser medium, we see that fo,
the sine of the incident, is always greater than x d^ the sine of
the refracted ray ; and if the ray p e be made incident at so great
an obliquity that its sine would nearly correspond to radins, and,
consequently, that the luminous ray could only graze the surface
of the medium a e b d. still a considerable portion of the light
would reaUy enter and be refracted. The converse of this propo-
sition is extremely remarkable: for if ke be a ray paasing
through the dense medium, a db, into a rare one, acb, the sine
of retraction will exceed that of incidence (1055). When ke is
incident on a b at a greater angle than that at which the sine of
the refracted ray would be equal to radius, the refraction of the
ray becomes impossible, and instead of entering the rarer medium,
it is reflected back again from the internal surface of the denser,
in obedience to the ordinary law of reflection (1033). This sudden
substitution of reflection for refraction is consistent with analysis,
and afibrds the only instance of totcU reflection with which we
are acquainted ; for if the ray be incident in a dense medium on
the suri'ace of a rarer one at a sufficient obliquity, it is totally re-
flected, no light bein^ lost, except from a few undulations lieing
absorbed bpr the medium itself. The angle of incidence at and
beyond which this internal reflection occurs, is termed the limit-
ing angle between refraction and reflection. This limiting an(de
may be found by dividing unity by the index of refraction of uie
substance ; and on looking for the quotient in a table of natural
sines, the angle corresponding to it is the limiting angle. Thus, a
ray cannot pass from water into a rare medium if the angle of inci-
dence exceed 43** 27*, for , ^^,. =8ine of that angle: nor can a
• Phil. Trans. 1858.
UVUBUAL BEFKACnOX. 699
ray pus from flint glass into tlie same, if the angle exceed
88^ 41', for JL » 0*625, the sine of that angle. The briUiancj of
the light thus reflected far exceeds that reflected from the best
metaluc mirrors. This may he readily shown by nearly filling a
wine-glass with water, and holding it^ np, so that the sorface of
the fluid may be seen from beneath : it wiU appear like a sheet
of bamished silyer, from the perfect reflection of the incident
light, and no object held above it will be yisible if the position of
the eye be beyond the limiting anele.
1061. When an object is yiewed throogh two or more strata of
different refractive powers, very curions results follow. This may
be often observed when an object, situated at or near the horizon,
is so far distant, that, in consequence of the curvature of the
earth, a right line could not connect it with the eye of the spec-
tator: it will be invinble except under some remarkable states
of the atmosphere, giving rise to the phenomena of unuautd re-
fracHon, For tiie production of these e£kcts, it is necessary that
the strata of atmosphere near the earth should differ considerably
in refractive power, either by one portion being more loaded with
vapours, or possessing a lower temperature than the other ; so
that, by the great degree of refraction to which some rays passing
from the distant object are submitted, they reach the eye in curved
lines, and the spectator sees an image of the object in the air, in
the direction of a tangent to these curved lines ; other rays from
the olject may be reflected at the common surface of two strata
of unequal density, and produce an inverted image.
Phenomena of this xind, constituting the mirage, or fata
moraana of the Italians, are occasionally seen in great splendour
in the Straits of Messina. Lithe north of £urope, and in several
parts of Great Britain, the mirage has been frequently observed,
and is by no means of very rare occurrence on the English coast,
in the evenings of hot autumnal days.
Borne of the conditions for the production of the mirage may be
obflnred by regarding a' small object through ^ ...
the point of mixture of two fluids of different '^'
densities, as syrup or alcohol, and water, when
images will be seen on a plane higher, and
in an inverted direction, with regard to the
original object. The same effect may be ob-
aerved by looking at an object across a red-
hot iron, as in fig. 675, or over a charcoal
chauffer ; or still better, on a cool day, by re-
garding a distant wall, or tree, over the lioiler
of a steam-carriase : tne wall or tree will ap-
pear to be divided into several portions, and
surmounted by inverted images visible for a considerable space
above the source of heat.
600
DIOPTRICS.
1062. The transition from partial to total reflection may be
beautifully seen in an experiment described by Newton.* Hold
an equiangular prism, in the
^•*^*' position shown m Fig. 676,
^^ before an open window, m such
^S^^ ^x^*N^ a manner that a line drawn
Nw A.y^^ ^\w» froni the eye may describe an
angle of about 40' with the
base of the prism. The base
A B D o, Fig. 576, will appear
to be traversed by a curvod
iris, YT, of a bluish yiolet
coloiy, the space between t t
and A c appearing of a sombie
hue, in which reflection is ex-
-^j^ treraely imperfect; but beyond
y T incloding the space y B D T,
the whole will appear shining with a metallic splendour, the clouds
and surrounding objects bein^ depicted upon it with great bril-
liancy. The ins y y thus divides the space between partial mid
total reflection.
1063. If a ray of light be incident perpendicularly upon tiie
surface of a refracting medium, bounded by plane parallel sidesy
as a plate of glass, it will undergo no disturbance ; if in any
other direction, it will be Te>
^. 677. fracted according to the l&wi
already detailed. Thus, if a b.
Fig. 577, be incident on such
a medium as a plate of glaaSi
c n. it will undergo refraction,
and emerge on the opposite
side, in a direction paraUelto
the incident ray; oecause it
will be refracted exactly as
much from the normal, on
leaving the ghiss, as it was
towarae that line, on entering
i.^ • -J * ^1. .« i^ « **• If diverging rays, as EPo,
be incident, they will, after refraction, emeree fiom cD pandlel
to their former directions, their divergence being the same, but
taking place fix)m a point nearer to the glass than before ; and if
converpng rays, as k l m, be incident on c d, and converging to o.
they will, after emerging from the medium, really converee to p.
at a greater distance from the class than q.
1064. A Pritm is a wedge-shaped portion of a refracting ioe>
dium, having two plane surfaces meeting at an edge. Prisms an
usuaUy made of glass for optical purposes, with their sides At
* Opkioe, Mpra otfa<. lib. ii exp. 16, p. l».
REFRACnOH AT A BPHBRICAL SURFACE.
601
varioiis angles of inclination, arc, Fig. 578, represents one of
which the sides are inclined to each other at angles of 60* ; c a,
are termed the refracting sides, and a r, the hase, of the
CR
pnsm. If a ray of lieht, d e, he incident on the side c a, it will
Die refracted towards toe base if the prism be denser, and towards
its apex if rarer, than
'/.
the surrounding medium. ^'
Let the prism be of glass,
and draw gel perpen-
dicular to Ao; nie ray
DB, on entering the
prism, will be refracted
towards the perpendicu-
lar OBL, and conse-
quently towards its base,
A B|^ in a direction e f :
which at one particular
angle of incidence will be parallel to the base A r, as in the figure.
On emerging from the prism at f, the ray will be refracted from
the normal to the point f, in a direction fh, and will conse-
quently deviate still further from its original direction. Hence,
in riewing ohjects through a prism, they always appear to be
higher or lower than they really are : for, if an object be placed
at D, it will appear^ to a person stationed at h to be at d, be-
cause the ray h f, if produced, will reach d, and objecti attoays
appear to he tUuaied in the direction of the ray a which eventuauy
reach the we (1036).
1065. When refraction takes place at a curved surface, it is
said to be direct, as in reflection (1039), when the axis of the pencil
is a normal to the surface ; otherwise the refraction is said to be
oblique.
In order to find the geometrical focus of a pencil of rays after
direct refraction at a spherical surfiMe, let p, Fig. 579, be the
origin of a pencil of light m- j^ ^^
cident directly on a sphen-
cal refracting surface, or,
of which o is the centre,
and OB the radius; then
o p is the axis of the re-
fracted pencil. Let p r be
any ray incident on o r at
B and reflected in a direction which cuts o p in 0 ;
i reflection) be the geometrical focus, or the
when R is indefinitely near to o. Let opi^tf, op=v, oc=r,
lines being considered positive, when measured from o in a direc-
tion contrary to that of the incident pencil ; ft, the index of re-
fraction of the medium 0 R.
and let f (as
position of q
602 DIOFTBIOB.
Now ii=-t , by the definition (1067),
^sinpRC , BinBC^ ^^^^ ^^^ ^^^ Rc^are identical.
sinRCP Bin^BC
PC B g } because two sides of a triangle are aa the
"bp eg' I 8™" of the angles they subtend ;
but ultimately b p, b g, c g become o p, o f, c p respectiTely, and
therefore! in the limit,
PC op_tt— r V
'*""op cf"" u v—r
whence M (l - ^) = 1 - ^. or M (J- ^) ■= — i :
It 1 li— 1 r* t
and by transposing, ^ "" « " r~ * '•"^
which determines the position of f.
When the incident rays are parallel, o p is infinite, and conae-
quently - = o, and calling/the focal length, as in reflection (1040),
u
we obtain fix)m [a], /= — ~ r,
which eives the distance of the principal focus from o.
It will sometimes be found convenient to express the relation
of the foci of inddenoe and refraction in terms of their distances
from the centre of curvature of the refracting surface : for this
purpose let cp^p, and cw=q] lines being considered poriUve
that are measured from o in a direction contrary to that of Uie
incident pencil. We obtain, as before,
_po ^ OF_ p
'^~op 0F*~2?— r q '
and by proceeding as before we obtain
q p r
from which Hm dtetaaoe of f from o is detennined.
Hie ipheiical aberration of the extreme refracted ray may be
detennined by the same means as in reflection (1047), and may be
shown to be propoitional to the square of the aperture of tbe r^
fracting surface : and the last circle of aberration has the same
position and magnitude as in a reflected pencil (1048).
1066. The observations that have alraady been made (1049),
concerning oblique reflection at a spherical surfitoe, and the fonna-
tion and position of the two focal lines, will in all inspects ap^y,
mutatis tnutandUt to the case of oblique refraction at a sphen<^
sur&ce : and the same values may be obtained for the magnitude
FOBMS OF LBHSEB. 603
and position of the circle of least confagion (1049): it is situated
midway between the two focal lines when the angle of the pencil
iM smaU.
1067. Caustic carves are formed by the intersection of refracted
rays, in the same manner as in reflection (1043). They may be
seen by holding a glass sphere, or globe foil of water, near the
candle, and allowing the refracted rays to fall, after passing
throagh the sphere, on a sheet of paper held nearly parallel to the
horizontal axis of the sphere ; a luminous figure, hounded by two
sharp curres, will be obserred, meetiugatthe point corresponding
to the focus of the sphere. These curves may be more distinctly
seen by covering a cylindrical glass vessel with black paper to
within about an mch of the top ; pour water into this vessel, until
it rises half an inch above the level of the paper. Cut a piece of
white card, so that when placed at the level of the blacK paper,
and perpendicular to the axis of the vessel, it may half surround
the glasis ; then hold the latter up to the sun, or before a candle,
with the card away from the source of light. The luminous ravs
passing through tne water will be refracted to a focus on tne
card : and a triangular luminous figure, bounded by caustic curves,
will be depicted upon it.
The formation of a caustic curve bv one refraction may be ob-
served in the same cylindrical vessel, by rendering the contained
fluid very slightiy turbid by a few drops of milk : on allowing the
li^ht to enter two or three inches of the depth of the fluid, and
viewing it in a vertical direction, the caustic curve will be seen in
the flmd, the light bein^ reflected by the diffused milk-globules.
1068. Lenses for optical purposes are generally constructed of
glass, but certain transparent minerals, of various kinds, have
occasionally been used for this purpose. Sections of the principal
kinds of lenses are shown in Fig.
580 ; and, if these be supposed to FSg. 680.
reTolve round the axis a a, each
will describe the particular lens of ^ ^ ^ -J"^ J J^
which it is tiie section. /\ (\\7 \\\\ \\\\
The tpherical lens, c, is a simple \~/\Ji\\7/\ j/Jj
sphere, as its name implies ; the v £-i^ /J
double convex lene, d, is bounded
by two convex surfaces, concave towards each other; the double
eoncave, e, has both its surfaces concave, their convexities being
opposed to each other ; these two lenses may have both their sur-
faces of unequal, or of equal curvature. A plano-convex lens, f,
is merely half a double convex, one surface being plane, the other
curved, as in the latter. A plano-concavCf o, is a lens having one
gnrCace plane, and the other concave. The lens h, termed a
meniscus, has one surface concave, and the other convex, and
these curved surfaces meet if continued, since the convex surface
has a greater curvature than the concave ; whilst the concavo-
604
DIOPTRICS.
convex lens, i, bas similar surfaces, but they do not meet if pro-
duced, as the conyei bas a 2tf«Mr curvature than the concave sorfaoe.
1069. The course of a ray refracted through a spherical lens
may be readily understood ; let ▲ b d c be a sphere, of which the
jf. ^. index of refraction is ii, and let
-^^ ***• the rays e, k, o, be incident
upon it : the ray h, being inci-
dent perpendicular to the sphe-
rical surface, will pass throuch
without refraction (1054). To
find the course of the ray k,
draw the normal at a, k a s,
and draw the line a c such that
si]iCA8:8inKAE::l:/c; the
ray a o is thus bent towards the normal k s. On reaching c,
the ray will emerge into a rarer medium, and will again suffer
refraction, being now bent from a line k o s peipendicnlar to the
surface at c, at such an angle that sin l c s : sin m cf : : 1 : ^. By
a similar process, the course of the ray o may be found. The three
rays will tnus meet at f, which is the joeus of the refracted pencil.
1070. In order to detennine the position of the geometrical
focus of a pencil of light refracted through a sphere, let p be the
distance of the focus of incident rays from the centre of a sphere,
and o,, 9, the distances from the same point of the foci of refracted
pencils after the first and second refractions respectively, and r
the radius of the sphere : then as in (1065),
1 ift tt— 1 r ,
-C =-C — , [a]
and from refr'action at the second surface, if the coarse of the
pencil be supposed reversed,
therefore[a]-r5], i!^-^=-.2-^, or 1-1= -2^5^^
whence the geometrical focus of the emergent pencil may be found.
If the incident pencil consist of parflJlel rays, p is infinite, and q
becomes/, the/oooZ length of the sphere ; then we have
-. = .. 2 - — t And consequently = ->'
If the sphere be of glass, ft may be taken to be nearly 1*6 (1057):
con«Kine»tly, /= -^^f= "jiIfs'"' "''^ " •"•
The principal focus of a glass sphere will consequently be at
the distance of half the radius from its surface: ^e negative
sign means that the focal distance must be measured from the
FORMULA FOB FOCAL LBHOTH8. 605
centre of the sphere, in a direction opposite to that of tho focns of
the incident pencil.
The course of the refracted rays, and conseqnentlj the position
of the focus F, will vary according to the refractive power of the
substance of which the lens is constructed. Thus, Sir David
Brewster* has shown, that in a sphere of Tabasheer, one inch in
diameter, of which the refractive index is 1*11145, the focal dis-
tance for parallel ra^ will be four feet from the lens ; in one of
^lass of a refractive index of 1*5, it will be but half an inch ; au'l
in one of zircon, of which the refriu:tive index is 2*0, it will
coincide with the surface of the sphere. The following rule results
from the above formula : to find the focal distance of a sphere from
its centre, divide the index of refrctction of the material of which
it is eowitntcted, by twice its excess above unity, and the quotient
will be the distance expressed in radii of the sphere,
1071. The course of a ray through a double convex lens may
be found in the same manner as that already explained in the case
of a sphere (1069). Let the ^, 582.
lens AB, Fijj. 582, be of the ,fS. y^
same material as the sphere, ^^^o^o^
and B, H, Q, three rays entering v— — — a^<*^
it ; K will pass on and emerge
without refraction. The ray, b,
will, on entering the lens, be
refracted towards the normal ^ ^
K o B : and on emerging from c " y^^Y^'
into a rarer medium, it will be y^ \
again refracted, but in a contrary directiuo, or from tne line sou,
a normal to the point of emergence. By a similar process, the
course of the ra^ o may be ascertained ; b, k, o will thus be found
to meet at f, which is the focus of the lens.
Supposing the incident rays b, k, o, to be parallel, and there-
fore F to be the principal focus of the lens a b, if the rays inci-
dent on the lens oe converoentj the focus will be nearer the surface
of the lens than f ; but it divergent from any point further from
the lens than p, their focus will fall beyond that for parallel rays:
if the incident pencil diverge from f, the refracted rays will be
parallel, and if from a noint within f, the refrttcted rays will di-
Terge. The coune of refracted rays through a plano-convex lens,
as well as through convex lenses of unequal curvature, may be
found by a similar process.
1072. In order to find the geometrical focus after direct refrac-
tion through a lens, let q, Fig. 583, be the origin of a pencil of
which the axis q a b passes perpendicularly through the centre
of a lens ; let f, be the geometrical focus after one refraction, and
F that of the emergent pencil. Let a q= v, bf=s9, and r, «, the
radii of the surfaces a and b respectively ; lines being considered
* Treatise on Optics, p. 37. London, 1831.
potitiee when iDeasnrsd in ■ direction oMtraty to Uut of tlie in-
cident pencil. After refraction at the fint mrUce (1065, a),
^■'^- n \ —1
i7;-i=V= [-1
Since v is the geometrical
Tocug of the rays aAer the
second refraction, coDveTaelra
pencil converging to f woiud,
after refraction at b, converge
„_1
m
If the thickneiB of the lena bo neglected, and it bo asnmied that
i?, = BP„th6n[al-|6],
from which the position of Fmaj be determined. The dtWancea »i
and V being Teciprocd, tho pdnti p, Q, are caUed emJugaUfod.
When the incident raja are parallel, u ii infinile, and ctnue-
quently -=0; abo o then becomes/, the /tfcolfen^oftholens,
and the point r the prineipal foau, and the preceding fomnlB
becomea y = ^~^l\r'^~i)''
and consoqnentl]', e ~ w ~ ?' ^^
When / is poutive, the leni ia thinnest at it* axis, and when/
is neaative, the Iuiih is the thickest at its aiis; Ihns lenses him be
divided into tno classes, dietin^shed by the sign of the focal
length, or, in other words, the direction from the lens in which the
focus lies. Those of which the focal length is rBsitive, are called
concave lenses, and those in which it is negative, convex leaaea.
Lenses may, however, hsive an infinite varietj of forms, bat still
the same focal length. The reciprocal of the focal lengih ia aonie-
times called the pmner of the lens.
As the lenses of optical instmmenta am almost nnivsnally com-
{losed of glass, it will suffice for the present to confine our atten-
tion to these; and as the index of re&actioD in glass may be taken
to be 1*5 in rontid nnmbeit (1057], the prect^ing formnla dien
which ma; be applied to particnlar cases.
BBFBACnOM THBOUOH COHCAYS LBK818. 607
Oonoex lemes, f negatiye. FormtUat for parallel rayt.
A. Doable con rex lenBes; « negative, /= — — •
B. If their cnnratare be equal, r =s *, ancl/= — r.
C. Plano-convex lenses,— = o, and/= — 2 r.
D. Meniscns lenses, /negative; and $ positive, f^
Formtdafor diverging rays.
E. Doable convex lenses u and $ negative, v = —, r — - —
° tt(r+»)-2rf
tf r
F. If tbeir corvatare be eqaal, r = s, and v =
tt— r
1 2 Uf
G. Flano-convex lenses, — =0, and v =
9 ' u-2r
H. Meniscns lenses, s positive, and v=^ ; p.
The fommlsB for converging njs may be obtained from these
last, by merely changing the sign of u.
1073. To find the coarse of ravs incident on a doable concave
lens, let b, f, o, Fig. 584, be, as before, the rays, of which h will
pass throagh withoat refraction, b,
on reaching o, will enter the glass, ^' "^
and be bent towards xs, a normal
to the surface at the point of inci-
dence ; and on reaching c, the ray
OG will emerge and nndergo a
second refraction, being bent from
the normal at the point c ; by which
its divergence wul be increased:
the coarse of the ray o may be 9\
found in a similar manner. Thus the rays b, n, o, if parallel, are
made to diverge by refraction throagh a concave lens, instead of
converging, as in a convex glass. The emei^ent rays b, s, t, will
diverge in the same manner as they would, if thev had proceeded
frt>m a radiant point at f, as shown by the dotted lines f c, f d ;
this point is the focus of the lens, and is a rnrtuai, or negative
focus, as in the case of reflection from convex mirrors (1042). If
the incident pencil be convergent, as b, s, t, but to a point more
distant than f, it will diverge after refraction ; if convergent to f,
the refrtusted pencil will be parallel ; and if convergent to a point
nearer than f, it will still converge after refraction, but to some
point beyond f.
608 DIOPTBIC8.
The oonne of refracted rajs through plaiKMsoncavei aad double
concave lenses of unequal curyature, may be traced by a similar
process. From an inspection of the last dia^m, it is clear, that
if the incident rajs on anj concave lens be divergent, the negative
focus of the refracted rajs will be nearer the lens than the prin-
cipal focus F.
1074. The negative focal lengths, for parallel rajs, of all the
varieties of concave lenses, maj be found bj means of ^e formnlis
alreadj given for ponvex lenses (1072, a, b, c, d). Their foci for
converging rajs maj be found bj means of the formulae for diveig-
ing rajs and convex lenses (e, f, o, h), and vice versd,
1075. The action of menisci and concavo^onvex lenses is
similar to that of convex and concave lenses of the same focal
length ; the foci in the former being real or podtive, whilst in the
latter thej are virtual or negative. The Tormuln for concavo-
convex lenses are similar to those alreadj given .for menisci (1072,
D, h). Both kinds are used in correcting spherical aberrations.
1076. Let anj number of lenses be placed so that their axes
maj coincide, as in Fig. 604, of which the focal lengths are /*„ /^
&c., let tt be the focus of rajs incident on the first lens, v, that of
the refracted pencil ; v, the focus of incident, and o, of refracted
rajs in the second lens, and so on ; then (950, d),
1 _ 1. ^ J_
»l
u
/,
l^ _
1
1
»t^
^l
=7;
&c.
= &0.
1
1
_ 1
Vft »!,-, ft
n
and bj addition, =-?+>+ ...+7-»
which determines the focal length of the combination, the thickness
of each lens being neglected.
As the reciprocal of the focal length has been called the power
of a lens (1072), it is thus shown that the power of a eornhkuition
is the mm of the powers of^ the eeparate tenses ; tnot is, the alge-
braical sum, due regard being paid to the signs of all quantities
emplojed in the several expressions.
1077. Images are fotmea bj lenses in the same manner as they
are bj mirrors (1044). Let ▲ b be an object situated at a consider-
able distance ; the rajs propagated from it will, on reaching the
convex lens e f, sufier refraction, and after emergence will paint
on a screen, placed near its principal focus (1072), the image o d of
the object, out in an inverted position, in consequence of the
crossing of the rays. If the screen be removed, and a piece of
ground glass be placed at 0 d, the eje placed behind it, as at o,
HlGHIFTHrO POWER OF CONVBZ LEK8E8.
609
will see the image vety distinctlj ; then let the glass "be remoTedi
and if the eje has been placed within the limits of distinct Yision,
^.685.
a picture of the object will be seen painted in the air, a little
beyond the principal focus of the lens.
1078. If the object be within a moderate distance of the lens,
its image will be formed on a screen as before ; and will be visible
most distinctly when the object and the screen are placed in the
conjugate foci (1072) of the lens. If the object be still nearer, and
it be viewed through one of the modifications of the convex lens,
it will appear larger, and if through a concave lens, smaller, than
it really is. This curious property of lenses entirely depends upon
the apparent angle under which the object is viewea. Taking
first the case of the double convex lens, as a b (Fig. 582), let the
rays b, h, e, be supposed to pass from an object placed near it, and
the eye be placed between the lens and its focus f ; under these
drcumstances, the object will appear to be larger than it really
is; fvr if the rays fc, fd be produced, they will diverge at a
considerable angle, ana, as bodies always appear to be placed in
the direction pursued by the rays which ultimately reach the eye,
the ra^s f c, f d, will appear to have passed from the object in
right hues, and the object will appear to the eye to be sufficiently
large to fill up the whole aperture of the angle. If, on the con-
trary, an object be viewed through a concave lens (Fie. 584), it will
appear to be diminished, because it is visible under a less apparent
angle *, for if an object be placed so that its rays e, n, o, suffer re-
fraction in the double concave lens, they will diverge, and the object
will appear to be situated in the direction of the right lines r f,
TF, ana included in the angle of convergence of those rays.
1079. The manner in which the eye judges of the size of an
object, according to the appa-
rent angle under which it is
visible, may be readily shown.
If the eye placed at e view an
object AB placed at such a
distance that the ri^ht lines
A B, BE, subtending it at the
eye, may form an angle of 20^
BR
Fig. 689,
610 DI0PTBIC8. '
it will appear of a certain magnitade. Let a b approach to the
position a 6, it is evident that it will appear under a greater appa-
rent angle than before, as a line c b, passing through it to the
eje, will, with b b, contain a larger ande, and, juasing of its
size from this angle, it will appear to be larger than wnen at a b.
If the object be placed at one half the distance, as €^b\it will
then subtend an angle of about 40**, and will appear to be twke
as large as when at A b.
1080. The magnifying power of a lens may be determined hj
the limit of distinct vision for minute objects, which is generally
about five inches, divided by the focal length of the lens. Thu
refers to its linear ma^ifying power, its superficial power being
obtained by squarine its linear, and represents the number of
times the whole surtace of the object appears to be magnified.
Thus the linear power of a lens of half an inch focus will be
5-T- i s 10, and its superficial power, lO**" 100.
1081. On referring to ^1047), it will be seen that the rays pass-
ing nearer the axis of the lens will be refracted to a focus at a
greater distance from the glass, than those which pass nearer the
circumference. On holding a screen of ground glass near the focus
of the central ra^s, a picture of an object will be seen on the other
side, very vivid in its centre, but less distinctly defined at its
edges ; on gradually withdrawing the screen, the maiginal ]^rtion
of the picture will become more vivid as the centre loses its dis-
tinctness. Hence, it is obvious, that no object can be seen with
perfect distinctness in every part through a convex lens at the
same moment, in consequence of this spkerieal aberratvm, as it
is termed. In a plano-convex lens, with its convex side towards
a distant object when used to form an image, or towards the eve
when used as a magnifier, this aberr<Mtion amounts to 1*17 of tne
thickness of the lens ; but when in the reverse position, to 4*5.
In a double convex lens, with equal radii of curvature, the aberra-
tion is 1*67 of its thickness, for parallel rajs: if the radii of the
surfaces be as 2 : 5, the spherical aberration will be the same as
in a plano-convex lens ; and if as 1 : 6, it will be a tninimMm,
heing then only 1*07 of the thickness, the most convex surface in
both these cases being towards the parallel rays j but it may be
still further reduced by means of a meniscus similarly placed.
Sir J. Herschel has shown that the aberration may be reduced to
one-fourth of that of a single lens in its best form, by means of
two plano-convex lenses having their convex surfaces towards each
other, and their radii as 1 : 2*3, and may be tehoUy removed by a
combination of a double convex, and a meniscus lens, with appro*
priate curvatures.
A single lens with a suitable elliptic or hyperbolic snifroe
would have no spherical aberration, out the difficulty of con-
structing such suifaces has hitherto proved an efiectual Mir to their
adoption.
611
CHAPTER XIX.
lioht;
CHKOUATICS.
1082. The rays of which a pencil of light is composed have
hitherto been considered homogeneous, and equally refrangible,
on their passage from one medium to another ; but the following
experiment shows that a pencil of light has not this uniform cha-
racter ; it admits, in fact, of decomposition, or separation at a re-
fracting surface, into a system of pencils, in each of which the
rays have a different degree of refrangibili^.
If a pencil of sun-light, s, Fig. 587, be admitted into a dark
chamber through a small
aperture in a shutter, d b, ^*9' 887.
and be allowed to fall per-
pendicularly on a screen,
a circular bright spot of
white light, w, will be
seen. Let this pencil be
now refracted upwards
by a prism of glass or
other refracting medium,
▲ B c, placed near to the
aperture, the direction of
the edge of which should
be perpendicular to that
of the pencil. If the
pencil be now received perpendicularly on a screen c H, an elon-
gated stripe of colours, v r, called the prismatic spectrum, be-
comes apparent. On turning the prism slowly about its edge, this
spectrum first descends, and then ascends ; and when it is station-
ary during a very small angular movement of the prism in either
direction, the pnsm is then in the position of minimum deviation
in which the mean path of the rays through the prism is parallel
to its base, ▲ b. If the screen that receives the spectrum be
placed at the same distance from the aperture, as that on which
the bright spot, w, falls when the prism is removed, it will be
found that the spectrum is of the same horizontal width as w,
bat its length is about five times as great, and it is composed of
successive hands of different colours, the lowest of which, or the
least refracted portion, is red, then orange, yellow, green, blue,
indigo, and lastly violet at the upper extremity, which is the most
refr^ted part of the spectrum.
R R 2
612
OHBOMATICB.
This remarkable experimeot was first made by Neirton,* and
is asaally termed the prismatic decomposition of b'ght ; white
light having been considered as composed of seven distinct and
homogeneous colours. But it is almost impossible to point out in
the spectrum, as it is termed, any distinct line of demarcation be^
tween adjacent tints ; for as the violet, indigo, and blue melt into
each other, the latter colour and green can scarcely be distin-
guished at their point of junction, and the yellow, orange, and
red are still more closely united. So that, although Newton
adopted seven, as the number of primary colours, it is better with
Euler to consider that, whilst the extreme violet is produced by
the greater number of undulations, and the red by the smaller
number, in a eiven time, there exists between these extremes
every degree of variation in the rapidity of undulatory movcmenti
and consequently an indefinite gradation of tints and colours.
1083. Aided by a friend, whose perception of colours he con-
sidered to be very delicate, Newton measured with as much ac-
curacy as possible the limits of the different coloured bands of
the spectrum ; he found their lengths, reckoning from the violet
to the red, to be nearly in the ratio of the numbers j^, |, f , {-, f , ^, ^,
a series nearly corresponding to the intervals of Sound in the
diatonic scale, or gamut (570). The following are the linear
measures of the spectrum made by Newton (who unfortunately
did not describe the kind of glass of which his prism was con-
structed), compared with similar measures made oy Fraunhofer
with a prism of fiint-glass, each philosopher dividing the entire
length of the spectrum into 360 parts :
Observer.
R.
0.
Y.
40
27
G.
60
46
B.
60
48
I.
48
47
V.
80
Newton . .
45
56
27
27
Fraunh()fer
1084. As in the experiment above detailed (1082), the violet
rays undergo the greatest, and the red the smallest, amount of
deviation from the original direction of the ray, a w ; the former
are termed the most, and the latter the least, refrangible rays.
When prisms of crown- and flint-glass are used, the following are
the indices of refraction (1057) of the difierent coloured rays :
OUu.
Red.
Oraoge.
Yellow.
Green.
Blae.
Indigo, i Violet.
Crown .
Flint..
1-5258
1-6277
1-6268
1-6297
1-5296
1-6350
1-5330
1-6420
1-5360
1-6483
1'5417 1-5466
1-6603 1-6711
* Optice, lib. i. part 1, prop. 3, exp. 7.
10S5. If a second prism apC of preeieelj the aame kind be
»pplied to the fir^t A b c, as shown in Fig. 587, the colours will
TsnUh from the screen, and white lii^ht will be reprodnced. This
is fermed tht reeompoiilion of tehite light ; and as a a i; r repr«-
KDta tha section of a pBrallelugram, it ia evident that wbcnevera
pencil of light is tranamitted obliquely throngh a plate of glass,
resolntion will take place at the firal snrface, and recompOBitinii of
»hit« light at the second; unless, .as in the Bpeclroacopc, iha
trangrerse width of the pencil be very small coai|jared with the
thickness of the plate, in which ciwe, bands of prismatic colonra
will emerge parallel to each other. The recombination of the
coloured raja maj be alao ahown bj holding a convex lens be>
tween the prism and the screen, which, If sufficiently near the
former, will bring all the rajs nearly to a focus, and repruJuce
while light.
It muit not, however, be supposed, Irrim the preceding obaer-
Tations, that the separated rays of any one Tery small pencil ure
reeombined, for, as the rajs refracted at diflerent angles on their
incidence, ai« each equally refracted in a contrary direction on
their ome^nre, it Tollows ihal each emergent very ^ ^^
smalt pencil will consist of parallel rajs of diETereut
colours as TK, Pig. 588, but as the same separation
of the coloared rays takes place with each aucccs-
■ive Tery bihbII portion of the incident pencil, the
overlappiug of the successive parallel spectra re-
produces white light. This may be seen to be the I
case by observing the pencil of light (1082) en a I
■creen alter oblique refraction through a thick plate I
of glass or a veaael of water with parallel (class sides, when Itie
npper margin of the spectrum will be observed to lie tinged blue,
and the lower mai^n red, while the inlennediate portiun coosists
ef white light ; Ihis evidently results from there being no rays to
combine with the extreme rays of the outermost spectra. For the
same reason a peacil of light, transmitted through an ordinary
lens, is observed lo be sarrounded bv a fringe of coloured rays.
1086. From a set of scciirnte admeasurcmentB made by New-
ton,* the following table, showing Ihe length and rapidity of nn-
dulationn producing the principal coloured rays of tSie spectrum,
has been constructed (see Table, p. 6U). Thus red light is pre-
sumed to lie canned by a little more than half as toanr osciilHtions
or undulations as are necessarr to generate violet light, and henca
the waves of the latter are a little. morv than half the length of
those ol red light, or more nearly aa 63 : 100.
1087. Fromsome researcbes of Sir John Heischel, in connexion
with the photographic powers of the spectrnm, it appears certain
that a hand of coloared light of (till higher ic&angitiility than the
• TreaMs* on Ufbt, io Sus. Uetoop., bf 8ii Jobs Haneb*!, S7G.
614
CHBOMJLTICS.
▼iolet exists just beyond the limits of that tint. This new band
is barelj luminous, and has been denominated the lavender
band by its discoverer.
Colours
ofrayi.
£ztreme red
Red .
Intermediate
Orange
Intermediate
Yellow .
Intermediate
Green .
Intermediate
Blue .
Intermediate
Indigo .
Intermediate
Violet .
Extreme violet
No. of waves
in an inch.
37640
39180
40720
41610
42510
44000
45600
47460
49320
51110
52910
54070
55240
57490
59750
Lenffth of
each wave.
00000266
0*0000256
00000246
00000240
0*0000235
00000227
00000219
00000211
00000203
0 0000196
0-0000189
00000185
0-0000181
0-0000174
0 0000167
No. of wares
in a second.
458x10"
477
495
506
517
535
555
577
600
622
644
658
672
699
727
t»
II
II
>i
II
II
II
II
II
II
II
II
II
II
1088. The seven colours of the solar spectrum are generally td-
garded as simplef because they cannot be separated into otnen
by a second reiraction through a prism, in which they differ from
the tinted light obtained by passing the sun*s beams through most
varieties of coloured glasses. When light passes through even the
most ti'ansparent medium, as water or glass, some of its undula-
tions are absorbed, and these vary in quality according to the
nature of the substance ; the transmittea undulations which ulti-
mately reach the eve, communicate the sensation of that colour
which is produced oy the undulations of white light ininu$ those
which msy have been checked or absorbed whilst passing through
the given medium. Thus, on holding a piece of smalt>blue glass
between the eye and the light, the transmitted rays will be of a
line blue colour and consist of a mixture of all those undulations
which have not been absorbed by the glass ; and if decomposed bj
the prism, will exhibit a spectrum (1082) deficient in those rays
which have been absorbed by the blue glass.
1089. On examining the solar spectrum through such a piece of
glass, which is best done by placing it before a prism, through niiich
the observer is regarding a hole m a window-shutter, Sir David
Brewster found that the greater part of the red and orange rays
had disappeared. The yellow band appeared greatly increased in
breadth, encroaching on the spaces formerly covered by the orange
on one side, and the green on the other. Hence, the coloured
glass had absorbed those rays which, when mixed with the yellow,
OOMPLBMSKTAJIT COLOUXS.
615
oonatitnte orange and g^en, and conseqaenti/ the green of tlie
Bpectnim becomes decompoBed into blue and yellow, and the
orange into yellow and red. This has been termed the simplifi-
cation of the spectrum bj absorption, and greatly corroborates
the views of those philosophers who have contended for the exis-
tence of but three primary colours, as red, yellow, and blue.
1090. The solar spectrum has by some been re^rded as com-
posed of three spectra of equal lengths oTerlappmg each other,
the red having its greatest intensity in the middle of the red
space ; the yellow, in the middle of the band of that colour, and
tne maximum of the blue between the band of that colour and
the indigo. Sir David Brewster has exhibited by means of
three curves the intensities of tint of the three spectra, which he
conceives to constitute the
solar spectrum. ; Thus, if oh, I^.6».
Fig. 589, represent this spec-
trum, the red curve b com<
mences abruptly at o, and
gradually declines to h ; the
yellow one t commences less
abruptly ; and the blue one
B begins with a very gradual
curve ; — the heights of these
curves, or lengths of their ordinates, represent the intensities of
the tints of these fnimary spectra in every part of c h.
Putting B for the primary red, b for the primary blue, and t for
the primary yellow rays, the following will be a view of the pro-
portions in which these rays have been considered to exist in the
spectrum, and in white light :
J*
•l -^
^ |*!/| *
Cokmr. Proportions.
White . 20R + 30Y + 50B
Bed . . 8 R
Orange. 7B + 7 Y
Yellow . 8 Y
Colour.
Green
Blue .
Indigo.
Violet .
ProportioBB.
13Y + 10B
6 Y+12B
12 B
15B-t-5R
Jn^m 690.
1091. Each of the prismatic colours has some other which is
said to be complementary to it, and which, when combined with it
produces white light. If we consider the indigo,
not as a separate colour, but as a deeper shade
of blue, the remaining six may be regarded as
composed of three primary, ana three secondary
colours. The complementary colours to eacn
of the former will be the compound tint made
by blending the other two, whilst the comple-
mentary tint to each of the latter will be tnat
primary colour which does not enter into its
composition. This may be seen by a glance
at toe diagram, Fig. 590, consisting of three
616 CHSOMATICS.
intorseotiDg circles, each re]^reaeiitin^ a primary tint. In the centre,
where they all overlap, white light is produced, and in the other
spaces the complementary colours are exactly opposite each other.
1092. Oorham^a Colour-top. — ^The phenomena exhibited by the
blending or intermixture of different colours, may be conTcni*
ently studied by aid of Mr. Gorham's colour-top :* this condste
of a short and broad spinning-top, the upper surface of which is
quite flat. This is furnished with a series of circular discs of
paper, which are white, black, blue, red, yellow, and green. Each
has a hole in the centre by which it may be placed on the stem
of the top, and a radial slit, by which they may be made to orerlap
each other, so as to bring into view sectors of any required angular
magnitude. The discs, when placed in any required podtioii, are
retained by means of a screw.
The blending of two or more colours, distributed on contigaons
surfaces, by a rapid rotation, is analogous to the well-known ex-
periment of whirling a hot coal, or otner luminous body, by the
nand, when an unbroken luminous circle is peix^eived. As the
luminous body can occupy only one point of its path at a time, it
is evident that the impression on the retina of the eye must last
for at least the period of an entire revolution. Ck)loared surfaces,
when rotated, form, in the same manner, circular areas of coloor,
the images of which being superposed on the retina^ the impres-
sion of mixture is produced, and a compound colour results. The
resultant colour is generally identical with that which would
arise from a mixture of the pigments in the same proportions :
but to this there is a remarkable exception in the production of
greens. This, as is well known, is produced bpr the mixture of
blue and yellow pigments in almost any proportion ; but there ate
no known blue and yellow which, when combined in any propor-
tion by rotation, will produce even a tolerable green. By cover>
ing one disc by another with an aperture of a suitable form, the
gradations of shade fh)m any given colour, to white or black, or
to any other colour, may be beautifully illustrated. It may be
remarked that no mixture of colours by rotation will produce
absolute whiteness, but a gray, or neutral tint, such as would re-
sult from some mixture of black and white, may be produced.
1093. Media of various colours absorb different primary rays ;
thus, the piece of blue glass already referred to (lOiBS), absorbed
the red and part of the yellow ; some pieces of red glass, or a com-
bination of blue and red, absorb eyery ray except the homogeneoas
red. A solution of the ammoniacal sulphate of copper transmits
the violet, but absorbs all other undulations ; while the ammo-
niacal oxalate of nickel absorbs the violet, and transmits the blue
and red. This remarkable absorptive power of different substances
becomes curiously modified by heat, as shown by the tints
* l^orotoopioAl Jonmal, Jan. 1860.
DISPER8ITE POWERS OP SUBSTANCES. 617
aesumed by yarious substances at different temperatures ; thns,
the biniodide of mercury tarns yellow, bincxide of mercury black|
and tbe salts of cobalt blue, or bluish green, on being heated.
1094. The absorption here spoken of occurs during the passage
of the rays through a medium, but an analogous phoBomexion has
been observed to attend the reflection of light from the polished
surfaces of certain metals.
The metals generally, except gold and copper, reflect nearij
colourless light, but after repeated reflections from the sur&ces m
the same metal, the reflected light becomaf coDodenXAj
coloured : —
iVom Copper, the colour is Scarieily
„ Gold „ „ Ked,
„ Silver „ „ Purs Yellow,
„ Zinc „ „ Indigo Blue,
„ Iron „ ,, Violet
Light transmitted through a very thin lamina of any metal, is
usuall^p^ of a colour complementary to that produced by repeated
reflection from its surfac^.
1095. On examining the solar spectrum (1082), the green rays
are observed to be placed very nearly in the centre, and are hence
frequently termed the mean or medium rays of the spectrum. If,
instead of using the prism referred to, one of the same kind of
glass, but of greater refracting angle, be employed, the length of
the spectrum, or distance of the mean rays from tbe extremities
will be increased ; and diminished, if the refracting angle of the
prism be lessened. But when the spectra produced by two prisms,
one of flint- and the other of crown-glass of equal angles, are
examined, that produced by the latter will be found to be shorter
than that by tne former; hence flint-glass is said to have a
greater disversive power than crown-glass, because it spreads or
disperses tne spectrum over a greater space. A hollow prism of
thin glass fiUea with oil of cassia, produces a spectrum of twice
the length of one produced by a pnsm of solid glass, on account
of the great dispersive power of that fluid.
1096. If the prism a b c. Fig. 587, be of flint-glass, and one of
crown-glass, a f c, be applied to it, the angles a c b, c a f, being
Buch that the deviation of the mean rays may be the same in both,
the spectrum will disappear, and the spot of light w will be repro-
duced, not colourless, as when the prisms were of the same kind of
glass (963), but elongated a little vertically, and tinted above with
purple, and below with green light. This arises from the unequal
dispersive power of the two prisms, which prevents the latter from
completely neutralizing the effects of the mrmer.
Tne course of the rays will perhaps be better understood by a
reference to Fig. 591, in which the rays are seen to converge less
coloured ntji emei^ from the ctowuImi
priam is the revene of vbat it would be if the
rari n-ere rcfrxcted bj thM prinn ■Icbc, iIk
red ray ■, b, being toward* the bue, kod tiie
violet rnjg, v, towardi theapexofthmtpruo.
Or if the nj were drawn inddeot poiModic*-
larly on the firat larisce, u at i>, it wovU
proceed Dnrafrscted lo iba amanum rarftoe of
the two priimi, the mean nj would tbeoo
pnnue its ori^nal conrae withoat re&accioB,
but the extreme rayv would diverge >t an an-
gle dependisg oo the difference of the dicper-
liTo powers of the two pnama. and emei^
parallel to the mean ray, at the aurface x r. In this Utier case,
chroniat" ''
change ir
1097. The diepersi . _
to ita index of refrsctioa ; it waa fonnerlj determined bj dindiug
the difference of the indices of relraction for the twl and nrlet
rajB, hj the eiceas aboie nni^ of the index of refiactioD of (he
mean njt. That, tlie disperaiTa power of crown-glaas ia CK)39,
for (1084) rMee-i-5a6e=oi)2oe, Md^^^-oflsg.
. turn
Oilofcaaua . 0'139 Oilofcaraway 0-M9 Ether. . . .
Fhoaphonig . . OlSB Flint-glsu . . 0048 Caator oil . .
Bimi1ph.carbonO'U6 Oil of juniper 00*7 Water .... (MOS
OilofcloTBB . 0-062 Oiloftarp. . 00*2 Flate-glMa . , 0-03i
Oilafaaaaafraa OOGO Amber. . . . 0*041 Sulphuric acid 0031
Rock aalt . . . 0-053 Crown-glan . 0-039 Alcohol .... fyi>i9
aioflhjms . 0-050 {Diamond. . , 0-038 RockciTMal . <yiSi
1098. Not onlj are the total lengthi of the epectis altered bj
the Bubetitution of priams of different dispetain poweia, bat ibi
•pace* occupied bj the colonred bandi are not pn^ioHioiial to tht
■Jtered length of the whole apectruln. Thia cuTiooa eliect if
termed the irraliotuiUiy of the apectral diapenion, and ia i»-
markablT well shown bj using two prisms, one of oil of cana
(1095), t^e other of sulphnric acid. If the spectra prodooed be of
the same length, the more refrangible colours, as the riolet, iodin,
and blue, will be found to occupy a much lai^r portioD of U*
entire apectrom in ibe oil than in the acid ; the reTcrae being the
case with the leas refrangible raja, as red, orange, and jeUow.
1099- If the solar rayi, admitted through a narrow alit in s
prauhhofbb's limes. 619
pUte of metal, parallel to the edge of the prism, be trauamitted
through a prism, a long spectrum traversea by numerous dark
lines will become visible ; and the late Dr. Bitchie found that if
a bottle containing nitrous acid gas be interposed between the
spectrum and the fight, those lines will increase so much that the
whole will present the appearance of a striped carpet. Two of
these lines were first observed by Dr. Wollaston, but they have
since been more carefully studied by Fraunhofer, Brewster, and
others. None of them exactly correspond to the boundaries of
the coloured bands, but they appear to be generally constant for
the same kind of light ; that is, for light derived from the same
source, through whatever medium it may be refracted. About
a thousand of them have been counted by Sir D. Brewster.
The relative positions of the more remarkable of these lines are
represented in Fig. 592, and of these the most important are com-
monly designated by the letters originally assigned to them by
Fraunhofer. Of these, ▲ is a well-defined line a little within the
red end of the spectrum ; at a a gnroup of several lines forms a
bfl;nd: b is a well-defined line of sensible breadth ; in the space
between b and c there are 9 very fine lines : o is a very dark
line ; and between c and d 30 very fine lines may be counted.
At D in the orange space are two strong lines, separated by a very
small interval ; between d and e about 84 lines may be distin-
guished. E lies in the green space ; it consists of several lines,
of which the middle one is rather broader than the others, but
I V| 30 I 84 I 80 I
thev are placed so close, that they appear to form one broad line:
on Doth sides of e are other groups of fine lines much resembling
E, but not quite as dark. Between e and b are about 24 lines ;
and at b are three strong lines, of which the two furthest fipom s
are very close together : these are the strongest lines in the bright
part of the spectrum : between b and f about 50 lines may be
counted, f is a strong line at the commencement of the blue,
between which and o about 185 lines may be distinguished ; these
are of various breadth, and variously grouped, g is a strong line
in the indigo, in the middle of a band of very fine lines ; and^ be-
tween o and H are about 190 lines variously arranged, h is a
strone line in the violet, in the middle of a band of fine lines, near
to which bnt further from o, a similar band is seen. From h to
I, the end of the visible spectrum, the lines are fainter, but very
numerous.
620
CHROMATICS.
1100. All these dark lines arise in all probabilitj from certain
undalations bein^r absorbed during the passage of the light to onr
earth ; those above referred to are constant only for the light
derived directly or indirectly from the snn ; for almost every fixed
star has its own system of lines. The line d, indicating the place
of a deficient ray, appears to be very constant in the hght of the
Elanets, and of many of the fixed stars. The spectrum from lamp-
ght appears deficient in three dark lines, d being replaced by a
double bright one ; the ray thus wanting in the solar spectram
appears to correspond with the homogeneous light evolved during
the combustion of alcohol, in which common salt has been dis*
solved, as in Brewster's monochromatic lamp.
1101. The great value of these fixed lines is in their presenting
definite means of comparing the refractive (1057) and dispersive
powers (1097) of bodies, since they are not liable to those vanations
of perception of colours which affect the position of tints in the
spectrum, as assigned by different observers. The following is an
abstract from the table of Fraunhufer's admeasurements of the
refractive indices of water, oil. of tarpentine, fiint- and crown-glass
for the lines b to h inclusive.
Medium.
f*B
f*o
fh>
A*«
fh
f*S
f*K
Water at)
18-76«»C.)
Oiloftarp.
Crown'glass
Flint-glass .
1*88006
1-47040
1*62431
1-60204
1-88171
1-47163
1-62630
1-60380
1*88867
1*47443
1-62708
1*60848
1*88686
1*47836
1*68137
1*61463
1*88780
1*48174
1*68434
1*62004
1*84127
1*48820
1*63901
1*68077
1-3M17
1-40387
1*65468
1-64C87
The dispersive power of any substance is now taken to be
measared by f^BZJtt but the resnltB thus obtained will not be
/"n-l
found to differ materially from those of Sir D. Brewster (1097).
1102. It appears from the experiments of Messrs. Gladstone
and Dale,* that the index of refraction always decreases with
increase of temperature ; but that the amount of this tempera>
ture-change of refractive power varies considerably, being only
0*0002 per 5"* G. in water, and the highest observed, 0*0042 in
phosphorus. The length of the spectrum also decreases with
increase of temperature ; with highly dispersive bodies as bi-
sulphide of carbon, and hydrate of phenyl, considerably; bat in
water the decrease is scarcely appreciable. In some substances
the dispersive power is diminisheo, in others it is augmented by
increase of temperature : this depends upon whether the nume-
rator or the denominator of the above expression for the dispersive
power decreases most rapidly. The amount of the temperature -
• PhiL Trans. 1868 and 1868.
niTBXBXTT OF UOHT IH THE 80LAB BPBCTBUU.
621
change of refraction, appears to bear some relation to the change
of density due to the same elevation of temperature.
If the quantity tt — 1 be called the refractive energy ^ and the
ratio of this to the density, the specific refractice energy, it
appears that the specific refractive energy of several liquids
observed is a constant, and not affected by 3uinge of temperature.
It appears also that the specific refractive power of a mixture of
liquids is a mean of those of its constituents.
Much further information on the relations existing between
refraction and chemical composition, as well as a copious table of
the indices of many fluids for all the lines from a to h will be
found in the latter of the two papers mentioned. An abstract of
this table containing the temperatures of the observations and the
indices of a, d, and B, of several characteristic fluids is here given: —
Liquid.
i«>C.
/'a
I'd
Mh
Water. . . .
160
1-3284
1-3324
1-3431
Ether ....
16-0
1-3629
1-3666
1-3683
Alcohol . . .
160
1-3600
1-3638
1-3761
Chloroform . .
10-0
1-4438
1-4490
1-4661
0. turpentine
Bensole . . .
240
1-4596
1-4653
1-4846
10-6
1-4879
1-4976
1-6306
Aniline . . .
21-6
1-6644
1-6774
1-6297
Beet. 0. cassia .
28-0
1-6649
1-6801
1-6244 (
Bisulph. carbon .
110
1-6142
1-6333
1-7090
Phosphorus . .
350
2-0389
2-0746
2-2267
(/««)
1103. Sir D. Brewster and Dr. Gladstone* have given a very
carefully drawn map of the solar spectrum, five feet in length : in
this is recorded the position of many lines not found in Frann-
hofer's map. Some lines observed only at low altitudes of the
sun are supposed by the authors to be due to the atmosphere.
1 104. Tne intensity of licht in the solar spectrum appears to be
S latest in the yellow band, and from that space it decreases to
th extremities of the whole series of tints. Fraunhufer has
exhibited these variations in the light of the different parts of the
spectrum by the curve bkl,
Fig. 593, the ordinates of which
indicate the intensity of light
in the difierent parts of the
spectrum bv, in which the
position of his lines has been
marked. Taking the ordinate
X If falling nearly in the boun-
dary between the yellow and
orange as unity, the following
^.683.
* Fhn. Trans. 1860.
622
CHROMATICS.
will represent the illmninating power of tlie different portions of
the spectram in which Fraunhofer*s lines are seyerallj situated ;
the red extremity heing indicated by r, and the violet by ▼ : —
Parts of the
■peotrum.
B
B
C
D
B
F
6
o-osi
H
V
Intenntie*
of light.
0-0
0-082
O-OM
0-64
0-46
0-17
0-036
0-0
1105. The calorific powers of the spectmm increase from the
▼iolet to the red extremity, and extend considerably beyond it, the
obscure space h, Fig. 587, beyond the red extremity possesising
a higher temperature than the red band itself ; so that it is evident,
that when undulations are propagated through a prism, a certain
number of them move with too little rapidity to communicate to
the eye the sensation of light, and are only to be recognised by
their calorific effects. These rays of non-luminous heat are less
refrangible than the rays of red light, and are therefore found in
thegreatest abundance beyond the band of that colour.
Inese calorific rays, like those of liffht, are subject to Taried
degrees of absorption, according to the refracting medium of
which the prism is constructed ; being, according to Seebeck, in
the greatest number in the yellow band, when a prism of water
is employed ; in the orange, with one of sulphuric acid ; in the
middle of the red, with crown-, and beyond the red, with flint-
glass. These phenomena are explicable on the supposition that
there exist in the solar beams, rays of heat of different refrangi-
bilities. Consequently the absorptive power for heat of the
medium of which the prism is composed will materially affisct the
dispersion of radiant heat over the luminous spectrum.
From the observations of Nobili and Mel loni, on a spectram
produced by a rock-salt prism, the highest temperature was found
beyond the red, and about as far distant from it on one side as
the blue band was from it on the other.
The following temperatures were observed by Sir H. Englefield
in the different coloured portions of the solar spectmm : in the
blue, 56" F.; green, 58'; yellow, 62'; red, 72'; beyond the
red, 79°.
1106. The chemical action of solar light, in prodocing oombina-
tion and decomposition, has been long known, and this, like the
heating power, appears to reside in greater intensity at one end of
the spectrum than the other. This may be shown by dippinff in
a solution of nitrate of silver a slip of paper, previously waued
over with a solution of common salt ; on drying this, and exposing
it to the action of the solar spectrum, a veiy remarkable e^ct
will be observed. In the course of a few minutes the chloride cS
silver with which the paper has been imbued, will become of a
deep slate colour in the violet, and in the sombre space beyond it ;
CiiLOSIFIC AHD CHEUIOAL RATS.
628
whilst in the yellow, orange, and red, it will remain scarcely
affected, its colonr being less altered in the blue than in the violet,
and still less changed in the green. Thus the chemical action of
the different rays of the spectrum appears to be most intense in
the violet band, and in the dark space beyond it, at the directly
opposite end to the seat of the principal calorific rays. There is
reason to believe that those undulations which are propagated
through a prism with too great rapidity to act on the oi*gan of
vision, possess the power of exertmg certain chemical effects on
many substances, in the same manner that calorific effects are
exerted by those undulations which move with too little rapidity
to produce the sensation of light. Granting this, we meet with
another circumstance in which the propagation of light and of
sound correspond : it has been already shown that to most persons
aerial waves moving with a velocity sufficient to strike the ear less
than 16, or more than about 24,000 times in a second, are Inaudible
(568) ; whilst luminiferous undulations, if less frequently rei)eated
than 458 millions of millions, or more frequently than 727 millions
of millions of times in a second, are incapable of acting on the
visual organs.
1107. If DB, Fig. 594, represent the solar spectrum produced by
flint-glass, and ab^ ds, the non-luminous portions beyond it, at
each extremity, the curves acd, grb, will give an idea of the
relative position of the calorific and chemical rays. The longest
ordinate of the curve e h b falls without the red ray b in the ob-
scure space beyond it, where the calorific effects are most manifest ;
and the longest ordinate of the chemical curve acd falls in the
dark space beyond the violet ray y, where the action on chloride
of silver appears to be most intense : both curves rise abruptly,
and gradually decline to zero at the opposite ends of the spectrum.
Spectrum Analysis. — ^The progress of this department of physics
has of late years been matly facilitated b^ the introduction of the
spectroscope (Ch. Xa.), an instrument in which a telescope for
viewing, and a divided circle for measuring, the lines of the
spectrum are conveniently combined with a pnsm, or combination
of prisms for separating them.
1108. The spectrum formed by electric light was first observed
by Frot Wheatstone to consist principally of a few bright lines,
624 CHE0MATIC8.
the cbaracter of which depends on the nature of the sahstanoes
hetween which the spark was transmitted ; but these obserrations
remained for many years unheeded, and it is to the researches of
Bunsen and Kircnon that the present great development of this
department of ph^^sical science is mainly due.
It was ascertained by those distinguished observers that all
simple bodies, when raised to a state of incandescent Tapoor,
emit light which, when submitted to prismatic analysis, does not
present a continnous spectrum like sun-light, bat one consisting
of isolated coloured bands, of various but definite degrees of le-
frangibility. These luminous bands are found to be constant for
the same substances ; and when two or more elements are com-
bined, no new bands are produced, but merely a superposition of
those peculiar to each. Tne existence therefore of a known band
of lignt in any given spectrum may be taken as evidence of the
existence of an elementary body to which it appertains ; and con-
versely, bands previously unknown demonstrate the presence of
some element, the existence of which has not hitaerto been
recognised.
It may be further remarked, that the quantity of anjr element
capable of producing a definite spectrum is so infinitesimally
small, as to be inappreciable by any chemical tests : hence, if the
spectra of all known elementary bodies be carefully observed, and
tabulated, spectrum-analysis becomes a fertile means of detecting
the presence of a new and unknown element.
To this mode of investigation the discovery of several new
metals is due :— of caesium and nibidium by Bunsen and Kircho£^
and of thallium by Mr. W. Crookes,* who in 1861 was occnpied
in examining a seleniferous deposit from a vitriol-chamber at
Tilkevode, in the Harz mountains. Availing himself of the then
new method of spectrum-analysis, he inferred the presence of some
new element, from the appearance of an unknown paie-green line.
He possessed a very small quantity of material for investigation,
but oy the exercise of much skill, and by persevering^ reference to
the spectroscope at each stage of his chemical manipulations, in
order to track the whereabouts of the stranger, he succeeded in
isolating a new metallic element, to which he ^ve the name of
thaUiumt from the resemblance of the colour of its peculiar band
to that of a sprouting twig (Gr. OoXXtfc)- This metal resemUes
lead in its external characters ; it has been found most abundant
in some varieties of iron pyrites, but not in greater proportion than
10 ounces to the ton. The position of the principal lines of
csesium and thallium is shown in Fiff. 595.
1109. From the observations of Mr. Attfield, it appears that
when an electric discharge takes place through rarefied atmo*
spheres of the compounds of 0, H, N, and G, the spectra prodoced
are the superposed spectra of the individual elements : wnile fnun
• Phil. Tnmi. 1862.
BPECTSUM-ANALYBIS. 625
thoee of HH. PUicker and Hittorf,* it is shovrn that the spectra
of ignited gases and vapours vair considerably in their character
with changes of temperature. For example, below a certain tem-
perature nitrogen emits only eold-colourea light, giving a spectrum
of gradually-shaded bands ; beyond this temperature, the light is
bluiiih violet, and the spectrum consists of channelled spaces.
At a still higher temperature, the light is whiter and more bril-
liant, and bright bands appear in the spectram ; these increase in
breadth, and at the highest temperatures approach the character
of a continuous spectrum.
1110. Messrs. Huggins and Millerf have observed that the
lines of the solar spectrum present different characters ; some are
sharply defined, while some, even with a narrow slit, are always
nebulous at their edges : some dark bands consist of a haze, not
resolvable into lines by any power applied, but others evidently
oonidst of double or multiple lines, placed too close together for
separate measurement.
The same observers have remarked that in the spectra of the
metals, several lines of different metals are quite comcident ; and
many others so nearly so, aa to be inseparable when superposed.
In several instances, particular lines of three different spectra
very nearly coincide ; m one case, four lines, and in another, five,
are also veiy nearly coincident.
A series of observations has been made by M. Janssenf on the
spectrum of aqueous vapours as manifested by the additional lines
vihible in the solar spectrum during the afternoon hours, and as>-
Bumed to be due to evaporation from the earth's surface, under the
influence of solar heat. This spectrum consists chiefly of two
bands at a, Fig. 592, of one below b, of several faint bands above
o, and of a broad and dark band below i>. This last accounts for
the relative prevalence of rays belonging to the lower end of the.
spectrum towards Bunset in clear weather.
1111. One of the most important results of spectrum-analysis
IB the insight thereby attained into the physical constitutioiL of
the Bolar and stellar photospheres. It has been satisfactorily proved
by experiment that a luminous vapour, capable of emitting rays
01 a given rdfrangibility will arreift or absorb those same rays,
when transmitted through it from an independent source of light.
This is most readily shown by transmitting through a prism the
light of a spirit-lamp with a salted wick; when the well-known
yellow double sodium line will be thrown upon a screen. If a
portion of incandescent vapour of sodium be now interposed in
the path of the refracted rays, the bright sodium line is replaced
by a dark band, which moreover exactly coincides with Fraun-
hofer's double line d, in the solar spectrum. It is thence infern/d
with a veiy high degree of probabinty that this dark line is due
« FhiL Trans. 1895. f Ibid. 1863.
X Beport Brit. A^iociatioD, 18(i«.
B B
to Ibe pretenoe of lodinm Tapoar in tlie immediabi riciiutj of tlta
tan; and a Birailnr carrespoDdenM aiiiU villi ragani to faanj
other metallic gpectra. For eiaiDple, mora than fiftj brufit
bands in the iran-snctrain bave their correipnnding r«pre«Dt»-
tivet in the dark liasi of the aoUr epectnim, one of which ii
Fraunhiifer'a line i : moreover c and r are hvdn^ea-linca, netthrr
of vhlch has been obaerred alone ; and b beloDfTa to mMgaatiuia.
It thui appears that hydrogen, Kidium, potaaiiimi, magneaiiun,
calcinm, aluialniuni, banum, chraminm, iron, copper, xjnc, loaii-
gnnese, and nickel in all probahililyeiutintheaolarpfaoloapfaerc:
and the eoilium lino haa alio been observed in the ipecln of Ibe
sUn Aldnbaran, Algol, Belelgeux, and Pollux. It farther appean
from tbe obsM-Tationa of Meisn. Hug^na and Miller* that ihe
Bpeclrum of AJdebarar cnmpriBea the lines of hydropen, aotGom,
nuigneaiuiD, calcium, iron, biBinulh, tellurium, antimony, and
mercury; tbtt of a Orionia (Belelgeui), ■odium, ma^nedom,
calcium, iron, and hiBmuth ; and that of Siriua, hydrogen, ■odium,
ma^neHium, and iron. They haie alto observed that tbe spectra
of eonis nebuln consiist only of bright linea ; while otbera preaecc
1112. A remarkable unknown itar described aa " Ten- briOiant,
of about tbe second magnitude'' was observed bv Ur. J. Biminp.
hum, of Tuam, on 1B66, May 12 (perbaps tbe saute aa tbai
observed by Sir J. Hersobel on 1842, June 9), in the rainatelUlioii
Corona Burealis : it was observed by Mr. Baiendell, of Man-
chester, on Mar 15, and thtn appennMl of about the third sM(:ai'
tude; and by Messrs. HuirgiiiiBanJ Miller,ton May 16, to whom 1 1
llien appeared (o be conaiclerablT below tbe third magnitude, but
presented a spectrum unlike any previuunly obumM, Fig. 596,
conaisting of absorption -bandi like the solar spectnmi and ■!« ol
bright bands, like the gaseous spectra. Of the four bri|clit baoda
the brifihlest coincided with r, another between r and o, and i
fainter band nearly coincided with o ; the fourtb orcopied nearli
the position of c in the aolar apectrum : hencs the preanKC »i
iucandeioent hydrogen may be inrelred. Of th« dark baadisi
• Fiiii. Tiiu. IMS. t BiU. uag.
FLU0BE8CBNCE. 627
the lower end of the spectrum the principal were two above c,
one below d, and one nearly coinciding with d ; in the upper part
of the spectrum they were veiy cloee and numerous. The position
of the principal groups shows that the light of the photosphere
after passing through the absorbent atmosphere is yellow ; but the
light of the green and blue bright lines compensated the loss by
absorption in that portion of the continuous spectrum, and to the
eye tne star appeared neariy white. It is remarkable that Mr.
Bazendell, without knowing the results of prismatic analysis,
described his yisual impression to be " as if the yellow of the star
were seen through an overlying film of a blue tint.*' On May 24
this celestial apparition haa declined to below the eighth magni-
tude; having afibrded a conspicuous example of the amount of
otherwise unattainable information to be derived from spectrum-
aoalysis.
1113. A very remarkable action is exerted by several bodies on
light, to which attention was some ^ears since directed by Sir
John Herschel, who observed this action in a variety of fluor spar,
and in solutions of salts of two organic alkaloids, quinine ana
cesculine : it is best observed in a solution of disulphate of quinine
in water acidulated with sulphuric acid. The fluid, although
really colourless as water, disperses a lively blue light, which,
when examined by viewing it through a prism, appears ouite
free from the pure red rays, part of the orange, and all the yellow:
this was termed ^poUc dispersion (JiriiroX^, a surfaced from the
peculiar action having been supposed to take place at tlie surface
of the liquid. But as subsequent observations have shown that
this effect is by no means conflned to the sur&ce of bodies, the
term " epipolic" must necessarily be abandoned. The light trans-
mitted throueh the solution of quinine has undergone a physical
change, and is no longer capable of developing toe blue tint in
another portion of the same Kolution, or in any other body pos-
sessing a similar property. This may^ be shown by filline a glass
trough with water, and placing behind it a tube filled with &
solution of quinine, taking care, by screens, to cut. off all side-
light; the blue dispersed light will be beautifully distinct. Then
replace the water m the trough by a solution of quinine, and th«
blue tint previously visible in the tube will no lon^r be perceived.
It was likewise observed by the same profound pnilosopher, that
some opaque substances appeared to possess analogous properties
of reflecting ravs not reflected from other surfaces ; when, for
example, Uie solar spectrum is received on a piece of ivory, or tur-
meric paper, the lavender band (1087) becomes distinctlv visible.
1114. These, and some similar phenomena observed bv Sir D.
Brewster, led to a careful investigation of the subject by Prof
Stokes, from which has resulted the most important recent dis-
covery in physical optics, namely, a change produced by certain
substances in the velocity, and consequent refrangibility of the
B 8 2
628 CHROMATICS.
rajTB of light ; and not only of the ▼isible rays, but also of inviable
rays, far more refrannble than the visible spectrani, which are
thus rendered cognisable by the sense of vision, and the existence
of which was previously unknown.
The following substances have been found to possess the greatest
power in changing the refrangibility of rays : —
The mineral called uranite ; some salts of uranium, and glass
coloured by peroxide of uranium, commonly known as "canary
glass.'
An alcoholic solution of chlorophyll (the colouring matter of
A weak infusion of horse-chestnut bark.
A weak acidulated solution of disulphate of quinine.
A particular green variety of fluor spar.
Various red sea-weeds, and their cold infusions.
An alcoholic infusion of the seeds of Datura stramonium.
Paper washed with a pretty strong solution of quinine ; or with
a tincture of stramonium seeds, or turmeric.
Glass is found to be perfectly opaque to many rays of veiy
high refrangibility, which are trausmisMible through quartz;
therefore prisms and lenses of quartz must be employed in order
to obtain an extensive invisible spectrum. When the spectrum
formed by a series of two or three qnartz prisms in a dark room
is permitted to fall on a piece of canarp^ glass, or on a solution of
qumine or sesculine (the active principle of the horse-chestnut
bark), the sudden illumination of the fflass with bright yellowish-
green light, and of either of the solutions with that of a pale
bluish tint, presents a truly marvellous, it might almost be said,
a supernatural appearance.
1115. The spectrum of invisible rays of high refrangibility trans-
mitted through quartz prisms has been observed to extend beyond
the violet rays to more than double the length of the whole visible
spectrum ; Fig. 596 represents a map of the fixed lines in the first
half of the invisible spectrum, given by Prof. Stokes, in which he
has designated some more conspicuous bands by italic letters.
Fig, 696.
Group H. Group L Group w. Group «. Group p.
It appears from some further observations of Pro£ Stokes* that
the spectrum of electric light is peculiarly rich in invisible rays ;
having been ascertained by fluorescence in uranium-glass to extend
to six or eight times the length of the visible spectrum.
• Phil. Tnot. 1863.
FLUORE8CE2ECS. 629
Altboogli glasa is opatjne to the rajs of veiy high refrangtbility,
it transmits a Urge portion of those belonging to the yiolet re^ou
of the spectram, which are convertible by compounds of oranium,
and by the solutions above mentioned; hence results a ready
mode of exhibiting the more striking phenomena without the aid
of prisms or direct sunlight. This consists in closing an aperture
in the window-shutter of a darkened room with a piece of purple
or deep blue glass (some specimens of which answer better than
others) which transmits a very small portion of the lower and
more luminous rays of the spectrum ; a piece of canary glass, or
crystals of nitrate of uranium, or the solution of quinine or ssscn-
line, when held in the transmitted light, become instantly self-
luminous by emission of the converted rays.
1116. For a detail of the many very interesting experiments, as
well as of the various means of observation employed, the reader
must be referred to the original memoirs,* but the following are
the more important results : —
1. True internal dispersion (which has been designated by
Pro! Stokes dA fivorucence) is a totally different phenomenon from
the mere reBection of ordinary rays from opaque suspended par-
ticles, which might be termed /oJM internal dispersion.
2. In the phenomenon of internal dispersion (properly so
called), the retraogibiHty of rays is changed, incident rays of de«
finite refran^bility giving rise to dispersed rays of very various
refrai^bilities.
8. The refrangibility of any given ray is never exceeded by that
of any of the dispersed rays arising from it.
4. The colour of light is in general changed by internal dis-
penion, the new colour always corresponding to the new refrangi-
bility ; and this is equally true whether the incident rays belong
to the visible or invisible part of the spectrum.
6. The nature and intensity of ligbt, dispersed by a solution,
appear to be entirely independent of the state of polarization of the
incident rajs. Moreover, the dispersed light presents no traces of
polarisation, whether the incident rays be polarized, or otherwise.
6. The power of changing the refiiangibuity of rays appears to
be possessed b^ a great maoy bodies, especially b^ organic sub-
' stances, in which it is almost always manifested in a greater or
less degree. Of this the crvstalline lens is a conspicuous example.
7. ^e phenomena of fluorescence oppose fresh difficulties to
the supposition that the luminous, chemical, and phosphorogenio
rays are of a different nature ; but they are perfectly conformable
to the supposition that the production of light, of chemical action,
and of phosphorescence, are merely different effects of the same
cause. The phosphorogenic rays of an electric spark which, ^ as
it is alread;|r Imown, are intercepted by glass, appear to be nothing
more than invisible rays of excessively high reuiingibility, whi^
• 7hiLTruH.1668,pwt8s and 18S3, pwt 1.
630 CHROUATICi*.
there is no reason for supposing to be of a different nature from
the rays of Hpfht.
1117. Both iluorescence and spectnim-analysis have been ap-
pealed to to demonstrate the wonderful rapidity with which
certain substances taken into the stomach are absorbed, and de-
posited from the blood in eyery part of the system, even in tho
non-vascular tissues. Thus quinine has been detected in the
human crystalline lens, remoyed on account of cataract, 2) hours
after it had been administered : and two grains of chloride of
lithium having been given to a guinea-pig, after 6 hours the
lithium lines were visible in the combustion of alt the tissues.
In another experiment no trace of lithium could be detected after
six days : it appears that the removal of any particular substance
from the various tifsuos by absorption is by no means so rapid «
process as its deposition.
1118. Photphoreaeence, — ^The property of emitting light, either
spontaneously, or in consequence of exposure to intense li^ht, or
to a moderate heat, is \ffTmeA phoaphorescenoA. The luminosity
of phosphorus, of the glow-worm and fire-fly, of various marine
moilusca, and of decaying animal and vegetable matter, is probably
of a chemical ori^n, and might be called cAemico/ phosphorescence
in contradistinction to phyncal phosphorescence, produced by the
agency of light and gentle heat. The phenomena of phospho-
rescence, produced by the action of light, appear to be intimately
related to those of fluorescence: while the phosphorescence
resulting from the application of gentle heat, as in the minerals
apatite and fluor spar, appear to be eoually related to certain heat-
pnenomena to be nereafter described, and which Prof. T^dall
lias designated as " calorescence.*' Of the latter substance it may
be remarked, that, after having once been rendeVed phosphorescent
by heat, it will not again phos^oresce under similar circumstance^
until an electric spark has been repeatedly passed over its surface.
In this fact ma^ be recognised the intimate relations existing
between electricity, light, and heat. It is very questionable
whether these phenomena of fluorescence by heat, and ordinary
incandescence are not identical ; differing only in the temperatura
at which in^ different substances heat-motion impressed on the
molecules is imparted as light-motion to the surrounding medium.
The character of the rays emitted bv a phosphorescent bodr
appears to be quite independent of the character of those by which
this peculiar property is called into actfon : thus, a portion of cal-
cined oyster-sneli, if placed in the red, yellow, or violet rays of the
solar spectrum, will present those respective colours to the eye — *
that is, the rays reflected by the superficial particles will be un-
altered ; but when removea from the spectrum, it will in either
case emit, for a short time, the same pide light, thus showing that
a portion of the various coloured rays has undergone the same
change by the molecular action of the substance on which they
CHROMATIC ABEBRATION. 631
impinged. The sulphorels of the alkaline earths, strontia, baryta,
and lime, are amongst the most powerful phospbori ; from these
M. £. Becquerel obtained, by change of temperature and other
appropriate treatment, the yarious clours of the spectrum in their
pnosphoresoent emanations. In some bodies, the property of phos-
phorescence is extremely transient ; but the existence of this pro-
perty in several of the most powerfully fluorescent bodies hcM been
ingeniously demonstrated by M. Becquerel, by means of the phot-
phorotcope. In this instrument, a small vertical cylinder about
one inch in diameter, and six or seven in length, and capable of
being put in rapid rotation, is so placed at the angle of a dark
chamber, that atwut one-fourth of the circumference is presented
inwards, the remainder appearing externally. When the chamber
was illuminated with the electric light (808^, and the surface of
the cylinder covered with any of tne eartny phosphcri above
mentioned, a moderately rapid rotation suflSced to render the whole
of the exposed surface of tne cylinder equally luminous. With a
little increased velocity of rotation, the same result was obtained
when the surface of the cylinder was covered (by evaporatinj^ h
solution) with sulphate of quinine, or with SBSculine. When simi-
larly covered with nitrate of uranium, a very rapid rotation (not
less than 300 revolutions in a second) was required to develope the
phoephorescent effect, and even then not more than half the exposed
surface of the cylinder was illuminated, the light appearing like a
beautiful band of lambent flame emanating from the aperture. It
would thence appear that in this substance the duration of phos-
phorescence could hardly exceed the thousandth part of a second.
The very attenuated media in some of the tubes prepared by
Geissler and others for exhibiting the remarkable stratifications
in the electric discharge (811, 812), are highly phosphorescent, as
shown at the moment of the cessation of the discharge : and the
luminous trace of a flash of lightning has been ascribed by
Faraday* to the phosphorescence of the oxygen in the atmosphere.
1119. When bght passes through a prism, it is resolved into a
series of coloured rays, of which the more refrangible are bent
towards the base of the prism (1082) ; but when it passes through
lenses, an analogous resolution into coloured rays is not so readily
observed, although it does exist, and to so great a degree as to
interfere most seriously with the perfection of microscopes and
telescopes, causing the image
to be tinted at its edges with ^' ^^'
various colours, the result of
ehromatic aberrcUum,
The section of a convex lens
may be represented by two
CUB A, B, Fig. 697, placed
to base, and that of a
• Phil. Magasine, June, 1867.
632 CHROMATICS.
ooncAT6 lens by two others c, d, with their apices id contBci. On
a ray of light being incident upon snch elementary prisms, it Qn-
dergoes refraction and resolution into coloured rays ; and the moat
refrangible, the yiolet rays v, y, are brought to a focus nearer the
lens, and the least refrangible, or red, b, b, to one at a ereater dis-
tance ; so that, on placing a piece of paper at e f, the image of
the sun or other luminous body will be seen surrounded by a Tiolet
or a purple border, which will be replaced by a red one on moving
the paper to o h.
1120. The greatest improTement ever mnde in optical instni-
ments consists in the discovery of achromatic lenses ; these are
formed by combining a concave and a convex lens, constructed of
substances of different dispersive powvrs
Fig, 698. (1 101). Thus, if a convex lens made of crown*
glass, of which the dispersive power is 0*089,
be combined with a concave lens of flint-glass
in which the power of dispersion is 0'048, a
componnd.lens will be constructed capable of
refracting white light to a colourless focus.
This combination would be perfect, if the coloured bands prodnoed
by prisms of these two glasses were respectively of equal breadth ;
but, in consequence of the irrationality of the spectra (1098), this
perfect neutralization of tint takes place only with toe extreme
rays, the violet and red ; the intermediate ones imperfectly de-
stroying each other, cause the object viewed through such com-
pound lenses to be bordered by fringes, which, however, are so
faint that for all ordinary purposes uie combination may be con-
sidered as achromatic. By employing certain fluids, as hydro-
chloric acid, confined between two lenses of crown-glass, Dr. Blair
overcame this remaining difficulty, and obtained a compound lent,
perfectly achromatic for the intermediate as well as for the
extreme rays.
In order to produce a more perfect degree of achromatiBm in
the object-glasses of telescopes, a combination of three lenses is
frequently employed : for this purpose a flint-glass lens is placed
between two of plate-glass ; the adjacent surfaces being cemented
with Canada balsam, in order to prevent the loss of light by
reflection from so many surfaces. The course of the rays will be
better understood by a reference to Fig. 591, supposing a second
and thinner crown-glass prism to be placed in the same direction oo
the contrary side of the prism of flint-glass. The rav in its passage
through the two prisms there represented is dispersea without being
refracted, and if the angle of the third prism be such that its disper-
sion will be just equal to that of the other two, but in a contrarv di-
rection, the ra^ will finally emer^ refracted, but almost colomieaSw
In the object-glasses of microscopes, owing to the large
difference between the angles of incidence of the central and
peripheral rays on the first surface, it is impossible to correct the
IKTEKFERBXCE OF BATS. 68S
aberrations by a single combination ; it has therefore been found
desirable in practice to employ two achromatic combinations in
" objectiYes*' of 0*7 inch or of any greater focal length, and three,
in tnoee of 0'5 inch, an^ all higher powers. The principal part
of the magnifying power is always thrown on the external or
anterior combination, and the correction of the aberrations on the
posterior ; the spherical, chiefly by the middle one.
1121. When two or more undulations act simultaneously on a
particle of matter, it oscillates with an intensity corresponding to
the combined force of the undulations : the same thing occurs,
provided the latter are of equal length, or differ by a given number
of entire undulations, even when they emanate from different
sources. But if the waves actine on a particle differ by any odd
nnmber of half undulations, they interfere and oppose each other^s
action, and thus actually produce partial or total darkness. This
may be rendered more intelligible by drawing two sets of waves
containing the same number of undulations, as ▲, b, Fig. 599 ;
any particle at o must be made to assume a
movement corresponding to the combined -ZV* fr-
action of A and B, and a corresponding
intensity of light will result. By alter- ^
ing the relative position of a, b, so that ^
A may begin one half an undulation later \^
than B. as at a', b', it will at once be ^
been tnat they will be always in oppo-
site phases, and any particle at d will
be acted on in opposite directions by a' and
tl ; for whilst the impulse at w is from right
to left, that at e is in an opposite direction,
and tnese mutually opposing eaOh other, the particle^ at c will
remain at rest — darkness thus results from the conflict of two
luminous undulations. If the waves of light, instead of meeting at
the end of an entire half-undulation, encounter at any fractional
part of one, partial interference will ensue, and colours will be
developed, bearing a relation to the length and velocity of the
undolations remaining undestroyed.
1122. It lias already been seen that the interference of sonorous
undulations produces silence (549—554) ; and in the extension of
this fact to luminous waves, there appears a striking analogy
between the oscillations of li^ht and those of sound, the difference
being rather in decree than in kind. The alternately increased
and diminished effect of combined luminous undulations, bears a
remarkable analogy to the beats in music (549), which are pro-
duced when sonorous vibrations, differing in their rapidihr by a
fractional portion of the. period of either wave, interfere, ana by so
doing alternately intensifjr and diminish each other's eflfocts.
1123. An ex^rimental demonstration of the interference of lu-
minous undulations may be obtained by an apparatus first pro-
634 CHROMATICS.
posed by M. Fresnel. He allowed a ray of liglit to fall in the direction
of p c, Fig. 600, npon
±tg, eoo. g^ prism, c, with an cx-
' -^ ceedingljobtuse angle.
^^x^^TSif^^j^-^vv-^ Then the eye placed at
^-'^'^'^^^^^Ty$wOO&'&0^^».AJLS&^^ A will see the radiant
"^^^ — >yVyW^X^^ point double, appa*
^*^**-v^^*^^^^^4^^^^ ^ rently in the directions
^^_^^^:J-it7r^^*^ A B, A D, and between
D- — '"" those two points, a
series of dark and
bright lines perpendicular to a line joining the two images. If
homogeneous light (1082) be employed, as that from a spirit-lamp
with a salted wick, the lines will be alternately yellow and black ;
but if common light be employed, they will be tinted with the
prismatic colours.
The explanation of these colours is not difficnit ; the two images
B, 1), may be regarded as the centres of two series of undulations;
but as has been shown in the case of two series of waves in water
(466), when these undulations meet in the same phase, lisrht is
developed, and when in opposite phases, interference is proanced,
and darkness or coloured light occurs, according to the interfering
waves. In the above figure, the dotted curves show the poaitions
of the opposing, and the entire curves those of the coincidinff un-
dulations. The series of dots show the points where lummoas
interference occurs. A fine eacperimentum cmcis, proving the real
origin of these dark bands, is made by covering up one half of
the prism ; the interfering rays are then cut off, and the bands
instantly disappear.
1124. An interesting set oF illustrations of the doctrine of lumi-
nous interference is met with in the phenomena of difou;tion dis-
covered by Grimaldi, a Jesuit of Bologna. To observe these pro-
perly, a l)eam of diverging^ light is necessary ; this may be obtained
by making a small hole in a window-shutter, and receiving the
light on a screen at the distance of some feet. If a convex lens,
of small focal length, be fitted in the hole in the shutter, the li^t
is refracted almost to a point, from whence it diverges in a manner
very well fitted for experiments on difi&action. For small experi-
ments, a p>|ramidal box, a b d c, Fig. 601, about two feet long, and
blackenea inside, may be advantageously employed ; at e, a convex
lens, of an inch focus, is fixed, on which, by means of the plane
mirror o, a sunbeam can be readily thrown. The light is refracted
by the lens to a point, and then diverging, is received on a sheet
of white paper placed at the bottom of toe box ; by means of a
door shown at f in the section, the bottom becomes easily visible^
without admitting any considerable quantity of extraneous light.
1 125. If any small opaque bodies, as hairs, pins, &o., he held in
the beam of diverging light, bl l, their shadows will be thrown
DIFFRACTION OF LIQBT.
635
Fiff, 601.
on the bottom of the box, wirrounded by coloured fringes. If h be A
section of a pin thus exposed, the fringes
nre seen surrounding its shadow, a^
though thej were produced by coloured
rays passing by its margin, notm straight
lines, but in hyperbolic curves, as shown
by intercepting them at different dis-
tances by a piece of card ; when their
decrease in extent will be found to be
much more gradual than if the light
passed by h, in right lines ; this dis-
turbance of the rectuinear course of the
rays is termed diffraction, Besidesthese
external fringes, there are internal ones
within the shadow, which, if the body be
narrow, as a pin, becomes completely
filled with them. These colours are, as
Lord Brougham* has long since shown, in harmonic proportion,
like those of the solar spectrum. The tints of the coloiued fringes,
reckoning from the shadow, succeed each other in the foUowmg
manner : —
1st fringe — violet, indigo, blue, green, yellow, red.
2nd fringe — blue, yellow, red,
3rd fringe — ^pale blue, pale yellow, red.
If homogeneouM light (^1082) be employed, the fringes will be of
the same colour as this li^ht, and their intervals will appear black.
The fringes are broadest in red, narrowest in violet, and of inter*
mediate breadth in the other colours of the spectrum.
1126. These phenomena admit of ready explanation on the theory
of interference (1121), forwhen the diverging rays which are infletied
on one side of the pin, meet those which are inflected on the oppo-
site side in the same phase of undulation (369), thev coincide, and
produce a line of white light, which ought to occupy the middle of
the shadow ; whilst rays which differ in their paths, as those
which pass obliquely past the pin into its shadow, meeting with
those which pass more directly on the opposite side, encounter
each other under different phases, and interfere, either producing
darkness, as when homogeneous light is used, or so interfering as
to produce a coloured frinffe.
1127. In shadows of this kind, formed by narrow bodies, the
middle is always occupied by a luminous Hue, as though the light
had passed directly through the centre of the diffracting body.
This very curious fact is best observed by holding a small disc of
metal on a slip of glass, in the diverging pencil (1125) ; the rays
passing bv its circumference are inflected, and meet after traversing
equal paths in similar phases in the centre of the shadow, producing
• Fhfl. Tnwa. 1796.
A brilliant spot of light ; tlie shadow thus precigely lewinUM lliat
of a circular diec perforated in the centre. This beaatiM «xperi-
tneut is best perfonoBd b; meuiB of a drop of thick black Ink, or
a mixture of Ump-black and siro, placed on a plate of glaio, ao aa
to form a circalar spot about the tenth of an inch in diameter :
this modificatioD ot the original experiment of F^«tnel waa Di-
gested b; the late Prof. Powell.
1128. Ifa diBc, perforated with a verT imatl hole in th« centre,
be held in the beam of diTerging light (IISS), the conTerae of tbe
Ust experiment will be obserted: forthoee andnlationa which paM
directl,v through the aperture, interfering with thon paaamg more
ohIiqaeN, produce a dark spot on that part of tha shadow cor-
responding to the hole in the disc. Thui we find light TirtaaUf
changed to darkness, and darknen to light, b; the tfuoord or om-
a/rd nf the luminous waves.
1129. If two knife-edges be held very near each otber in the
diTorgent beam, beautimllT coloured fringes will be obserTed to
bordur their shadow, and a lUrk line will, IT thej be
tfj. Wi. sufScientlj near, be seen to occnp; the middle of tbe
space, at which the; are reall; aepaiate. This result
of luminous iolorference may be readily sbown by
placing a slip of lin-foil On a plate of glass, dividing
It longitndinall;, and very slightly separating the
divided portions at one end, so that they mar fonn a
very acute ansla with ea<;h other, as in Hg. 602.
Let this be held in the diverging light of the apparatus
before described (ll!d), about six inches rran tbe
10 that it may form a well-deGned shadow. Tha a
ponding
hns will be covered with a beantiful set of fringes
diverging from each other as they approach Uie
Bpeiofiheacnteangle, r, Fig.60iS,andbi ' '
on eacb side b; hyperbolic curves, with
parts of the carved fringes oorrespond to the apex
of the angle fbrmed by the slips of tin-foil. In
the figure, raa represents the pngeotioo of tb«
slit in the foil on the paper on wnioti tho ahadaw
falls. ThiBexperiii>entiBaneM7,althiMwhRingji
mode of repeating Hewton's obaenation* with the kniie-edKoa.*
1130. The explanation of the production of colours bydiDndMn
(1125) is well illnstrated by placing a card on onende, and cm a
pUne above or below the bed; h, Fig. 601, so as to intercept
some of the incident or diSiiicted light: the fringe* then dis-
appear, because one set of the undutaijona producing intsrfemiee
■Optlog. Ub.lil.pusf. obLia
DIFFRACTION OF UOUT.
637
Fig.Wi.
has been cut oiT. If a transparent body be substitated for the
card, the fringe# lindergo a remarkable change, from the retardo'
Hon of those uddalatious which are propagated through the traus^
parent screen.
1 131 . The beantifnl phenomena of diffraction may be easily ob-
served by Yiewiog, in a darkened room, through a piece of the:
finest copper-wire gauze, a series of objects, Fig. 604, fixed in a laree
screen of black pasteboard, so placed as to prevent any light reach-'
ing the eye, except such as passes through the object.
The following give particularly interesting results.
A. Fix six sewing-needles over a hole cut in a
Siece of blackened wood, taking care their mutual
istances correspond to the thickness of a needle.
On viewing this object at a proper distance through
the gaujse, each needle will apnear transparent, the
centre of each being occupied oy a line of reddish
light.
B. Examine in a similar manner a piece of tin-
foil, in which a fine slit, about one-twentieth of an
inch wide, has been carefully cut. The slit will
seem widened from light entering the shadow, its
centre being occupied by two vertical lines of
bluish black, whilst a series of coloured bands will
extend to a distance of half an inch on each side
of the slit
C. Make a line of small holes in a piece of tin-foil
by means of a fine needle. Each hole will appear
bordered with a reddish margin, whilst a series of
spectral coloured openings will appear in the foil
for half an inch on each side of the real aperture.
D. Perforate a piece of tin-foil with a very fine
needle, as shown in the figure. Each opening will,
when examined as above, present nine coloured
squares like a window, and the spectra will be so
numerous, that the foil will appoar full of holes,
admitting li^ht of different colours.
A convenient form of apparatus has been de\ised by Mr. Bridge,
consisting of a lens placed in an aperture in a dark screen, on
which direct sun-light is reflected by a mirror. When the image
of the sun in the focus of the lens is viewed, by a telescope, through
transparent apertures of various shapes in an opaque film of col-
lodion, produced by photography, the effects produced are ex-
tremely varied and beautiful.
1132. The brilliant tints of soap-bubbles, and of thin plates of
various transparent bodies, afford further examples of interference
of light : for the undulations reflected from their first surfaces
interfere with those reflected from the second (1034); and upon
the amount of retardation thus experienced by the luminous
638 CBB0MATIG8.
wayeB, the yarietieB of colours observed in these thin platos depend.
The colours of soap-bubbles are well seen bj boiling a small
quantity of soap with distilled water in a bottle, and corking it
whilst boiling hot. The whole being secured from air, is allowed
to cool, and on adroitly shaking the bottle, a laige bubble, pre-
senting the coloured bands with great beauty, maj be readilj
fonned ; this bubble is permanent for several hours, and afibrdi
every facility for examining its tints. Soap-bubbles may be blown
from a mixture of soap and glycerine which will last for a oood-
derable time without bursting.
1133. The colours of thin plates of air may be obsenred by
{>reRsing a convex lens on a plate of glass, and nolding it in the
ight, so that rays reflected from it will pass to the eye. At the
point of apparent contact with the lens and ^lass, a black spot
will, under these circumstances, be visible ; this is surrounded oy
a great number of rines of different colours, each series of tints
consisting of fewer colours as they recede from the centre. On
holding the glasses between the eye and the light, a set of rings
will be observed, differing in colour from those seen by reflection ;
and complementary (1091) to them, each ring possessing that
colour, which by mixing with the tint of the corres]^ndiDg re-
flectea ring, would produce white light. The following are the
colours of the rings, observed by reflection and transmission, com<
mencing from the centre or point of apparent contact, as given by
Newton.*
By reflection; — ^Black, blue, white, yellow, red, yiolet, blue,
green, yellow, red, purple, blue, green, yellow, red, green, red,
greenish-blue, red.
By transmission; — ^White, yellowish-red, black, violet, blue,
white, yellow, red, violet, blue, green, yellow, red, violet, greenish-
blue, red, bluish green, red.
1134. Tlie following table contains the thicknesses, expressed
in millionth parts of an inch, of plates of air, water, and glass, re-
quired to produce the different coloured rings : —
By aid of this table, the thickness of thin Alms of air, water, or
glass may be readily determined by observing the colours they
reflect. The comparative thickness of plates of two substances,
reflecting the same colour, is in the inverse ratio of their indices
of refraction (1067).
These rings may be exhibited by merely placing together two
plates of window- glass, about four inches square, and pressing
them in the centre by means of a pointed piece of metal. The
different coloured rings, somewhat eccentrically arranged, will
appear with great beauty around the point where the presaare is
applied.
1135. When these rings are observed by homogeneom light,
they present the same hue as that of the light itself, alternating
* Optice. Lib. ii. pars S.
C0L0UB8 OP THIX FLATE8.
639
Berieaor
orders of
ooloors.
Oolonra leen bj relleotion.
First . . <
/Very black
Black
Blackish
Pale sky-blae
White (like polished silver) . .
Straw-coloar
Orange red (dried orange-peel) .
^Red (geranium sanguine um)
^Violet (vapour of iodine) . . .
Indigo
Blue
Orange (fresh rind of oranges) .
Bright red
vDusky red
Third
Purple (flower of flax) .
Indigo
Prussian blue ....
Grass-green . . . .
Pale yellow . . . .
Bose-red
V Bluish- red
Fourth .
Fifll>.
Sixth .
Seventh
'Bluish-green .
Emerald-green
YKllowishgreen
,Pale rose-red .
fSea-g^en . . .
Pale rose-red . .
(Greenish-bine . .
Pale rose-red . .
(Greenish-blue
Pale reddish-white
Thioknesa of pUtes
producing them.
Air.
0*50
1-00
2-00
2-40
5-26
711
8-00
900
1117
12-83
1400
1612
16-29
17-22
1833
19-67
2100
2210
23-40
26-20
27-14
2900
3200
3400
3629
3600
40-33
4600
52-60
58-75
6600
71-00
77-00
Vaster.
0-38
0-76
1-50
1-80
3-88
6-03
6-00
6-76
8-38
9-62
10-50
11-33
12-20
13-00
13-75
14-75
15'75
16-57
17-66
18-90
20-33
21-75
24-00
25-50
26-50
"27-00
30-25
3410
39-38
44-00
48-75
53-25
57-67
Glass.
0-33
0-66
1-30
1-55
3-40
4-60
517
6-80
7-20
8-18
900
9-70
10-40
11-11
11-84
12-66
13-06
14-25
1610
16-26
17-50
18-70
20-66
2200
22-80
23-22
26-00
29-66
3400
3800
4200
45-80
49-66
J
with dark and almost non-luminous rings; and thej appear to have
the greatest breadth in red, and the least in violet light. These
640 CBBCUATICS.
rings appear to be larger, in proportion as they are seen io a
more oblique direction ; this is best seen by examining the rings
produced, when the slant side of a rectangular glass prism is pressed
on the surface of a convex lens of yenr small curvature.
The coloured rings thus exhibited by thin plates, are produced
by the interference of the light reflected by the first surface with
that reflected from the second, for when either of these reflected
rays is intercepted, the colours cntirelpr vanish.
1136. The rings seen by transmission are produced by those
undulations which are not reflected, and are consequently pro-
pagated through the thickness of both glasses. Those luminoos
ravs, which, when combined with the reflected rays, produced
white li^ht, being propagated through the glass, produce the
transmitted, or complementary (1133) rings.
From Newton's table (1135), it appears that air, at or below a
thickness of half a millionth of an iuch, and water and glass at a
thickness of about ou,e-third of a millionth of an inch, cease to
reflect light, and ajvpear, consequently, black. Films and fibres of
quartz, so minute as to be incapable of propagatinglnminoas undu-
lations, have been met with and described by Sir L>. Brewster.
1137. It is by no means necessary that very thin plates should
be used to exhibit colours, for plates of any thickness, so arranged
as to cause the interference ofluminous undulations, will produce
the same effect. This may be shown bv fixing two slips of plate
glass about 0*1 inch distant from each other, by means of two
pieces of wax, and then by pressing one end of each plate together
they may be so fixed as to describe a very acute angle with each
other. On looking at a candle through that part^ of the plates
nearest each other, numerous reflected images of it will become
visible : the first of them appears crossed by a series of beautiful
bands or fringes. These increase in breadth by diminishing the
inclination oi the plates ; they are produced by the interference
of the waves of light reflected from tne two surfaces of each plate,
1188. The coloured rings observed by regarding the sun, or
other luminous body, through a piece of glass covered with minute
particles, as of dust, lycopodium, &c., or of water, by breathing
on it, are all owing to the interference of luminous undulations
infli'cted round the particles (1125). A similar explanation will
apply to the colours seen by scattering fine powders or dust on,
or before, a mirror exposed to the solar rays. The beautiful tints
presented by mother of pearl, and other natural or artificial sub-
stances of which the surfaces are marked by minute strise, are all
explicable on the hypothesis of interference ; all that is requisite
to produce these colours being, that the depression shall be of anch
a aepth as to cause an alteration in the paths of rays incident
upon them, equal to some aliquot part of tne length of an undo-
lation.
1139. If a series of reiy fine and close parallel lines be roled
THB SAIKBOW. 641
apon a sar&cei the reflected light will be oolonred hv thoee tajB, of
the length of the undaUtionB of which the inter valB of the lines are a
multiple, the others being partially destroyed bv interference ; the
colours thus produced have been ornamentally applied in the
manufacture of what were well known as Barton* t btUtons; but
philosophy in buttons not having been appreciated by the public,
it has long been difficnlt to obtain a specimen.
An elegant confirmation of the undulatory theory is due to the
ingenuity of M. Robert, who has succeeded in ruling bands of lines
on glass so exceedingly fine, and truly equidistant, as to reflect
well-defined prismatic colours. The bands are about 12 in number,
of which the widest reflects the coloora of the lower end of the
spectrum, and four or five of the narrowest reflect no colour, being
multiples of the wayes of invisible rays. If the piece of glass,
which is suitably bevelled at the edge, be now so j^aced that the
incident rays mav pass through the edge, and be received by the
eye after internal reflection, the ravs are retarded by their passage
tnroagh the denser medium, and the higher rays of the spectrum
(using the terms higher and lower in relation to the refrangibility
of rays^, are now reflected from the narrower bands, all the coloured
rays aavancin^ in the series of bands, according to their degree of
retardation. Nobert's lines form an extremely interesting micro-
scopic object.
1140. Among the natural phenomena which serve to illustrate
the laws and principles laid down in this and the preceding
chapters, the well-luiowii rainbow, and less frequent mirage,
especially deserve attention. The former consists of a coloured
arch, apparently suspended in the sky, and opposite to the sun, and
is frequently composed of two bows, termed primary and secondary,
and sometimes even of other supplementary arches. The rainbow is
never seen unless a shower of rain «^^ ^f^^^
is falling, or the spray of water,
as from a cataract, rising between
the spectator and that portion of
the sky opposite to the sun. To
explain the cause of these bows,
let B, F, Fig. 605, be two drops of
water, and sb, sf solar rays inci-
dent npon each of them, then those
which enter near their centre will
be refracted to a focus, as in a
sphere of glass (1069) : but those
which enter near their upper part
suffer refraction, daring which the light is resolyed, as in pris-
matic refraction (1082^, and colours are consequently produced.
And such of these refracted rays as are incident at the back of
the drop, within the limiting angle (1060,) there undergo total
reflection, and emerge at the lower part, as c, in the drop b. But
T T
642 CHBOUATICB.
most of these small pencils of coloured rays are diTereeDt
after they emerge from the small aqueous spheres, and blending
with one another, reproduce white light. There is, however,
one particular angular position for each prismatic colour, in which a
globule of water may be placed in relation to the sun and the eye,
uiat a parallel pencil of that colour may emerge parallel, and reach
the eye ; and as that position will vary for the different colours,
in proportion to the aifference of their refractive indices (963),
the colours will, as in the prismatic spectrum, be presented
in succession to the eye. Moreover, all points in the circumfe-
rence of any circle described round a line joining the sun and eye,
as an axis, will be similarly situated in relation to the sun and
the eve ; consequently, the several colours will appear in a circular
arc, of which the centre is some point in a line drawn from the sun,
through the position of the eye, and present to the spectator a
bow of the prismatic colours, bounded alK>ve by the red, and below
by the violet rays.
The several coloured solar rays that enter the lower hemisphere
of the drops of rain in certain positions, as at o, B, are retncted
to the back of the drop, undergoing the same resolution into
coloured rays, hence they are successively reflected to the top, and
to the front of the drop, whence, in one particular angular posi-
tion of the aqueous globules for each colour, thev emerge parallel,
and reach the eye, presenting to the spectator the appearance of
a second bow, exterior to the first, and with its tints much fainter ;
and reversed in position, in consequence of the rays having suffered
two reflections m o, h, whilst in k, f they underwent but one.*
The phenomena of mirage arise from the reflection of terrestrial
objects from the common surface of two or more horizontal strata
of atmosphere of different densities. When the rays fall on llie
surface of a rarer stratum at a very large angle of incidence, as
when a spectator is looking towards the horizon, they will fre-
quently exceed the limiting angle (1060), and total reflection
will then take place. The objects from which the rajs proceed will
then appear to be inverted ; sometimes both an erect and an in-
verted image of a distant object will become simultaneously visible.
* For a fall in^f stigaUon of the phenomena of the rainbow, and of the
sparious bows frequently observed to acooropany it, the reader is rererred to
a memoir by Prof. Miller, in the seventh Toliime of the Caaibrid|{e Philo-
•ophiotl Transactions.
643
CHAPTER XX.
OPTICAL 1N8TBUMENT8.
1141. Optical instniments maj be divided into the catoptric,
including thoee depending upon reflection ; the dioptric^ or those
acting by refraction; and tnose depending on the combination
action of both effects, or cata-dioptrie instruments. Of optical
instruments depending on reflection, the various forms of mirrors
already described constitute the most important. The common
looking glass, the theoretical action of \Knich has already been
explained (1036), is too well known to need description ; and the
convex mirror, formerly a common ornament in large rooms, is
chiefly employed on account of the diminished images of objects
which it produces (1045), and thus the whole extent of a landscape
becomes, as it were, compressed into the space of a few square
inches. The concave mirror is a very important instrument, and,
besides its application to science, it has formed one of the most
valuable resources of charlatans and jugglers, on account of the
power it possesses of forming in the air an image of any object
5 laced beyond its principal focus (1044). Thus, if any object, as a
agger, strongly illuminated, be held towards a concave mirror, an
image of it will be formed in the conjugate focus, so vividly and
periectly painted in the air, that the person who holds the dagger
can scarcely believe that the weapon which advances to meet mm,
is but a spectral image of the one with which be is armed.
1142. The most important application of concave reflectors is
to the construction of telescopes, in which the image of a distant
object, as one of the celestial bodies, is formed in the principal
focus of a concaye mirror, and magnified by means of conyex
lenses (1080). The sim- jp. ^^
plest reflecting telescope is
that constructed by New-
ton in 1666: this consists
of a concave parabolic
(1050) metallic reflector
AB, fixed at the end of a
tube CDB Fig. 606. A
small plane mirror, in-
clined at 45", or, still bet-
ter, a rectangular prism p, is fixed in the tabe, between the specu-
lum A B, and the image formed in its focus. The image is thus ro-
T T 2
644 OPTICAL IHSTXUlfERTS.
fleeted towards an opening in the side of the tnbe, where it is
viewed through a convex lens for the purpose of magnifying it.*
The advantage of a prism over a plane mirror, for the purpoM of
reflecting the ima^ of the distant ohject towards f, is sufficiently
ohvious, for, hy tntenuU reflection (1060) from the back of ^m
prism, nearly all the rays are reflected to the eye; whereas, if a
plane metailic speculum were substituted, about 45 per cent, of
the rays would be lost (1034), from the undulations producing them
being absorbed on reaching the surface of the metal. For the
purpose of preventing spherical aberration (1081) from interfering
vnth the distinctness of the image, Newton placed a plate of metal
pierced with a small hole between the eye and the convex lens,
through which he viewed the object
1143. The Gregorian reflecting telescope was invented in 1660,
by Dr. Gregory, but not actuaUy constructed until some years
subsequently to that of Newton. In this instrument, the iDcoooh
venience of taking a lateral view is avoided ; it consists of a oon-
cave speculum fixed in a tube, but pierced in the centre with a
hole, through which, by means of a lens, or a comlnnation of
lenses, the image of the object is viewed. The rays after fonning
an image of the object in Dr. Gregory's telescope, are received
on a small concave mirror, placed opposite the aperture in the
larger one, and form a fresh image which is viewed through
that aperture. The observer, in using this telescope, is placed
in a line with the object, whilst in Newton's, he is at right
angles to it.
1144. When a cohvex mirror is substituted for the small con-
cave one in Dr. Gregory's instrument, we have Caasegrsin's
telescope. In this, the magnifying power is less than in either
of the preceding, but the image is more (tistinct than in any other
construction, as but one image is formed ; and as one speonlnm is
concave, and the other convex, they have a tendency to ooirect
each other's spherical aberration.
1145. In astronomical reflecting telescopes the Newtonian cod>
struction has generally been adopted. The oUiq^ue mirror has
hitherto been supnorted by a single stem from the side of the tube,
the vibration of wnich in some degree impairs the definition of the
image : in telescopes of very large dimensions, as in those of Sir
W. Herschel and the Earl of Rosse, this inconvenience is obviated
by placing the speculum a little obliquely, and thus bringing Ibe
image to the side of the tube ; but a sl^ht loss of definition from
oblique reflection is thus introduced.
Astronomical telescopes of lai^ size have generally what is
called an eqiuitorial mounting; in this the telesoope is attached
transversely to a rotating axis, which is acy^B^d parallel to the
polar axis of the earth. If this, the polar axis, be made to rotate
(frequently by clockwork) in a direction opposite to that of the
* Newton, Optioe, lib. i. prop. 8, prob. S.
645
eutb's rotation, and nilh u equal velocitT, it ia clear that • fixed
hsavenl^ body, as a »tar, once m tbo field of the ingtrumBnt will
remain ia it ax ioog aa maj be required ; bnt if ■ montUe ^oif,
tm the mooD or a planet, be under obeenation, theta Itt owti Mn^
tootion, aa weQ ae the earth's rolatioa, must be compenmltBd by k
notion ^vea to the inatnuQent, in order that tki objeei t!a*^
AatroiKHDioal reflecting tele- Flf-ttt.
derate coat hare been c
■tmcted hj Hr. Browning, one of
which ia repreaented in f>g;. 607.
The Btand here ahown ia called
an "eqoatoHal moDnting :" a
firmly fixed iron pillar, a, mp-
porta the honr-cinJe, c, which
1 by meai
-,- ._.„ , - The j_
clination-aiia, d, ia at right angrliM
to the polar axii ; to one end of
thia the teleacope ia attached, tai
a bMTy maaa of metal, B, at &
cnunteipoiae to tha other end I
the position of the teleeoope ia de-
termined by the KjadaalAil circle
•nd vernier, h. The specnlom, a, ~
U pUced in the bottom of the teleacope, and the ej^pjece, ■, u
Kiewad into a rotating cylindrical cap, in order that it may be
tnmed in any direction moat conTenlent for obeerration. A amall
t«leacope, r, i* attached paiallel to the aiia of the laiver one,
haying croas-wirea in the centre of the field of view ; this la called
■ " finder." When correctly adjnrted, ar^ object brought to co-
incide with the ptnnt of croaaing will be m the field of the large
teleacope.
1146. The apecnlnm employed in thia teleacope ia one of silvered
glasa (1062), the bottom of which ia made perfecCly plane ; thin ia
clamped by a ring into a oell (be bottom of which is also tralj
plane; the apecnTaiii may therefore be removed fur cleamDg or
— I'lhiag, and replaced, wilhont doatroying its adjuatment
„„ „ -,Fig,6 ,
diao it along bv three piecea of strong obronometer-balanoe-ipring
' wire, pUoed edgewise towarda tbs Bpeooliim ; and to this diso the
oblique refleclor i« nttached bj Bcrewg Tor the pncpoae or&ii^uttiitg
j^ jm tlio direction of the reflectine aurfaca. The
niirTor is also capnble of being sdjoHUd laUrall;,
bv means of the screWB, c, c, c ; aad it is much
lc9B liable to vibration than with llie ordinaiy
ninglo Bupport.
1147. If, instead of ^mtiUlng the immge to
be painted on the lelina of the efo, it be re-
ceiied on a screen, then Ihe iDBtniment bocomM
either a cajneni obscurfl, or a Bolttr roicroBcopa,
according to the wrangement emploj-ed. If a
coniei lena be lized in a hole made lu one end
of a box, made a iitlle longer than the focal
IvDgth ot the lens, and painted intcrriallj with some black
pigment, fur the parpoae of absorbing all eitranpoua light, the
image of a landscape, to which the lens is presented, will be
beamifullj end vividly painted, in an inverted position, on a aheet
of paper Gied at the end
"«■«*■ of the boi oppoflite to tbe
I lens. SomelinieB, inalead
of receiving tbe image on a
»lieet of paper, it is in-
flected by a plane mirror,
A, Fig. 609, placed at aa
angle of 45°, towanla the
upper part of the box ; *
sheet of white paper, or »
pioee of ground glsas, n, being there jilaced to receive it. In thi*
mode the image appears erect, and inverted only as regards the
right or lefl portions, and is usually preferred for the purpose of
RKetching distant views. As the lateral portiona of the picture are
indistinct from spherical aberration, a meoiscna (1068) is preferable
to any other form of single convex lens, for the porpose of reducing
1148. If any small object, strongly illominated, be placed oat-
side a csmera obacura, having a lens of high power, and a little
beyond tho principal focus of the lens (1065), an image of tbe
object will be depicted on the paper screen at the end of the box.
An instrument tlius arranged has been termed • Megaaoope : hat
it is not olUn employed.
1149. The best form of camera obscnra is that in which internal
(1060) instead of specnlar reflection is employed to prevent the
loss of light attenduit on the latter. The box is then made of a
pyramidal form, a B c i>, Fig. GIO, and a rectangular prism, having
one of its faces, a, convex, and another, b, concave, is placed over
an aperture in the top of the box. The rays from a distant object
will be made to converge afler impinging on the convex sniface, o,
and being reflected in the interior of tbe priam, will pats into the
647
box, and paint the image on t. liieet oF paper placed at the
botiom, c D, to rccoiTe il. The picture thae obtained is extremelj
vivid, Irom tlie perfect reflectioD of raji _
from the bock of the priam, and tniiD
tiie spherical aberration being to a grea
extent CDunteracted by the cuncave fac
of the pmm. Aa these meniscua prism
•re not re>dilj obtained, tliej may b
advBnt»geoitslj replaced bj a rBctanKDliir
priBm baniig a planD-cDnvei and a plano-
concave leng, of suitable focal lengths,
cemented by Canada balsam on two of
its faces OS shown at e, in the figure.
1150. Wlwn a virid pencil of light,
before being made to convBiga bj re-
fractioll through a lens, passes through
a amall transparent body placed before it, at a distance a little
greater tben ita focal length, aa enlarged image of the object will
be painted on a screen placed at a proper distance behind uielenB;
this is the principle of the solar microBcope. The simplest form
ot this inBtrnment consists of a pyramidal box abdc. Fig. 611,
fbmisbed with a door at E, like the camera
obscura. The solar rays falling directly, or "*■ *^''
reSecled by a common lookiiig-glaaB
plane ntiiTDr F, are tranamilted t
plano-convex lena a, whste they nndeipj
refractioD, and fall on an abject placed a
E, nearly in the principal Iccas of a. Thi
light then passes through two plane
convex lenses, each of about half an inch
focal length, at l, moveable by met
radiWDrk at h, forming a widely diverging
bundle of convergent pencils, and paints a
highly magniOed image of the object at the
bottom of the box, where it may be viewed
through the door k. To prevent as macb
as possible sphGrical aberration (lOHI), a diaphragm of metal,
pierced with a small bole, sbonld be placed between the two
lenses at h.
if the mirror P be removed, and the direct light of on Ai^nd
lamp be incident on a, this becomes the tucernaT, and if the light
of lime ignited by mixed oxygen and hydrogen gases be employed,
the oiy-hydrogen microscope. Where high magnilVing powera are
required, more complex object-glasses must be employed.
1151. The magic-lantern differs scarcely at all in principle from
the three last-described instruments. The light of a lamp, placed
in a tin box, is reflected by means of a concave mimir, tuiQ oon-
dented by a lens, on figures pointed in vivid transparent colonn
648
OPnOAL nrSTBUMBKTB.
Bf.611.
^;
I
A
\
/
>
*
_ff
on a Blide of glass ; the light then is refracted through two convex
lenses placed near the object, and capable, hy a sGding tube, of
bein^ a<^'usted to such a distance as to cause the image, when
received on a white opaque screen, to be as vivid and distinct as
pOMible; the aiagio-latitem being nothing more than a lucernal
miorotcope of low magnifjing power. If the screen on which the
object IS painted be transparent, and the spectator be placed behind
it tlie ittAg« will, in a dark room, appear to be painted spectie-
like in the air, constituting the well-known phantasmagoria.
116t. A very valuable mstrument, termed the eatnera luada,
for taking drawings of landscapes, &c^
depending upon internal reflection, was
contrived bj Dr. Wollaston : this ooii>
sista of a quadrangular prism, a b d c,
Fig. 612, the angle b, being 90*, D
67•5^ and c 185°. Bajrs ns, evolved
from any distant object, will, after in-
cidence on oi>, be reflected in the in-
terior of the glass to c a, and thence to
the eye placed above the edge a. And
as all objects appear to be placed in the
direction of the rays which eventually
reach the eye, the image will appear to
be painted on a screen or sheet of paper
at o n ; and if « perforated piece of metal be placed on a n, so that
one-half only of the aperture be over the angle a, the image and
paper will both be visible to the eye placed over the aperture; and
a sketch of the object may thus be taken with extreme accuracy,
by simply oopylng the oullineB of the figure, as it appeara depicted
oii«a.
The most convenient mode of using this instrument with the
miotoscope is to nlaoe the body of the latter horizontally, and to
view the image airectly, and the paper by reflection.
115S. A very exoellent instrument, advantageously leplacing
the oamera lucida, especially in making microscopic drawings, is
the mirror of Soemmering : this consista of a small round plane
speculum of steel, about one-fourth of an inch in diameter. Tbia
b^ing fixed before the e^e-glass of a mioroecope, at an angle of
45** with the axis of the instrument, a person looking into it (the
body of the microecope being arranged horiaontaHy) will see a
reflected image of the table, but from the small size of the mirror,
a portion of the rays proceeding from an image of the object enter
the eye simultaneously, and thus the image appears superpoeed
upon a sheet of paper placed on the table, and with a little ma-
nagement, the outhnes of the image may be readily traced with, a
pencil on the paper.
1154. Instruments designed for presenting to the eye a mag-
nified image of an object aie called miero$cc>pe$» When single
TBI UHTU MIOIOBODra. M9
laiMU are tutd for nmpla nucTOKopeij it ia imporfamt to diioinuili
■pberical ftberraticn tw mncb na pooaible, br pennittiDg onlj tboas
raTB vbicb pua Dear tbe ceatre of the leni to reach the tje.
I Thi«ni»j,to»gre»leil«nt, be effected by Dr. WoIImIod'i metbod
' of [Jscing between two plano^oiiTex lenaea, a piece of metal per-
I fonited in the centre, commonlj called a dK^ragm. A better
mode of obtainiag tbe ume effect it b; grinding away the
eqaatorial poitiona of a apberioal lena, aa ia the well-known Cod-
dui^on iena, which ia tfie moet ptn^t ample lena hitherto
conatincted.
The Stanhope leni ia another toit nsefol modiEoation for the
purpoae of a simple microacope ; thia ia a thick lena with two
Bphericsl anrfaoea, so amnxed that the foci of all pamllel peDcila
Kfracted at one mrTaoe ahall approximately ooincida with the
other anrface ; conaeqnenlly aoy minate oinecta depoaited on the
latter, will be diitinctly bmq mnch magnified, on viewing them
thmogli the former.
If the ocnlar aurfaM or the Stanhop* lens be tnnwd towarda ft
•oorce of light, and the atgect be liewed by a Coddington lena ot
■hart rocm, a high magnifying power will be obtained, accompanied
bj extnmely good definition, owing to the yerj aconiale manner
in which the object ia illuminated Xl 165).
: 1156. One of the beit forms of aunple microBcopc for a certain
olaaa of olgnctt, on account of the great dis-
'■ tinctneaa of tbe image, is the doublet of Dr. '"'* *"-
WoUaaton. Thia conaiats of two small
filano-eonTei lenses, of which the focal
engths are as 1 : 3, fixed in the braaa cop a, «
Hg. 613, the leaat oonrex lena being nearest
tbe eye. The hrssa labe b ia about six
inches long, famished below with a plane
minor St p ; a circular apertnre is made in
a diaphragm placed aboTe it, throogh
which the light reflected from r passes
throngh the conrei leas B, so as to (ona a
distinct circalar image of tbe apertnre at
the distance of about O'B inch from i.
Tbe object to be examined ia placed on a
slip of glaia on p P, and the lensei in a are
adjnstedbymeans ofa screw at s. By this
instrument many delicnie morkinga, and fine strinon rei^minale
cbjects, may be clearly seen.
In all simple microscopes, the centre and edges of the magriifled
image are never equally dialinct, ^m the spherical aberration of
theleBses (1061). To remedy this, diaphragms are placed in the
body of the microeoope, to exclude thoae raya which are refracted
from the edgea of tbe lenses. Y ' ""^ — "^ ■*
lensea oontrtTed by Sir John I
650 OPTICAL IS8TEUMENT8.
prevent this aberration from interfering with the distinctness of
the image.
1156. A microscope, composed of two or more lenses, is nerer-
theless termed fimpUf provided they are combined to form an
image, as in the Woliaston doublet ; but when an image formed
by one lens or combination is magnified by another, as the object
itself would have been magnified, then the instrument is called a
compound microscope, and is commonly preferred to the simple
instrument, from its having a larger field of view, and, when pro-
perly constructed, not fatiguing the eye so much as those consist-
ing of a single lens, or combination, of very short focal distance.
In the compoand microscope, in its simplest form, a magnified
image of an object is formed, by allowing the rays passingthrungh,
or reflected firom it, to be refracted through a lens, or combination,
of short focal distance, called an object^uiss ; the image thus pro-
duced is further magnified by a second lens of much lower magni-
fying power, called the eye-glast; hence this instrument requires
much more care in its construction, to ensure an accurate and
perfect ima^, because the eye-glass magnifies the errors of aber-
ration existing in the image formed by the object-glaaa, in addition
to the similar errors that itself introduces. Let a b o. Fig. 614,
Fi 614. ^ ^ ^^^ ^^ brass, blackened inside to abeorb 8up«r-
^' fluous light, and provided with a small lens at c ;
^ an object placed in its focus at p, strongly illnmi-
^^ , nated by light reflected from a mirror placed below
' ^'^ it, will have an image formed in the focus of the
eye-glass A at /*, and a ^rtion of this may be
viewed through a, by which the rays divei^ng
from f are made to enter the eye in parallel pencils.
For tne purpose of increasing the field of view, a
third lens b, called the JiMylass, of less ma^ify-
ing power than the eye-glass, is generally mtro-
duced ; this causes the converging rays going to
form the image to converge still more, and a
smaller imase, as shown by the dark arrow, is
formed at ^ The distance of the object-giase c
from the eye-glass a must considerably exceed the sum of their
focal lengths.
1157. A compound achromatic lens, the construction of which
has already been described (1120), forms an excellent object-glass
for a compound microscope, giving a nearly colourless image of
the object, which will bear a higher magnifying power in the eye-
glass than an image formed by an ordinaiy lens of equal focal
length. Among other advantages presented by an achromatic olgect-
glass, is the fine illumination of tne image, arising from the larger
pencil of rajs which can be admitted into the bodv of the inatrti-
ment. This may be readily understood by a reference to what
▲CHEOMATIC OBJECT-GLASSES. 651
has been already stated with regard to the use of diaphragms or
stops, in the constmction of optical instruments. These are per-
forated pieces of metal so placed as to cut off the more external
rajs of a pencil passing through a lens, and thus permitting only
the central rajs to reach the eye ; and in this manner many of
the aberrations of a lens are practically reduced to a minimum,
although at the expense of a great loss of light. The achromatic
construction, by allowing the transmission of a larger pencil of
rays, enables us to use high magnifying powers with a perfection
of illumination previously unknown.
The advantages of a large angle of aperture in an object-glass
are not by any means confined to the increase of the quantity of
light transmitted: in many colourless and transparent objectSi
such as the siliceous shells of the various diatomacese, the
structure is indicated by differences of thickness so minute, that
no visible di.sturbance of the transmitted rays takes place, unless
they pass venr obliquely, or in other words, none but very oblique
rays will render the structure visible. The truth of this observa-
tion may be readily shown by experiment : — ^many difficult test-
objects may be discerned by a small margin of the peripheral rays
of an object-glass of large aperture, the central rays being stopped
out ;^ although when the object is much more iUuminated by the
admission of the latter, and the exclusion of the peripheral rays,
the structure previously recognised will remain wholly invisible.
1158. As it is a matter of great practical difficulty to balance
the chromatic and spherical aberrations perfectly in a single com-
bination of lenses, a great advantage is gained by the union of two
or three combinations, in which & aMrrations of each are mu-
tually balanced. Object-glasses constructed on this principle
have, in the experienced hands of Messrs. Boss, Powell and
Lealand, and Smith and Beck, been brought to an amount of
perfection which could scarcely have been anticipated. In the
nigher powers three double, and sometimes even triple, com-
binations are employed : and the larger amount of magnifying
povver being obtained by the anterior or external combination, its
positive aberration (1048) is corrected by an exoess of negative
aberration in the two posterior or internal combinations. When
three combinations are employed, the middle one is generally a
meniscus (1068) for the purpose of correcting the spherical aberra-
tion, or curvature of the image, in order to produce a flat field.
1 1 59. So delicatel V are the aberrations of a well-made achromatic
object-^laas balanced, that merely covering an object under ex-
amination with a piece of thin glass, or mica, is sufficient to inter-
fere with the perfection of the image. This effect is practically
perceptible only when object-glasses of high power are employed ;
and we are indebted to the ingenuity of the late Mr. Andrew aoss
for a knowledge of the mode of correcting it.
Kf.eit.
tood by a n . . .
F1^. 616, ia vhich, let f be the focas of u
I object glisa, corrected for ancoTered ob-
■ peripb«rKl rar, ftod Bra nj
ixii of the pencil, cr. If a pUu
I of thin glau di be now inteipoeed, ihe
I rajB A F, B r will be refruted on CDteiiaK
I the plate, in the directiona Ao, ■&, and
J *ill emerge in the directions af^ bf,, and
a certain amonat of ncKatiTB aberratlMi,
repreaeDted b; rf,, will be introdnced : in order to con«ct tl^
or to make f, anii ^ again coincide with r, it ia neceaaaij to
incnase the refractian of the raja a and b, but of a tnoch more
than B. This is effected br approiimating the anterior to the
two posterior combinations, bj which the raj a is compelled to
paaa Ihrongh the anlenor lens nearer to its margin tbaii before,
and therefore throoKh a portion forming a more obtnie wedge,
where it consequeDtlj taonn mone letiaction.
The otgect^lasses posseasing this erBat improvement are cod-
■tmcted with a mecbaniBm ahowa ia !ng. 616. The two poaterior
Bchromatio lenses are fixed in tbs end of the tnbe, b ; Dp«i this
slides a cvlinder, a a, carrjing at the
lower snd the thiid or anteKor leu,
which, hj taming the screwed ring,
o c, maj be approximated to, or
separated from, the olber two lenna.
The proper diitance fiir the a<)j<»^
ment of theae lease* fbr nncorered
objects is known b^ a Ene marked on
the tabe, a, coinciding with one on
the tnbe, b ; and, when ohjecta ate
examined which «■« corend with
S'»m, or immetaed in a flaid, the
stance of the third lens froqi the
other two ii altered bj turning the
ring, cc, no^l the bant definitioo i^
the object is obtained.
1160. The image of an object thns formed bran achnmatie
combination of lenses i> eiamloed throngh e;e-piece9 of different
Diagni^ing powera ; these are rariooslr constructed, bnt Ihe moat
approved ore the Ho^enian, or mgataie, and Bamsden's, or the
potitiee eje-piece. Of these the ncgatiTe eje-piece ii b; far the
most frequently omplojed ; it consiata of two lenses, ■ a, and r r.
Fig. 617, each being plano-conTei, with their ooDTexitiea lowanls
the object.glass. i B ia termed the eje-glass, and r f the field-
gloai, for reasons alread; poinled out (I lOS). A perfbrmted H^
or diaphragm is placed at b b, to cat off ths extreme raji that
ooxbubd AonoH of thb btv-pisoe axd OBJBcr-oLAfla. 658
Jiy. 617.
might interfere with the perfection of the ima^. It most fortn-
nately happens that the arrangement of lenses m the Hnygenian
eje-piece possesses the property of correcting
not only tneir own aberrations, bat ako those
of the object-glass, as first pointed out by
Mr. Ross.*
The positive eye-piece also consists of two
plano-convex lenses, but the convexity of the
field-glass is upwards, and the principal focus
of the combination is external to tne field-
glass. The ^ principal use of Bamsden's
eye-piece is in the construction of a micro-
meter ; for thispuipose, a scale marked with
a diamond on a plate of ^lass is placed in
the principal focus, and its image, being
consequently superposed on that of the object, serves the purpose
of measuring the magnitude of an object, when the value oi the
divisions of the scale is known.
1161. All that is essential to the construction of a perfect mi-
croscope is then, a good achromatic combination of lenses to form
an image of an object, and a well-made eye-piece to magnify this
image. It is obvious that the mi^nifving power of a microeoope
can be increased in two modes ; by increasing the magnifying
power of the object-glass, and thus forming a larger image ox the
object, or by examining this image with a deeper eye-piece (t.e.,
one of higher magnifying power). The first mode is undoubtedW
the most accurate, as by tne second we magnify any errors which
may enat in the ima^ formed by the object-glass, as well as the
image itself: still, with good and trustworthy obiect-glasses, we
may not inconveniently examine the image with different eye-
pieces, and thus avoid the necessity of altering the position of the
object, or removing the object-glass. Acconiingly, some of the
continental microscopes, as those made by Oberhauser, are pro-
vided with a series of six eye-pieces of different magnifying
powers ; English microscopes have, however, seldom more than
three or four.
When very high magnifying powers are required, as 2500
diameters and upwards, there is a third mode of augmenting the
power, which is sometimes very available, namely, bv increasing
the length of the body, or the distance between the objective and
the eye-piece. It is evident from what has preceded (1079), that
if an im^e be formed at any nven distance firom a lens, the
diameter of the imaee will m doubled, if it be formed at double
the distance ; hence ue power of a microscope will be doubled by
doubling the length of the body. With any given objective, and
at any rate the third eye-piece, it is ouite obvious that both defi-
nition and illumination are improved by lengthening the body in
* P«imy p7olop«dia. Art. Miorosoope.
65* oPTi
prefersBce to using a deeper eje-pi«ce, ad3 the wnter it incKn«d
*« beliere, from repeated oWrvntian, that bejond » certain limit,
better reeolt is obtained by lengtlieDiiig the bodr, tlun hj ei
bjectiTc, that limit being probablj aa objectii
an inch focus,
few Tran, Bchromatic ohjectiTea ofi'^hofaa
ioch focuB were the higbest powers eiistlog; objectives of -^th
Rnd Ath of an inch focus were Enit conBtraclBd by Mr. Wenbam,
and have aince been made by Messrs. Powell and LeaUnd ; objrc-
tivOB of 1°™ (^Atli inch) focus have also been made by IL
Hartnack of Paris, and in America; but the writer has not aeen
any equal in defining power to the English glasseB.
1162. The greatest recent improrement in the tninvscope hu
been the appliEBtion of binocolar viaii — —' •■" "■" -' ' ■-
n relief (see Stereoscope). This
._. 1.., means of prisms, hut
Wenham pitsduces unqueatioDBbly the best r
requires description. A priam, of which A b C D,
Fig. 618, is a side view, is ao placed just above
diflerent ways by means of priams, hut as the method da« to the
ineenuity of Mr. Wenham pi-oducea unqueation " - '' ' ■
suits, that alone requires description. A priam,
Fig. 618, is aside riew, is so .
Flu. ms, ,1,^ object-gU™, that the edge meeting the angte
In, binects the bundle of pencils proceeding from
the object, so that one-half of the penciu pass
directiy up the body in the direction E p, white
.. , . , nied by 0 , .
horizontal surface of the prism d c, and being
twice internally reflected, emerges perpendicu-
larly, and thercrore without rerraction, from the
surface a b. and proceeding towards H, enters a
second body inclined at a suitable angle to the original body, and
is there magnified by a second eye-piece. In order to suit tho
TBriouB distancoa between the centres of the pupils of the eye in
different individuals, the distance between the centres of the eye-
pieces is B^JUBts^ hy elongating or shortening the divergent bodies,
shown in Fig. 619, by means of two racks and a pinion-
By this arrangement the images received hy the two eyes an
very mnch the same as they would be if a similarly enlarged ob<
ject were viewed by unaided viaion ; and it is well known that
the mental combination of two dissimilar pictures produces the
Krception of rilUfin a solid object. It is evident that the per-
il ^fiuition of the object must in all probability be slightly im-
paired hy tho tranamission of half the pencils through the nriim,
but as the other half reaches the eye undiatnrbeil by reflection w
refraction, a perfect image is perceived ; justaa a goal atereoacop'o
effect may be produced by the combinatioD of a feeble with a well
developed photograph. In all other binocular arrangementa, both
halvea of the visual rays aro equally submitted to some kind of
prismatic diatturbance, and the low of definition thereby i« *«ry
perceptible.
1 163. TKetnccluiiical anangementB of a microscope are ecarcelj
of lesa importiuice tlian tbe perfection of lenaes. As a nneral
rule, tbat form of support wliicb combines sMbititj with tlie
ETeatest facility for the necessarj adjustmeDte is to be proferred.
Ths eta^ ahnuld alvaja be a fiitnre, and the ndjustnieiitB to
focus eSecled hy moving the bodj of tbe instrnment. Many forms
of support for the optical part of the microscope have been con-
ilructed by Messni. Pritchard, Powell, Koss, and Smith and Beck,
ID thiH country, and each has probably, in the opinion of variona
observers, its peculiar ncommendationa. Aa a really verking
inetnunent, capable of I>eing applied to any purpose for which a
microscope can he employed, and compriainE all the most recent
improve men Is, [he one cnnttructed by Mr. Boss, lepreaented in
Fig, 619, ia prubably the best.
Tlie whole instrument is supported on tvo standards, resting
on » firm triangular foot. The double body, a. isconnecledby an
arm, b, to a rectangular piece that tlidea within a box, c, attached
by an axis to the two lege, in order that the instrument may
656 OPTIOA.L IH8TBITMBirT8.
be placed in a oonvenient inclined position, as in the ^gnre. A
Sair of e je-pieces, b, b, are inserted in the upper end of each cylin-
er, and an object-glass, o,li8 screwed on to the lower end, wiiich
is brought into adjustment hj raising or lowering the sliding piece
by means of a rack and a pinion attached to the milled heao, d;
this is called the eoarBe aajustment. The >Sne adjuMtment^ which
is necessary for the high powers, consists of a lever enclosed in the
arm, b, acted on by a very fine screw, f, which moves by a vexy unall
quantity an inner sliding tube to which the object-glaas is screwed.
The binocular prism, a o. Fig. 618, is inserted in a sliding frame,
p, so that it may be interposed in the path of the rays proceeding
from the objective, or withdrawn at pleasure. When the priam la
withdrawn, the whole of the rays enter the vertical tube, as usual;
but when it is in its place, the pencil, b f, enters the vertical, and
o H, the oblique tube. In the use of tiiis instrument it is desirable
first to bring the object into focus with one eye, then to adjust Uie
eye-pieces to the distance between the eyes, so that both images
may be seen $imvltane<mtlyt and lastly, to readjust the focus.
llie binocular effect is most satisfactory with the low powers ;
with those above the half-inch, the prism is comparatively useless.
Messrs. Powell and Lealand have adap'ted a prism to higher
powers hj inserting it in the tubular portion of the objective, but
the defimtion is somewhat interfered with by diffraction.
A double nose-piece, d, designed by the writer, is shown in the
figure ; in this two objectives are attached to the ends of a bar,
turning on a pin at d, bo that either may be brought into position
at will without the trouble of unscrowing. When a hign power
is employed, it will be found a great convenience to have also a
low power in use for finding the object. The foundation of the
object-stage is firmly united to the box, c ; on it a plate traverses
in the diracdon of a normal to the box, c, by a rack and pinioa,
and a second plate moves transversely on this by means of a
scrow with several threads. Above this is a sliding plate and a
clip, on which the object to be examined is placed : these adjusting
sti^e-plates are mounted on a toothed ring, that has, by means of
a pinion, a circular motion in its own phme, concentric with the
optical axis of the instrument, so that an object in focus may be
rotated, without being carried out of the field of view, lliis
arrangement is very convenient for many observations made
with polarized or oblique light. Beneath the stajge already
described is a ieeondary ttage, l, also furnished with nonnM,
transverse, and rotatory movements, and attached to a pieoe
sliding in a dove-tail groove, in which it may be nused or
lowered by a rack and pinion. The secondary stsge is designed
for supporting any kind of illuminating apparatus, as o, by which
rays reflected from the mirror, m, or cominff directly from a lamp^
are brought to a focus ; and by means of the adjustments that
locos may be made to coincide with the olgect.
MAQKIFYIira POWEBA.
657
A stand, b, is attached hy a sliding piece to the triangnlar foot,
at the top of which is a joint carrying an arm with a condensing
lens, K, at the end of it, for accumulating light upon an opaque
object. The stand, s, also supports a lamp and reflector, by which
a strong light may be thrown on a disc of white enamel, placed
at the bacK of the mirror, v. ; this afifords a softened light for illu-
minating transparent objects, that is peculiarly grateful to the
eye, when long employed continuously.
1164. The magnifying power of a compound microscope depends
jointly on the focal length of the object-^lass, and the power of
the eye-piece ; but the definition of the image depends on the
accurate adjustmeut of the object-glass with regard to chromatic
and spherical aberrations : and in tlie practical application of the
microscope it must ever be borne in mind, that an object is by no
means necessarily better seen by makina it look larger; on the
contrary, the lowest power under which the eye can distinguish and
separate the aeverat parts of an object is always to be preferred.
The following table contains the different approximate linear
magnifying powers obtained with different eyepieces and object-
glasses, as constructed by the best makers. The eye-pieces are
commonly called a, b, c, &c.
Object-
glassei.
A
B
C
D
E
3 La.
2 ,.
U»
1 «
} .,
i „
TfF it
i n
i ..
i „
i ,,
19' ft
Tt it
At tt
13
20
25
37
65
95
140
195
250
320
420
600
800
1260
2500
20
32
40
60
102
153
220
310
370
510
670
880
1180
1860
3700
35
55
70
105
145
265
370
540
590
700
900
1200
1600
2500
5000
56
90
112
170
270
420
650
850
1000
1120
1300
2000
30OO
5000
9500
84
135
168
256
405
630
975
1275
1500
1630
1820
3100
4600
7500
14600
1165. It is an axiom scarcely appreciated by many micro^
scopists, that in order to obtain the most perfect definition in the
image of an object, a careful adjustment of the rays incident on
the object is not less important than the due adjustment of those-
u u
658 OPTICAL XK8TRI7UENTS.
ihaX proceed from the object to the eye. In many instances tlie
parts of an object will be distinct!;^ defined, when it is pioperly
illuminated by an aplanatic pencil (that is, a pencil free from
aberration), that would be almost if not entirely lost, when the
common mode of illumination is adopted, namely, a pencil re-
flected obliquely from a spherical mirror of short focal length, and
which, consequently, possesses a very large amount of spherical
aberration. Any optical combination placed under the object, by
which a pencil of light may be directed upon it, is called an
illuminator or condenser^ which ordinarily consists of a combina-
tion of lenses so placed that their common axis may coincide with
that of the microscope ; and when properly adjusted, the object
and the source of light are the conjugate foci of the combination :
when this is the case, the image of the source of light will be
superposed on that of the object. When this adjustment has been
made, the object will generally be best seen by slightly depressing
the focus of the condenser. A Kelner's eye-piece, in which the
eye-glass is an achromatic combination, answers very well for
an iUuminator. A Wollaston doublet (1156) answers very well
for low powers, or a deep eye-piece may be thus employed;
but a moreperfect aplanatic combination is required for the high
Sowers. Tne next lower object-glass is sometimes used as a con-
euser, but the frequent shifting of apparatus is so troublesome
that it is better to employ some distinct illuminating apparatoL
of which the most complete and comprehensiye hitherto proposea
will presently be descnoed ( '67).
When a lamp or gas-burner is the source of light employed, the
flame should be protected bv o pjound-glass shade, for if the object
be transparent, an image of ti.o flame itself, when supei^oaed on
that of the object, will almost, if not entirely, extinguish it. The
best source of light is any opaque white unpolished sarfaoe,
strongly illuminated : that from a white cloud opposite the son
reflected by a plane mirror, or still better, internally reflected by a
rectangular glass prism, is of all illuminations the best ; but aa
this is rarelv obtainable in the murky atmosphere of a large ci^,
a plate of plaster of Paris, or a flat surface of powdered carbonate
of soda, placed beneath the condenser, transyersely to the axis of
the microscope, and strongly illuminated, forms a convenient arti-
ficial white cloud ; but still better, a plate of white enamel, with
a finely-ground surface, the brilliancy of which, when soiled, can
be immediately restored by washing the surface with soap and
water. The light should be placed laterally, and in front of this
white surface ; and as large a concave silvered reflector, as can
conveniently be employed, placed behind the light, so as to accu-
mulate as much li^nt as possible on the plaster or enamel disc
1 166. By a modification of the illuminating pencil, a singular,
and with some objects, a remarkably beautiful image is produced :
the method is known as the dark ground illumination. In this
the otgect, inilead of bein^;, m oeaat. reoagnised bj obBlmcting
some of ibe raji that illuminate k bright space, !b traced ia linei
oF'lieht OD a dark ground, Bimtlarly to the black diogranu by
vbicb man; of these pages are ilIa^<t^ated ; and whicb, like the
dark-ground iliutDinHtioD, are best suited
fijT otjeols marked by itrong ontlines. _ *"■ **"■
Fig. 630 rppresenla the mode of prodnc-
ing this kind of iliominatinn, in vbioli
AEB ia the i>xt<:rior aurface of a cnne of
rays iasaiDg from the itiuminatur. nnd
neetiog in afocua atn, vhere the ot ject
is plac«l. If a conBiderable piirtiim aJ
the centre of this cone, as ci>, be ebul
oat hj an opaqne stop, then a coaicat
tJitU of raTB only will full upon the object,
tbroDgli the sBTerol portions of nhicb
the; will be Torionsl; reflected sad re
fnicted. Now if fq, the object-glasi
employed in viewing the object, be such
that IF, Eo, the extreme ra7a that cnn eater it, lie within o b and
DE produced, which are the innermost of the illuminaliDg r*;Bi >t
is eTidunt that no portion of these rays can enter tbe object-glus,
and ccnseqnendy where the Reld of view is vscHnt, it is perleotlr
dark, the onlv isja entering the object-glass being those the path
of which is altered in pasaing through the object
This mode of illumination may be effected either by plaoiDg an
opaqne stop in the centre of the pencil incident on an aplonatio
comtnnatioD, a b ; or the pencils abc, deb, may be reflected from
the inlerior of a truncated cuncare parabolic mirror with an opaque
stop in ^centre, as proposed by Mr, Wenham; or inlemally re-
flected from the sur&ce of a truncated paraboloid of glass, the
truncated end terminating in a hollow spherical surface of which
the focDB of the parabola is the centre, in order that the rays
reflected towards the focna may emerge from the glass without
Tefraotion, as contrived by Hr. SbodboTt.
By cementing a small truncated paraboloid of glass to the under
■"'^^ce of a slide, Mr. Wenbam has sacceeded in brilliantly iliu-
kting the shells of diatomacete mounted in Canada balsam by
■uriace of
'iCondenter. — TheulilityofanymachtneiggTBatl;
augmented by rendering the interchange of moTeable parts as
easT of accomplishment as possible; and microtcopic obiiervers
will Bed this to be especiollT the cs«e with regard to the TarieCies
of iljominatiag apparatus Uiat have been propoead ; of all these,
however, the condenser, Ti^. 631, designed by Mr. Oillett, appears
to be the most comprehensive in its application. The base, A b;
fits into the rotating ring of the secondary stage, i., Fig. 619;
D d2
the optical arreDfcemeDt, a, is that of a | inch abject-gUu (ai ■«-
preaented at tha aide of the figure) aod Iraaimita a pencil of aboat
B0° aperture: bat the great improrement consiati in the adap-
tatioD of a wuoer-»h»ped disc, the rim of
■'*■■*'■ wbicb is « truncated oone, aud so placed
@ that the aarface of the rim may paaa per-
peodjcntarlj ibrouf^h the commoa aiie of
the leuBea c, aud immediately below (be
Burface of the lowest lans. Varioiu aper-
turea. for modifying the illutninatii^
E pencil, are placed ronad tha circumferenoe
of the rim, each of which may be placed
central!; under the lensea, by meana of •
. atop, r, at the end of a aprine, e r: tbeae
oonsiit of a serias of circular oalea, which
iHry in diameter from o, which tranmiita
thaeuttM pennl, toi, which tranamita one of only about 10'';alao
apertures, la C, d, with oeotral discs, aupported esch by a slender
Mr, of which the lar^n^st, d, will produce a daii ground (1166)
with the i or ^ inch object-glass, and the other, e, with the
powers below these. A tourmaline, t, ia fitted into another aper-
tnra, by which a polariied pencil may be obtained, with no nioiv
trouble than that of turning round the cone with the Guger and
thumb. Another aperture, o, is stopped ap, all except a amall
lateral portion, which transmits an oblique pencil inclined about
8&° to tDs ails, and which may be made to tiavel round the axia
by the rotatory movement : thia modification of the ill aminating
pencil, to which we shall presently recur, is sometiniea canTGDJent.
If a pencil of perfectly colourlesa polsrlzed light be required, it u
focnd oonvenient to psss up a amsil Nicol's priam (lOSS) in tha
iuterior of the tube beaeath the lenses. The coaveniencea which
thia inttniment affords for the examination of objects either on ■
bright or dark ground, or by ohiiqae, or polarized light, witboat
any disturbance of a^ioatmeDtB, can acarcely be overesdmatod bj
mietDBcopic obserTcrs.
The writer has found that a more effectiTe illumination fbr rerj
high poirera ia prodoced by employing the optical combination of
the AinchobjecliTO of Mr. Eoea, inplacHof I
as aboTO described : alao if a plane horizenta
la combinations of the apertures and stops, every requidta
modification of the illuminating pencil may be oblained.
1168. OUiqtie lUumination.— It haa already been stated (1 167}
ID eiplanation of the practical nlility of object-glasses haring »
lai^ angle of aperture, that the more minute structure of aome
ob.|acts is cognizable otUj/ by ita infiuence on rays traTeniag tite
olyect with conaiderable obliquity. In default of paaaeMUig aodi
(lt1jec^glA■set (wliicli are difficult of constmction, and conae-
queotlj eipeniiTi), o umilar reBiilt niaj be much leu perfectlj
Btlained bj illaniinatin^ the object b; a pencil of oblique nja
only : but oblique light is at best a treocfaerous (UI7 of (he micro-
scDpiBt, and never to be depeudMi upon, as tbe appearances pro-
dncod are eitramel; fallHCious, but vhich neTertheless serve
occBsianaill/ to indluats what might be revealed bj more perfect
optical appliances. Eieveral modea of producing an oblique itlumi-
nation have been devised, of ivliich tie pnEm of Amia la the be«t :
this, as coiDmonly emnlored, is an obtuse j^. gu.
iaoscetea prism, of ivbich the two equal sides
have apherieal surfaces ; this is placed late-
rall; beueBlh the Btaze,the back of ihe prism
being nearly parallel to the plane of the
stage. A pencil derived from a light placed
aidevajB is refracted through one spherical
surface, and after Internal refiection at tbe
back of the prism, Is Bgaiu refracted at the
second apberical surface, mi at the second
surlace of a convex lens, and comes to a focus
at the under side cf a transparent object.
The writer has found the niOBt convenient
mode of mounting an Amici priam to bo that reprosonted in
tig. 623 : in this case tbo section of the prism tfg must be a
right-angled, or an acute, in place of an obtuse isosceles triangle.
It is attached to the top of a tube, he, (which fits iato the rotating
ring of the secondary stage, l, Fig, 619,) by two anpports, c, so
placed aa not tointeirupt the pencil of light, and between whifji it
maybe moved, so an to vary the inclination ofthebackof thepHsm.
Another prism, atcfl, equivalent to two right-angled prismi,
a be, oca onited, is placed beneath, bj which a central pencil, t,
is transferred to the side of the tube d ■ after two internal re-
fleclione ; it is then in succession refracted at the spherical surface
fg, reflected from the back of the priam tg, and refracted at ef,
whence the raya come to a focua at b, which point may be brought
to coincide with the axis of rotation, as tbe support, c, turns on a
oenlre, and is attached to a dove-tail piece, sliding in a groove in
the upper surface of d. This arrangement aSbrds a much larger
pencil than that mentioned in the preceding description of Gillett't
condenser, and, by the rotatory raBvement, the aiis of the obliquo
pencil may be made to rotate nmnd the axis of the microscope :
it moat howevor be boriie io mind Ihat obliqae illnraination is,
alter all, to be considered ^t as a Uet Msonroe.
It is Dttneceeaary to describe l)i« In^iooe prism of Kachet de-
signed for tbe aame fmt^mM, M tt is not eq«ai In llhiniiniitinK
power to that alreadv described, and moreover the inclination of
the oblique pencil i> mvariable.
Beflecting microscopes, on the same principle as Newton'a
fi62
OPTICAL INBTRUMK^tTS.
telescope (1092), have been constructed by Professor Amici of
Modena, and olners. In these instruments, the object is placed
in one focus of a small and finelj-polished elliptical speculum (e.
Fig. 572), and its image formed in the other focus is examined by
means of a magnifying eje-piece, consisting of one or more lenses.
1169. The reiracting telescope was invented in the thirteenth
century, although the discovery appears to bave been nearly lost
until the sixteenth. The simplest telescope is that employed for
astronomical purposes, and consists of a c«>nvex lens of long focal
distance fixed at one end of a tube, and exposed to the object, the
image of which, when formed in the focus of the lens, is examined
by a second convex lens, or eye-glass, of shorter focus. These
lenses should, for distant objects, be placed at a distance from
each other corresponding to the sum of their focal lengths. In
Fig. 623, A B is the object-glass, and o d, which must always be
of a shorter focus,
^'^^' the eye-glass, and
placed, it the fiicua
of the former were
eight, and that of
the latter two
inches, at a mutual
distance of ten inches. To accommodate this instnjment to olgecta
at different distances, the eye-glass is usually fixed in a tube
which slides within that containing the object-glass, and thus
permits a ready a^ustment of the instrument. In this telescope,
the object appears inverted from the crossing of the rays after
refraction through the object-glass, and hence its use is limited
almost entirely to astronomical purposes. An erect image may be
obtained by adding two other convex lenses, of the Fame focal
length, behind o d ; these are called erecHnff-glaue$, but a loss of
li^ht is necessarily produced by their use. Aberration maj be
diminished as much as possible, by the same means as those em-
ployed in the construction of compound microscopes. The mag-
nifpng power of these telescopes is found by dividing the focal
length of the object<glass by that of the eye-glass.
1170. In the refracting telescope as ordinarily constructed, the
erecting-glasses are interposed between a negative eye-piece and
the object-glass ; the course of an oblique pencil of rays is traoed
THE 8PBCTB0BCOPE. 663
from tlie object to the eye in Fig. 624, by which the constraction
of the instrnment will be rendered intelligible. The lenses a, b,
form an Hajgenian eye-piece, and together with the erecting-
glasses, 0, d, constitute the erecting eyepiece. The rays incident
on the object-glass, o, form an inverted image at o, whence they
traverse the erecting glasses, crossing the axis of the telescope at
some point near o, where a stop, or diaphragm, is, is placed with a
small hole in the centre, which cuts off the more aberrant ra^.
The pencil then traverses b, and forms an erect image at f, which
is viewed by the eye-glass, a.
1171. If a concave eye-glass be substituted for the lens cd in the
astronomical telescope, we have the Galilean telescope, which
exhibits objects in an erect position and with very great clearness.
The lenses in this instrument are placed at a mutual distance,
equal to the difference of their focal lengths, and hence telescopes
on this construction are much shorter than those in which both
lenses are convex. The magnifying power of this telescope is
found by the same rule as that already given for the astronomical
telescope. From the smalluess of its field of view it is chiefly
limited to the construction of opera-glasses.
1172. Opera-glasses have of late years invariably been used
coupled together, by which the visual power is augmented, as an
image is formed in both eyes simultaneously, and thus the im-
pression of relief f due to binocolar vision (1234), is obtained. In
these instruments the eye-glasses are both together adjusted by a
screw. In some, three eye-glasses are attached to a shifting frame,
80 as to present three different powers, which is very convenient.
A similar binocular arrangement has likewise been applied to
telescopes, but, it is believed, with much less advantage.
1173. The Spectroscope. — This instrument has been already
(1107) alluded to, as the means by which many important dis-
coveries in Bpectmra-analysis have been made. Fig. 625 repre-
sents a very convenient and well-arranged instrument constructed
664 OPTICAL IKSTBDlfENTB.
by Mr. Browning, on Bansen^s and Kirchoff^s model. A tripod
stand supports a horizontal circular plate on which are mounted
two prisms, A, each of 45", having the means of ac^jastiog their
edges normally to the plate. In the figure some fluid contained
in a test-tube, s, is supposed to be under examination. Light pass-
ing through the fluid enters an adjustible narrow slit at s, and
diverging from thence falls on a lens at d of the focal length d k,
bj which the pencil is rendered parallel (1065). The prisms are
«o i>1aced that each receives the pencil at the angle of minimum
deviation (1082), and the colourea rays being thus separated, tlie
spectrum is viewed by a telescope, b c, naving an achromatic olgectp
glass at B and a negative eye-piece (1160) at o adjustible to focus
by a rack and pinion. The telescope has a radial motion, and
may be adjusted to any required part of the spectrum by a tan-
gent-screw at F : by means of which also the angular measure-
ments are taken.
Sometimes, instead of one or more prisms of dense flint glass, a
hollow prism containing bisulphide of carbon, which possesses a
very high dispersive power (1097) is employed. This is efiected
by grinding away portions of the sides of a glass bottle in the
direction of two planes containing an angle of 60**, and cementing
on two pieces of polished plate glass.
Spectroscopes have been construct-ed with as many as ten prisms
arranged in a concentric circle on the plate ; by means of which a
much wider separation of the lines of the spectrum takes place.
For the purpose of comparing the lines of any two given
spectra, a portion of the narrow slit is covered by a right-angled
prism, bv means of which a pencil of light falling laterally on one
face of the prism is intemaUy reflected from the oase, and pasacp
onwards through the tube e d, parallel to that which paases di-
rectly through the remaining portion of the slit.
1174. A ngid spectroscope, in which all the parts are immo-
vably fixed, has been constructed by Mr. Browning for Mr.Gassiot,
for the purpose of determining the variation of refraction, if any,
due to change of the force of gravitation. In this instrument a
pencil of light passing through a slit is rendered parallel by an
achromatic lens, and heing transmitted through two and a half
prisms is reflected back perpendicularly from the silvered outer
surface of the half prism, and naving been again further separated
by the other two, tne spectram is brought to a focus bpr the same
lens, and being then reflected at right angles by a n^htrangled
prism, the image is examined by a micrometer eye-piece. The
separation of the spectrum is thus equivalent to that produced by
a train of five prisms each of 45**. Tlie prisms are adjusted to
the position of minimum deviation for the hue d of the solar spec-
trum. Sufficient time has not yet elapsed for any decisive results
to be obtained by means of this instrument.
1175. Straight Spectroscopes, — ^Two diflferent modes have been
devised by which the mean rays of the spectrum emeige in the
665
ction ss the incident pencil, knd in wbicfa conMqDtnlly
oacope becomes a atnight Inatrument, vkich is more
t for ma.ay purpOHOB of obKrvatioD, aa it maj be mora
Tected to the lonrce of liglit.
these is a red Qpli cation iy Hr. Browning of a priini
by Sir J. Henchel.
u A DC, Fie. 628, is "* ""
;1ed at A ; toe pencil |
ut D, is iDternall; I
at E and p, and I
at o in a direction I
tu that of its inci- |
it tUoa paninei the
■ L-durse through a aecond similar priiDi placed in a re-
>0!>i tion, aa in (he figure, bj which the separation du« to
prism is doubled.
c other mode of construction two rigbt-ai^«d flint-ghus
,. Fig 627, «.!.»,. „
Kitween three of crown-
r anglea of I
6. Tbia latter form has been adopted by Mr. Biowning in
nstrDclion of his ipeelro-mia-04cope, in wbich tbe ipectro-
ie adapted to the ejepiece of a _ _
land microecope ; into tba body
b is adiuttible for rayi of diffs-
refrangibil
. The combini
. , placed above the e^e-
1. Forthe poipose of comparing
difierent ipectia, a portion of the
is covered bj a rigbt-angled pHsm,
If taj traiuparent bodj the spcc-
n of which it required, for eiaiuple
lU conUining a coloared liquid,
plated in the fruno " ''
i^bl re
M passea throngh the apertur
i being intBrnally reflected upwards by p, passes tbroogb the
iljBiag piiitni telween b and c, b^ the eicio of the pencil pro-
iJinglniDilheohjflct nnder the mioroMopo. By theae means
e peculiar ipeclnini may be obtained from verr mioate qaan-
iei of any nibataiice (M of blood, for example) either diy, by
Beded liAt, or nhen sospended in a fluid, by transmitted light.
1117. 1m £(l)wlat.— In sooie inTestigatkna on points of pby-
666 OPTICAL 1X8TRU1IEKT8.
Bical optics it is desirable to maintain a solar beam continuooalj
direoteil to one point, for a certain length of time. This object is
attained by means of a heUostat^ in which, hj means of some
mechanism, the rotation of a mirror is made to compensate the
rotation of the earth. In the simplest form of this instrument,
the heliostat of Fabrenheit, a ^laiie mirror is jointed to the end of
an axis rotating by clockworJE onoe in twenty-fonr boors, and
placed parallel to the earth's axis, and is so inclined as to reflect
a sunbeam in the direction of the axis produced ; this direction
of the reflected pencil will consequently be maintained by the
rotation of the mirror. This pencil falls upon a second stationary
mirror, by which it may be again reflected in any required direc-
tion. The inconTenience of this instrument is the great loss of
light occasioned by two consecutive reflections ; and in conseqnenoe
more complicated forms of mechanism have been devised, in which
the required direction of the reflected rays is obtained by one re-
flection. These are all constructed on the obvious principle that
if a normal to the mirror be made constantlv to bisect the angle
contained between the line of incidence and that of the required
reflection, the direction of the reflected ray will remain constant.
In the heliostat of Sgravesande of Leyden, the earliest of this
kind, and in those of Gambey and Foucault, the required result
is obtained on the principle that in an isosceles triangle a line
bisecting the vertical angle is perpendicular to the base ; if then
one of two equal sides of the triangle be fixed in the reaoired
direction, and the other be maintained in the direction of tne in-
cident beam, and by making the plane of the mirror coincide with
its base, the bisecting line remains always a normal to the
mirror, it is clear that the desired result will be obtained.
1178. These instruments are more complicated and consequently
more costly (although sometimes a little smoother in tlieir action)
than that of M. Silbermann, which is constructed on a similar
principle ; namely, that in a trapezium of which the sides are
equal m pairs, the line joining the angles contained by the equal
sides bisects them both. In this instrument a circular base, a b.
Fig. 629, rests on a tripod adjustible horizontally by levelling
screws ; this supports, by a horizontal axis passing through d n, a
clock movement, c c, the central axis of which makes one rotation
in 24 hours. A line drawn on the base, a b, at right angles to the
direction of d e, must be made to coincide with the meridian Hne
of the place of observation, and clamped in that position by the
screw, F ; and if an arc of latitude, I ^ attached to the clock be now
set to the latitude of the place, the axis of the clock wiU necessarily
be parallel to that of the earth. A fixed tubular stem rising per-
pendicularly from the surface of the clock, carries the hour cijirJe,
A h ; through this the axis passes, carrying an index on the hoar-
circle, and terminated by a square block, p, with a damping-sciew,
through which passes a declination-arc, d i. Outside the tubolar
stem IS a seoond tube, q, having at its upper end a block and
w, throngfa whicli pauea aDothar Are, b h, oonoentna
dd.
, H M, is 10 placed that the centre of iU nHectinjc
lea with the centre of the *iiM,di t^vA hh'; it u
7^. m».
it 0 and a' to tvo framei taming freel v on radial atema
the two area at H and d, to that in &tl poaitiona of the
itre of the mirror, k, loaintaina a fiied position. A
le mirror at i ia conetrained to bisect continnonslj the
ned betwann the directioDS of its supports, hk, iIi,
ezial airangement before alluded lo. For this pur-
ed bar, or, ii attached perpendicnlarl; to the pbine
or at Uie point a; two equal linka (162), pr, or, are
{oal distances Irom o to tne tvo fr&mes, and the two
innected with a pin sliding fraelj in the slot of the
B clear, tberefore, that the angle p o 17 ( ^ 8 xd^iBKT)
be bisected by 0 r, and conaequentlj a k t bj e n, a
le mirror; hence if bk be the direction of the incident,
hat of the reflected beam.
to adjnst the instrument the declination-arc must bo
n's declioalion ; but if that he not knuwn, it maj be
liing to the arc, dd, a small plate, m, with a cross
anothur, «, with a Btoall hole, so placed that a line
lole and the oentre of the crosa may be parallel iat.d;
an be nored antil a eonbeam paasing through n fall
I at m, it is clear that the direction of the incident
illcoinadeTithEdl The an HB'mustnoir beahifled
668
OPTICAL INSTRUMENTS.
and tamed abont the axis, until the reflected pencil, k t, is in the
required direction, 'n-hen the arc must be clamped to its snpport,
o, and that to the surface of the clock, br a screw, r. It is evident
from what has preceded that the reflected beam must aJwaji
remain in the direction h k, which is now a fixed ]ine. In order
that the reflected beam may be free from jerking and unsteadiness,
it is necessary that the joints at o, p, q^ and r should all work
freely, but at the same time without any lateral motion ; and to
this end, the iinns op, 09, and the links, should be aa long as
they can conveniently be made.
n79. The construction of some of the mors impoTtaiit optical
instruments having been explained, the student will be enabled,
from the preceding obeervations, to understand the mode in
which the eve acts upon lights so as to prepare it for oommo-
nicating to the eensonnm the perception of surrounding objects,
and thus to develops the sense of sight. The following ol^erva-
tions, it must be borne in mind, apply only to the eye, consi-
dered as an optical instrument of the most perfect kind, and
unconnected with the phynologiosl relations of tne suljeot, except
SQch as are essential to a knowledge of the phytd<^ te^oo of the
organ of vision. Hg. 630 reprssents a honsontal sectiMi of the
left eye (human), mMe bv a plane passing through the line of
iunction of the eyelids. The form of the eye is nearly spherical,
four-fifths of its ciroumference, a b jl, being nearly circular, the re-
maining fifth, A A^ constituting the transparent portion, being more
convex^ and forming a curve of a lesser sphere. After removing
the muscles attached to the eyeball, the most external coat he-
comes visible : this is a tough, pearly, opaoue membrane, termed
^^ 030^ the aderotie coat, extending
from the entrance of the
optic nerve, o, Fig. 630, on
the nasal side of the optic
axis, CD, to A A, whers it
f. terminates in a circular open-
ing, furnished at its margin
with a grooved edge into
which fits the transparent
cornea, in the same manner
as a watch-glass fits into the
grooved circular frame of metal made to receive it. The oomea
18 as transparent as glass, and is about one-third of a line in thick-
ness. A delicate mucous membrane, termed the conjunctiva, is
expanded over the cornea and sclerotic, and thence reflected to
the inner surface of the eyelids. Lining the sclerotic coat is the
choroid membrane, extending from o to the anterior part of the
eye contiguous to the mar^n of the cornea, where it terminates
in the cuiary body, constituting a bond of union between the
choroid sclerotic, and iris. The choroid coat is here thrown into
8TBUCTUBE OP THE BTB. 669
a number of puckered folds, the interior surfaces of which, as veil
as of the whole extent of the membrane, are cohered with a black
pigment. The optic nerve, o, enters the eye on the nasal side of
the optic axis, and expands into a third coat termed the refiaa,
which passes towards tne anterior part of the eye, and terminates
in a well-defined edge. The retina is the membrane upon which
the images formed by the refracting structares of the eye are de-
picted : a delicate transparent double membrane, termed Jttcob^B
membrane, intervenes between the choroid coat, and the retina.
A delicate fibrous contractile structure, named from its various
colours the iris, is suspended vertically from the ciliary ligament,
havine in the centre an aperture, termed the pupil, which is
capable of being enlarged or diminished involuntarily, under the
stimulus of light. The iris is shown in the section at i, i ; the
space between it and the cornea is termed the anterior chamber
of the eye, and is filled with a fluid known as the aqueous humour.
Behind the iris is suspended in a capsule a transparent double
convex lens, l, of which the posterior is greater than its anterior
convexity : this is termed the crvstalline lens. The remaining
portion of the ball of the eye is filled up by a refracting structure,
termed the vitreous humour, in the anterior portion of which the
lens, L, is embedded : this is made up of a fluid contained in the
convoluted folds of the transparent hyaloid membrane. The
total length of the eye, along the optic axis, o d, is about 0*91 of
an inch.
1180. From the investigations of Sir David Brewster, the fol-
lowing are the refractive indices of the different transparent struc-
tures of the eye, when light is incident upon them from air, or
from each other: —
From air into the aqueous humour ^=1*3366
From air into the vitreous humour „ 1*3394
From air into the crystalline lens „ 1*3839
From the aqueous humour to the crystalline lens . „ 1*0353
From the vitreous humour to the crystalline lens . „ 1*0332
Bays of light, on impinging upon the eye, are refracted through
the transparent oomea, those incident on the sclerotic being re-
flected or absorbed. The cornea may be regarded as constituting
the anterior surface of a meniscus lens, of which the posterior
surface is formed by the anterior capsule of the crystalline lens ;
the aqueous humour forming the refracting medium of this fluid
refractor. The rays of light which thus tend to be refracted to a
focus, pass through the pupillary opening of the iris, those passing
too near the margin of the lens formed by the anterior cnamber
being reflected or absorbed : the iris, answering the purpose of the
perforated diaphragms in microscopes (1 1 55), and telescopes (1 170),
and being capable of varying its aperture, possesses advantages
altogether unattainable in rigid diaphragms. The pencil of rays
670 OPTICAL IK8TBD1IEKT8.
having pasaed through the flaid meniscus, impinges on the ayt-
talline lens, and is there considerahly refracted ; tois refraction is
modified by the action of the vitreons humour, the last medimn
into which the pencil passes ; and finally an inverted image of the
object, from the several points of which the rays of light are pro-
pagated, is painted upon the retina. All rays which are reflected
in the intenor of the eye, or pass too obliouely for distinct visioii,
are absorbed by the black pi^nent, with wnich the interstices and
folds of the choroid coat are imbued.
It appears from the observations of Mr. Templeton* on the
distances at which objects could be distinctly seen through aper-
tures of different sizes in a thin sheet of brass, that the angle
subtended by the diameter of the aperture at any point of Sie
object, should be about 19^ or a little more : hence whenever by
an act of volition the eye is directed to a near object, the iris is
observed to contract, in order to limit as far as possible the visual
pencil to the aperture most suitable for distinct vision. The same
observer has also inferred that the diameter of the space on the
surface of the retina occupied by the image of any point of an
object is about 0'041"««
1 181. The refracting structures of the eye thus act upon light,
and produce an imaee of any object upon the retina in the same
manner as a convex lens, with the advantsge of increased clear-
ness of the picture from the absence of both spherical (1081) and
chromatic (1119) aberration, produced by the curved form of the
retina, and by the structure of the crystalline lens ; the refractive
power of its centre beinggreater than that of its surface, in the
ratio of 1*399 : 1'377. The diminution of aberration is also a^
sisted bv the pupil, which acts in the same manner in preventing
spherical aberration, by being placed between the fluid meniacus
and the crystalline convex lens, as does the perforated diaphragm
in the Woilaston doublet, or the excavated sioes in the Coddington
lens (1 154). Chromatic aberration is doubtless to a certain extent
compensated in the eye bv the different dispersive powers (1101)
of its several structures ; but this oivan is by no meana perfectly-
achromatic, as mav be shown bv the spec^td colours observed
fringing minute boaies held near the eye : some of which, however,
are probably due to diffraction fll25).
1182. The eye in all warm-blooded animals is formed upon the
type of that of man, with the occasional addition of supplementary
structures, better fitting the organ for the perfoimanoe of vision
in the particular animal. In fishes, residing m a medium of nearly
the same refractive index as the aqueous humour, thecnrvatnreof
the cornea becomes useless, and the aqueous humour is replaced
by a viscid secretion of greater refractive power. The ciystalfine
lens is, in these animals, nearly spherical, and placed very near
to the cornea, and the iris, which is close to the latter, is imdilft-
• PhiL Mag. sad Anaals, I>eo. 18M.
DiVERTED lUAOE OK THE RETINA. 67l
table. In insects tlie eye is yerj simplei consisting of a lenticular
cornea, placed in front of a nervous expansion. The curvatures
of the several refracting surfaces of the eye, and their relative
distances from the retina, have been observed to vary considerably
in the different orders and classes of the animal kingdom.
1183. Although it is demonstrable that images of external
objects are formed upon the retina, it has been doubted by some
whether the latter membrane is the seat of vision, as in certain
species of cattle-fish an opaque membrane is found between the
vitreous Humour and retina. The choroid coat, and vitreous hu-
mour, have each been supposed to be -the true seat of vision. It
is a curious &ct, that the point where the optic nerve enters the
eye is absolutely incapable of visual perception, and the image of
any object falling upon it ceases to be visible. This mavbe shown
by placing three watisrs on the table about two inches distant from
eacn other, and having closed one eye, look at the outside wafer
on the same side as the closed eye : at the distance of about eight
or ten inches from it, the two outer wafers will be distinctly seen,
whilst the middle one will be qjuite invisible.
1184. When an object is viewed with both eyes in a healthy
condition, it appears single, whilst it is obvious that a distinct
image is paintea upon each retina. This is readily explained by
tiie fact, tnat the two images, formed on corresponding portions of
each retina, are mentally combined, and virtually produce but one
impression. If the optic axes be not brought to coincide at the place
of the object, the two images do not correspond, and then, as in the
case of SQuinting persons, the object appears doubled, or confused.
1185. Many mgenious arguments have been used to explain
why objects appear erect, whilst their images painted upon the
retina are inverted, although a little reflection on this circum-
Btanoe renders it probable that such must necessarily occur, from
the law, that all objects appear to be placed in the direction pur-
sued by the rays which eventually reach the eye. If a b, Fig. 631 ,
bean object from which i.. ...
the rays following the ^*^' ^^*
direction of the lines
shown in the figure pass
into the eye, they be-
come refracted towards
the retina, and paint
upon it the image c d.
Tnen if the retina be
aasomed to be the seat of vision, the impression communicated
by it to the sensorium is that of an erect object : for the part d of
the image will appear to be placed in the direction of the rays d a,
and the up]^er part, c, will appear to correspond with the lower part,
B, of the ooject, which will appear to be situate in the direction of
the rays o b. Consequently, although the image painted upon the
672 OPTICAL IVSTRUMENTB.
retina is really inverted, it canyeys to (he mind the impression of
an erect object; obtained, probably, in the first instance, by the
comparison of Yisual impressions with those commnnicated by the
sense of touch.
1 186. The Eye. — ^The wonderful perfection of mechanism in the
eye, as an optical instrument, is most conspicuous in its power of
adapting itself to various focal distances. It is well known, that in
viewing objects through a telescope, the distance of the lenses from
each other requires to be altered by drawing out, or thrusting in,
the slides of the telescope, whereas the eye appears intuitively to
accommodate itself to the various distances at which objects
happen to be placed. The quiescent, or ordinary state of the eye
when in its perfect or natural condition, is that of culaptation to
Sarallel rays, that is, to the perfect vision of objects at considerable
istances ; the alteration of focal distance takes place in adapt-
ing the eye to the distinct vision of near objects, and consequently
consists in an increase of that distance. There can be little donbt
that this change is effected chiefly by a displacement of the crys-
talline lens forwards, eflfected probably by the distension of the
vessels of the ciliary processes, which constitute an erectile tissue ;
and partly bv the contraction of the muscular fibres of the ciHary
ring, the probable effect of which would seem to be a slight elonga-
tion of the axis of the e^e, and an increase of the convexity of th«
cornea : all of these actions would evidently tend to bring mrwards
the focus of the refracted rays. The contraction of the iris that
invcariably accompanies the adaptation of the eye to near objects, is
probably only accessory in cutting off the peripheral rays in which
the aberration is necessarily greatest, and not the essential means
of adaptation as was supposed by Sir C. Bell.
The variation of focal length of the eve has been attributed by
some physiologists to an alteration in the form of the ciystallina
lens bv the contraction of its own fibres ; but the structare of
muscular fibre is so completely identical, from whatever part of
the animal kingdom it may have been obtained, and so essentially
different from the fibrous structure of the lens, that it appears
difficult to conceive the existence of muscularity in that organ ;
moreover, in all the higher orders of animals, the muscular struc-
tures are copiously supplied with blood-vessels and nerves, neither
of which have been detected in the crystalline lens. It has been
very confidently assumed by Prof. Donders* to be proved that the
acconmiodation of the eye to the distinct vision of near objects is
chiefly effected by a passive increase of the convexity of the ■
crystalline lens, due to external pressure on its peripherv ; but he
fails to explain how this is, or can bo, effected. This point is con-
sidered to be placed beyond controversy by an experiment of
* On the Anomftliea of Aocommodation and Befraotion of the Eye. 18M.
(TraQBlation pnbliahed by the Kew Sydenham Society.)
Cramer,* in which Ihe relative poaitions of the images of a candle
placed at a diatance of 30° from the optic axis, and reflected from
faces of the lens, are compared, as seen, flnt, when the eje is
a state of qiiieBcence ; and aecondly, when accommodated to a ne
nhject. These images are thua represented bj the observer : a,
Fig. 633, is the image formed at the
surface of the cornea, and b, e, those at
the anterior and posterior surfacea of
the lena respectively, when a healthy
eye is in a quiescent state ; and d, e,/,
are the correaponding im^iea, when the
eye has been adapted to a near ol^ect.
It is imdonbtedly obvions from tbe ap-
proiimatioD of (he image e to i^ that
the anterior aurface of the less moat have approached the cornea ;
bat it is b; no means eqaalij obvioua frma the change of the form
of the image from bto «, that the curvature of the lens has been
incisaaed, or from the comparatively slight change from e tof. that
tbe poater^or surface of the lena has not been brought forward, and
that the axis of the lena muathave been elongated. These pointa,
however, require a much more elaborate consideration than apace
here permits: all that can be taken sa really proved by this ex-
periment with resard to the mechanism of adaptation is, that Ihe
anterior sorioce of the lena is appraiimaCed to the cornea.
IIBT. In order that vision ma^ be distinct, it is necessary that
corresponding points of the oliject and of the retina ahoald be
conjugate ibci of tbe eye ; or in other words, that the pencils of
rays diverging from each point of the object, and entering the
pnpil, should converge to a focns on the retina. If, aa in Tnyopic
or abort-sighted persons, Ihe rays convei^e to a focus before tjiey
roach tbe retina, from the too great convexity either of the lens or
coniea, they impinge on that sensitive organ in a atate of diver-
gence, then the pencils proceeding from conliguons points of any
oliject are superposed upon, and conaequentlj confuse each other:
and the fnrtlier the focoa of incident rsya is fram the refracting
aurfaces of the eye, the farther the focus of refracted rays will be
from the retina, and conseqneotly tbe greater the confaaion :
hence, with persons thus affected, the difhcult; of diaoeming ob-
jecta increases with their distance from the eve. The very term
" myopic" is derived from the effort naturally made to diminish
the ajwrtnre of the transmitted pencils, and consequently the
confusion, by partially closing the eyelids: on the same principle
that an oliject placed very nearto the eye may he seen distinctly
through a pin-hole in a card. A concave lens of suitable power.
ij increasing the divergeoce of the incident pendl (1073), will
' Hm aeeoBnudatle-ietswgin, ph^olnciieh toegeliotit. Haarlna, 1853.
674 OPTICAL ISSTRUMRMTB.
diminish the convergence of the refracted ra.VB, and conseqnentlj
carry back each focal point towards the retina ; and this is tM
kina of leiis in spectacles worn by short-sighted persons.
In presbyopic^ or Umg-aighted persons, on the contniiy, the lens
or the cornea is not sufficiently convex, and as the foci of refracted
pencils are consequently situate behind the retina, a similar super-
position and confusion of contiguous pencils ensues. In this case
It is necessary to diminish the divergence of the incident Tays,
which is effected by a convex lens (1071), and the convergence of
the refracted pencils being thus increased, their foci will be Drought
forward to the surface of the retina. As the t^irm "presbyopic"
implies, this is the state of vision incidental to old age, and arises
{>rincipally from the diminution in the convexity of the crystalline
ens tnat naturally takes place in advancing ^ears ; but partly
also from the. diminished power of accommodating the eye to the
distinct vision of near objects.
1 188. A remarkable defect of vision has occasionally been ob-
served, which consists of a want of agreement between the refrac-
tion of the eye in some two perpendicuUr planes passing through
the axis of the organ; this has been termed tutigv/uUitm, and
cannot be remedied by a lens of any kind having spherical surfaces
only: the excess of refraction in one direction must be reduced by
a suitable concave cylindrical surface, or its defect in the perpen-
dicular direction augmented by a conoex cylindrical surface.
This defect of vision was first recognised in bis own person by
the present Astronomer Royal, and to his habitual sagacity we
are indebted for the appropriate remedy : he found that one eye,
from the imperfect vision of which he apprehended serious incon-
venience in the discharge of his important functions, was more
short-sighted in a nearly vertical than in the perpendicular direc-
tion ; and vision sufficiently perfect for ordinary purposes was re-
stored by a lens with a concave spherical surface of Z\ inches
radius, and a concave cylindrical surface of 44 inches radius, which
was turned from the eye, and its axis placed a little obliquely.
The following method of correcting astigmatism has been pro-
posed b^ Prof. Stokes : — Let a plano-convex, and a planoKM)ncaTe
cylindncal lens, of moderate but equal curvature, be each set in a
circular frame with their plane surfaces in contact, so that one
may be made to rotate on the other : when the axes of the two
surfaces are parallel, no refraction will take place, as the united
thickness of the two lenses will be everywhere the same, bat when
the axes of the surfaces are perpendicular to each other, the com-
pound lens will be convex in the direction of one axis, concave in
that of the other, and neutral at 45"* between these directions.
As the angle contained between the direction of the axes is dimi-
nished from 90° to 0", the difference of the refractive powers of
the compound lens in two perpendicular directions will be reduced
from its maximum value, to nothing ; and some intermediate poei-
IHCIDEHTAL OB SPECTBAL COLOURS. 675
tion may be found, in which the coniponnd lens will correct the
astigmatism of any given eye, and when combined, if necessary,
with a spherical concave nr convex lens of suitable power, will
produce correct vision : and from the curvature and relative posi-
tion of the cylindrical surfaces, and the curvature, if any, of the
spherical surfaces, the requisite surfaces for a single lens, that
will completely correct the given visual errors, may readily be
determined.
1189. The impression of an image npon the retina lasts for an
appreciable time after it has ceased to be formed, and hence the
eve may be rapidly closed and opened, without losing sight of an
object. If a burning stick or red-hot ball be made to revolve so
rapidly, that the whole revolution may be completed in about
0'14P, an entire luminous circle is produced. Phe impression
thus vividly excited upon the retina appears to continue about
one-seventh part of a second of time. The duration of this impres-
sion on the retina varies considerably with the intensity oi the
light producing it ; with direct solar rays, or those of a powerful
electnc light, an image of the source of light will remain for
several seconds imprinted on the retina.
The persistence of visual impressions has been made the basis
of many optical toys ; thus, if the figure of a horse be placed on
one side of a circular card, and another of a rider be suitably
placed on the other side, on whirling the card by two bits of string
attached to the two ends of a diameter in the proper direction, the
rider will appear mounted on the horse, from the two impressions
being constantly superposed on' the retina. Also if a number of
pictures representing successive phases of the same action be
placed equidistantly round the circumference of a rotating disc,
and they be viewea by reflection through an equal number of aper-
tures in the same disc, so placed that each in succession may be-
come visible only at the moment that it occupies the place of the
preceding picture, the successive visual impressions are mentally
blended mto a continuous action ; thus a steam-engine in motion,
boys playing at leap-frog, couples waltzing, &c. &c., may, with
sufficient attention to the successive positions depicted, be very
vividly represented. This apparatus was termed tae p?tant(ucope.
1190. It has been shown (1091) that the rays of ordinary light
may be resolved into two sets, producing upon the retina different
colours complementaiT to each other ; or which, when^ entering
the eye together, will produce the impression of white light.
When any person gazes upon a red wafer, strongly illuminated,
for some seconds, and then suddenly turns the eye to a white
surface near it, a spectral imase of the wafer, but of a green
colour, will become visible. If the wafer be yellow, and placed on
a black surface, the spectral image will be deep violet when viewed
on a white ground ; in the same manner a white wafer is attended
by its black spectral figure. Thus wafers, or other coloured ob-
zz2
676 OPTICAL IHSTEUUENTB.
jects, produce spectra of colonre complementary to their own.
The complementarj tints thus produced are termed incidental
colours, and may be found by reference to Newton's list of colours
in thin plates of air (11 33), the reflected and trauBmitted tinta
being complementary to each other.
1191. The most complete mode of demonstrating these coloors
is the following, for which the late author was indebted to the late
Prof. Cowper. Cut in a folded piece of cardboard a series of holes,
^.633. ^^^ whole resembling open lattice-work, and
provide some sheets ot thin tissue-paper of
Tarious bright colours, selecting those present-
ing strongly defined tints ; place one of theae
between the folds of the cardboard and hold it
up to a vivid light, keeping the eye fixed on the
lattice- work, whilst the -light penetrates the
coloured paper : in a few seconds the whiteness
of the pasteboard will disappear, and be re-
placed by a strongly-marked tint complementary to that of the
paper placed in it ; thus with yellow paper the framework will
appear violet, with blue it will be orange, and with red it will be
green. The illusion is so complete, that it always excites sur
prise in those who see it for the first time.
1192. These incidental tints have been explained by Sir David
Brewster in the following manner : — The eye being strongly ex-
cited by gazing on a coloared body, as a red wafer, becomes
partially paralvsed to the action of undulations producing that tint,
and this influence on the retina spreads itself bevond the
spaces actnallv impressed ; on then allowing white light to im-
pinge upon tne eye, those undulations which produce npon an
nnexcited eye the sensation of a colour corresponding to that of
the wafer or coloured paper, are without action on the partially
and temporarily paralyf^d organ : and the remaining sets of
undulations are alone active, producing on the retina the impres-
sion of a tint complementary to that of the coloured surface.
1193. A remarkable case of the resolution of light into its com-
f>lementary tints, by unequally exciting the eyes with colouriess
i^ht, has been described by Mr. Smith.* On holding a slender
slip of white paper vertically about a foot from the eyes, and
fixing them upon an object at some distance bevond it, so as to
see tne paper double, if the light of a candle be allowed to act
vividly on the ri^ht eye, without affecting the left, the left-hand
image of the stnp of paper will appear to be bright green, whilst
the other will exhibit the complementary colour, or red : if the
contrary eye be now illuminated, the position of the compk»-
mentary tints will be reversed.
1 1 94. Individuals are not unirequently met with, whose eyes are
as insensible to certain tints, as the ears of others are to particular
* £dm. Joom. Sdence, iii. p. 1.
BIKOCOLAR YIBIOir. 677
soandfl. Sereral cases of this kind have been described, in which
the following colours haye been confounded by the persons affected
with this curious defect of the visual organs :*
Bright green, with grajish-brown and flesh red :
Rose red, with green and gray :
Scarlet, with dark green and hair brown :
Sky-blue, with grayish-blue and lilac-gray :
Brownish-yellow, with yellowish-brown and grass-green :
Brick- red and rust-brown, with deep olive green:
Dark-violet, with deep blue.
This remarkable ^ate occasionally occnrs in disease, and disap-
pears on the patient*8 recovery, llie late author once treated a
case of cerebral disease, in which vision was previously perfect, but
during the attack the patient confounded several tints with each
other. The colours mistaken for each other in this instance were
in general the complementary ones ; red being mistaken for green,
ana orange being confounded with blue : of the physical cause of
this remarkable state, however, nothing is known.
1195. If an image of a picture be formed in a common camera
obscura (1147), it matters not in what position the instrument is
held vrith regard to the picture, the relative position of all its
points will be the same iu the image ; but when a solid object, as
a house or a tree, is depicted in the camera, this is notoriously
not the case, but the relative position of the several points of the
image will vaiy with the position of the instrument, just as we
see the object under a different aspect, whenever we change our
point of view : that is, a different picture is painted on the retina
with every different position of the eye. Consequently two
slightly different pictures of any object in relief will be simulta-
neously impressed on the two eyes ; the amount of difference of
the two pictures depending upon the amount of relief, or relative
distance of different parts of the object from the eye. That this
difference of the two pictures actually exists may be proved by a
very simple experiment : place a lighted candle about three feet
in front of the face, and hold up the fore-finger between the candle
and the nose ; the finger and candle appear on the contrary sides
of each other when viewed by the two eyes separately. A correct
knowledge of the principles of binocular vision^ that is, of the
mental impression derived from the combination of two simulta-
neous visual images, is due to the ingenuity of Prof. Wheatstone,
who first directed the attention of phvsiologists to the fact, that
the mind derives the perception of relief or solidity from the com-
bination or superposition of two dissimilar visual images, simulta-
neously depicted on the two retinae ; and this he has proved to be
the case by showing, that if two pictures of an object be taken in
the direction in which it would be viewed by the two eyes sepa-
* Beebeek ; Pogyendorf, AniuJen, zliii. 177.
Tstclj (for whlcli pnn>os« none can be bo perfect u photngnpha).
ftnd these picturea be lo prenntei] to the two eveii, tbat llieir
— c_ii __ .„__ 1; — — «i™. „f fj^g ratJDK, the
1196- The inatrumcnt by which these tralj magic eflecta uv
produceii is called Ibc tttTeo4eopt, and is of tiro kind*: Prod
WheaCstoae's oiiginal inBtrument, the reflecting BtcreoKop«,
rD. Brewater: a
modification Grst, it is believed, proposed by Sir D. Brewati
portable little inatrunient, and Dow generally applied to si
Bcopic pholograpbs of the ordinary email aite.
The reflecting stereoscope, Fig. 634, coDsista, in ita simpleat
form, of a horizontal board, about 15 inches long by 4 Or 5 inchea
wide, in tile middle of which are two Bmall plane mirrors, placed
Tertically and at right angles to each other, and at the enda are
two Tertical frameB with grooves, iu which the two pictures, dravn
or mounted on stiff paper or card-board, ma; be placed. The eyes
iV.ai.
[*?.
^
of the spectator are placed in front of the mirrors, so at to aet one
of the pictures reflected from each, and the pictures are adjusted
by monng them backwards or forwards in the grooves, until their
images exactly overlap each other, when the tlereoscopie effect
will immediately be discovered ; spectacles will be necessary for
those who are accuntonied to use them. The apparent realtt; of
the impreasion derived from the superposition of the two imazc*
tia ess. ^" ''^P^'"^ "" '''* correctness with which
the pictures are taken at the visnal atigle;
'* that of about 7" or 8° is commonly feund to
answer the purpose leiy well.
The refracting stereoscope consists either
of a pyramidal box, as more frequently ceo-
Btrucled, or of two parallel boards, separated
6 or 7 inches by a third placed perpendicn-
ilarly between them, as represented in fig.
6SS, and when these ai« connectod by hingea,
they ma; be folded up, for the sake of porta-
bilitj. Id either c«m the pictures are placed in the botttm of the
THE 8TERBOSCOPE. 679
instrament, and viewed through two priflmatic lenses, ▲, n, contained
in short tubes placed at the distance of the eyes from each other
at the top of tne instrament The rays proceeding from the pic-
tures p, o, in the directions o a, d b, are refracted through the
prisms a b, and entering the ejes in the directions e a, e b, apoear
to coincide at e. Two pieces of a common double conyex lens
cut in quarters were originally proposed for this instrument, and
answer sufficiently well for common purposes, but a far better effect
is produced by an achromatic combmation of two prisms, haying
different dispersive powers (1096).
Two simple figures have been selected for illustration, as they
are both susceptible of an easy explanation. In the reflecting
stereoscope, Fig. 634, the pictures are circles with a vertical dia-
meter, and an arrow placed obliquely across it; but it will be
remarked, that the arrow cresses the vertical diameter in appotUe
directions in the two pictures, and as such images can only be
formed in reality when the arrow is inclined to the plane of the
circle, that inclination is mentally inferred from the superposition
of the two images. The reader may readily satisfy nimself of
this fact by drawing a circle and a diameter on paper laid hoii-
zontally, and placing a pencil or any other thin straight object
through the centre, when, if the line and rod are both in a vertical
plane passing between the eyes, the images of the rod presented
to the two eyes will be on opposite sides of the line.
In the refracting stereoscope, Fig. 635, the pictures are squares
containing smaller ones placed eccentrically, with lines joining
their corresponding angles. These present the impression of a
square pyramid, the truncated apex of which is placed towards or
from the eye, according as we combine the pictures f, o, or o, p.
This will be rendered more intelligible, by considering the relativo
position of the, rays proceeding from the several points of the pyra-
mid to the eye.
Let tlie pyramid, a, Fig. 636, placed on the table, b, be viewed
by the eyes of a person looking perpendicularly down upon it,
and suppose that lines drawn from the «^
several comers of the pyramid to each ^'
eye be intercepted b v a screen, c, placed ^ ■■ ^ mr _
{)arallel to b ; of these the four back **Tri \^
ines onl^ are drawn in the diagram. i\ \ \ ,fj\
"By joinmg the several points where i V\\ '/f;\
these lines pass through tne screen, we j ^^^^-j^H c:\
obtain the figures drawn on it, which * — : — \ ,w : — r —
are the projectiont of the pyramid, and .J(' .At, 1
which correspond with the images pro- i /Jf — v\ \
duced on the two retinae. It will be . U^-yy-\S4 1
seen from the relative position of the / [/""jl \^ B \
lines drawn from the two eyes to the / \
pyramid, that the projections on c will
be necessarily dissimilar, the interior squares being nearer to the
contiguOnK aiileB of tlie outer oats; and it iafrom tberape ,
of these disaiiuilftr images that the mind infer* the elsTation (rf'tke
rnnaller iquaro aboTC, or in front of, the larger one, and «■«-
qnently, the trao relatiTe poaition of the linea drawn from one to
the other, farmtng tbe Intenil edge* of the prnimid. If t)w pv
sitiun af the pyramid were reversed, the relative poaition of Uk
iDiier and outer xgunres in the projectiorM uraold be icTeraed like-
wiBo. M at (i, V. Fig. 635.
The line (liagtnmB for the refracting stereoacope are conmMBl;
CD a black ground, of which Fig, 637 ia a apecimeii ; the projec-
tions A,^wbe ....
cone resting od itg base, and the projectioiu a, a, that of the h
cone in an inrerled poaition. Hint of onr r«adeni maj aneeead in
Huperpoeing the two adjacent image* withont the aid of a ttereo-
HCiipe, hy pbcing thisdisRraru about 12 inches from the ojea, aiid
loofcintc Bteadil; at the point of the finger held about fmr incbea
from the paper, aad betvncn it and a point midwa; between the
eyes ; if the attenlioc be then directed to the diagram, bat with-
out moriag the eyei, the slercoiicapic eBect will probably be
produced ; and \>y a little practice, the same may be e&ct«d
Hilh any ordinary slereoscnpic slide, without the aid of a itcreo-
acope, iu which case the slide will appear to cooaist of thi«e
pictures, tlie centre one being in relief
1197. The Fseudoicopt. — Another important fact in tbe phy-
siology of Tiaion baa been demonstraled by ProC WlwatxtODC.
namely, that the perception of the relative distaoces of otjects
depends upon the de^o of convergence of the optic axea, wheo
the eyes nre Buccossivcly directed to them. If the eyes be di-
rected to nn object placed at a, Fig. 638, the linai of tiana] di-
rection of the two eyva will evidently meet at a laiver angle ibaa
when they converge to a more remote point, b : and tbe acDaorial
power probably appreciates the variHtioa of angle by the amoimt
of nmscalar action necessnry to produce the reqaired chaon of
position of the eyes. This is proved to be the caae by an inn-
nioUB inalrument, the pttudoteopt, bj wbich the relative direcaao
□r rays reaching the eyea is inverted, and a correipiHidiDg in-
pressian of inverted relative poaitiou of the difleisnt parta of ao
object ii produced ; and the illuaion ia moat eitraoidinary, a can.
ce, M » bowl, g
If I
i«ctaiigular
Pis- ex.
priomi, c, □, be iiit«rp<wed between tbe ei^
and A, B, in tbe position repreneDted ip t ig.
638, eacb of tbs n;s from the point* i, B,
being refracted at tbe lint aurbces, then re-
flected intemalJj at the backs of the prisms,
and again refracted at their second surfaces,
will enter the ejes in reversed positions;
that is, the ra^s proceeding from a will eater
tbe eyes in tbe directioDS so, and appear to
come from b, while thase from ■ will similar];
appeiLT to come fnmi a, and thos the relative
poeition of the rays is inverted.
The delusi ve impression is not immediatelj
produced iu some iniliViduals, in whom the
judgment appears for some time to contend
BQccessfull; with the visual impressioi
sooner or later the judgment givea way, ana
the object suddenly appears to be turned inside out : Ihns com-
pletely falsifying the old adage, that " seeing is believing." for
we are unabls to resist the visoal impression, although we know
it to be eiTOneooB.
1198. The OphtKalnloteoi>t. — This instrument has formed an
important addition to the visaal means of investigating morbid
conditions of the various atructares tbat enter into tbe tormation
of tbe eyeball. In its simplest form it
consisU of a small concave mirror with 'V- •a»-
an aperture in ihe centre, ihrougb which
the intericr of the eye is examined (with
the aid of a lens, if required), when
that cavity is illuminated by the rays
from a lamp retifcted ^m, and con-
densed by, the mirror. A more conve-
nient form oF apparatus designed by
Liel)reich, and somewhat improved by
Messrs. Becli, is represented in Fig.
639. In this a stand, A, supports a
rest for the chin of the peison nnder
observadon, the height of which is
adjusted by a sliding tube, b, and a
ctunping screw, c. Tbe opbtholrao-
ecope is moontedonaboard, d, runoing
on three small castors, that it may he
moved readily in any direction, for ad-
t' istment. The tulw, B, is supported
oiiiontally on a stem having a rack and pinion a4f'"^ont
nt F, and hat at one end a perforated concave mirror, a,
682 OPTICAL IKBTBUHEHTB.
and at tbo other a lenr, h, both being moveable or Tertical ajEes.
The lamp, i, is supported by a horizontal aim, k, from the
upper part of the stem, so that the horiaontal position of the
plane of incidence and reflection is unaffected by the Tertical
adjustments ; and the arm, k, also supports a shade, l, by which
the eye is sheltered from the direct rays of the lamp. In order
to steady the eye for the examination of its different parts, it is
directed to be fixed on an ivor^ ball, m , which is capable of being
moved in any required direction by means of a sliding tube, x,
and a ball-and-socket joint, o. The eye is illuminated by rays
reflected from the mirror, o, and condensed by the lens, h, which
serves also to give a magnified imase of any part observed
through the aperture in the centre of the mirror.
1199. Some other instmment8 have also been devised for the
visual examination of interior living structures. The larfpngo-
scope, for examining the actions and condition of the interior of
the larynx, consists of a concave reflector, with a central aperture,
for illuminating the larynx, and a small square plane mirror with
rounded comers, placed comerwise at an angle of 45** with the
direction of a straight stem ; this is held, by means of the stem, at
the back of the open mouth, when the opening of the glottis, and
the action of the various parts during the production of vocal
sounds, is readily brought into view, through the aperture in the
mirror. This instrument has served to show the fallacy of the
ideas that have long passed current, as to the mechanism by
which the glottis is closed, as in the act of swallowing : the chorda
voceUes, instead of being, as was supposed, everted, are folded in>
wards, and protected by the closure of the mucous membrane over
them.
'J'he interior of the urethra has been sucoeaafnlly submitted to
ocular inspection by means of the endoaeope; this iustrament
consists of a straight silver tube capable of introduction, which
is widened oat at its outer end, so as to receive a small perforated
plane mirror, placed at an angle of 45** with the axis of the tube.
The light of a lamp j>laced laterally is reflected by this mirror to
the further extremity of the tube, and a small portion of the
mucous lining of the canal may be distinctly seen on looking
down the tube, through the aperture in the centre of the mirror.
683
CHAPTER XXI.
POLARISED LIGHT.
1200. Iir order that the ytirious properties of light might he
■acceBsfuUy inyestigated, the rays were in the first instance con-
sidered homogeneous, to obtain the general results of reflection and
refraction (Ch. XYIll.) The heterogeneous character of luminous
rajs has subsequently been considered (Ch. XIX A but without
any reference to the physical structure of the meaium on which
they impinge, or through which they pass, or to the influence
of that structure on tne course of the reflected or refracted
rays, or on the direction in which the undulations of any
given ray take place : it now remains^ to consider some very
remarkable properties of refracting media, and of reflecting sur-
faces, by which the nature of the rays is variously modified.
The intimate and constant relation between the nature of the
wave-motion transmitted to the eye, and the molecular arrange-
ment of the substance on which the waves impinge would seem to
indicate that the motion must necessarily be transmitted by the
molecules of the body itself, and not by any hypothetical fluid
existing in its interstices: it is very difiBcult to conceive tho
physical pro^rties of this fluid to be so extraordinarily modified
Dj mere contiguity ; and the hypothesis is needless, if the mole-
cules of the body suffice for the transmission of the waves of light.
One of the most familiar phenomena is that of double refrac-
Hon; a power possessed by certain crystallized substances of
separating an incident pencil of light
into two portions, differing from each
other in their physical properties. Let
A H, Fi^. 640, be a rhombohedron (24, Y.)
of calcite, or Iceland spar, resting on one
of its faces, a line joinmg a, ii being the
axis of the crystal ; and let a ray of
light, BT, be incident perpendicularly
upon one of its surfaces, a h ; i.nstead of
passing through without refraction, as it
would through glass, it will be divided
into two rays, one of which, to, pursues
the direction of the original ray, bt, and
is consequently unrefracted, and another,
TB, which is bent or refracted in the
1^.610.
plane a h h b, in which the aziB a x lies : the former is called tha
684 POLABIZED LIOUT.
oriHnary^ and the latter the extraordinary ^ rar. If the direction
of the ray b t, instead of being a normal to one of the faces, were
oblique, it would, on entering the crystal, be separated into two
rays, one of them the ordinary ray, obeying the general law of
refraction (1054), snd the other the extraordinaiy ray, following
a different law, being refracted from the axis an; or, in other
words, being so refracted, as to form a greater angle with the
direction of the axis than the ordinary ray.
1201. The double refraction of the incident ray may be readily
obseryed by viewing a small circular hole in a card through a
crystal of calcite : on looking throneh the thickness of the crystal
at the card, two holes will be visible, from the li^ht that enters
the aperture in the card dividing, whilst traversing the CTystai,
into two rays, which reach the eye separately. On taming the
crystal round, whilst the object remains fixed, one of the spots of
light will appear to revolve round the other ; the fixed spot corre-
sponding to the ordinary ray. Any object, as a line drawn on
paper, will appear double, in consequence of this property of double
refraction ; and if the rhombohedron be gentl v turned round whilst
on the line, one of the two lines at first visible will gradually ap-
proach the other ; and when the line on the paper lies in the same
plane with the axis of the crystal, they will overlap each other,
and appear to form a single line.
The plane, ahnb, passing through the axis of the crystal, and
dividing it into two triang^ar prisms, is termed the prindpai
section of the rhombohedron. It two sections be made perpendi-
cular to the axis of the rhombohedron, a ray incident perpendi-
cularly on the plate thus formed, will suffer no refraction ; that
is, no double retraction takes place in a direction parallel to the
axis.
1202. The property of resolving transmitted undulations into
two series differing in velocity, and conseouently producing double
refraction, is not confined to the crystals of calcite, but belongs
in general to all crystals not comprised in the cubio system (24, 1.).
In all doubly refracting crystals, there are either one or two
lines of direction in whicn no double refraction is observed to take
place ; these are termed the optic axes. In the case of calcite,
there is but a single optic axis, which coincides with the axis of
the rhombohedron (Fig. 641). In uniaxial crystals, the optic
corresponds with the geometrical axis, a line around which the
constituent molecules are symmetrically arranged. It occasionally
happens that no double refraction exists in some particular direc-
tion, in consequence of the presence of two doubly refractive forces
neutralizing each other, as in mica ; this is then termed the re-
euUant axis, in contradistinction to a real optic axis.
1203. Crystals with one optic axis, corresponding with the
geometrical axis of the crystal, termed untoxtoi cirstals, include
all those which belong to the Pyramidal and KhombohedFal
systems (24 ; II., V.). The relation of the extraordinary raj to
the optic sxiB is constant for eitch uoiaiifil crystal, b
fnicted either loiearilii it, aa in qaarti, or from it, as in
Tboee crjatala in wliich tbie rayis bent tovanis the axis
to have a poiiiive, and when bent from it, a negaiinf opi
The quality of the rollowiag crystala baa been oOBerred :-
Axit Piiniine.
Diaplase.
Apopbjltite.
Salphateorin
|jotasb.
Boracite.
I FeiTocyanate ofpotaah.
Phosphate of magneda
Axil Negativi
Calcite. I Emerald.
TourmaliDe. Mica (some Tsrieties^
Sapphire. I Cyanide of mercury.
1204. With regard to the ralalive velocity of the two seta of
nndulatiooa into which light incident on a doublr refracting
ciyatal is raaolTed, Hujgeaa hai demimatrated that the iliiTtronce
between the squarea of the velocitieB ie equal to unity divided by
the square of (he aina of the angle fonned by the ray with the
axis. Id calcite, the ordinary ray therefore moves with a greater
velocity than the eitraordinarj one.
1205. The law of double refraction will bo more readily under-
itood by BnpposiLig a rhombohedron j^, ^41.
of calcite, tig. Ml, of nhieh
axis is AX, to & formed into a ap
of wbich o ie the centre, and c
diameter perpendicular to A x.
found that wben a pencil is tr
milled along the axia a i, the index
of refraction ia l'€54, and ia the
same for both ibn ordinary and ex-
traordinary raya ; for a ray in the
direction d o, the index of refraction
is (he name for the ordinary ray, bu t it i a l'4Hit Ibrtheoitraotdiaary,
the diSerence of which is 0171 : tho difference of the indicea is a
maximum at the equator, and ia found to diminish graduitlly to
either pule. It ia aJao obaerved that the index of extraordinary
refraction is the same for all radii of the eqiiaturial plane { and
that it ia the same in the direction of all linKa joining o, and a
point in the circumference of a circle parallel to the ei:
-. Tha
8 for
finding the index of exlraordinnry refraction at any reijuired point
of the sphere. Produce e o, Fig. 530, to e, d, take
ocoro<J:OA: : 1-654: 1-483,
and describe an ellipae through the points a, (i^ x, e, of which a
I ia the minor axis. If the eitraorUinary index, i, be required
in the direction of a line ao, cutting the ellipae and circle in a
and b respectively, it will be found Irom the formula,
1 = 1-654 X—-
The t«nii negative 11 applied to crjstaU in which the eitnor-
dinarr ie less than the cirainarj index 1 vhsn greater, the; ban
a ponliue optic axii (1303). _
irthe CTjstal have a podtiTe axis, at qaartz for examplfl, the
ellipse deacribed aa in the preceding Rgure will lie wiMim the
^' m^or axis. The ordioan index in qnarti is
I1'54S4, and tLe eitraordinarj at the eqaator
V D IB 1-S583 ; take therefora
oeOTod-.ot.:: I-M84 : 1'558!.
The extraardinarT index, i, 'm a direction
pOTpcndicnlar to either of the triangntar facea,
fonning the prramidal guniioiu of Hg. 15,
which are iDclmed at an angle ofSS' iC to
the iixis, will be found bj drawing oab.
Disking tlio suue angle with OD, and
i = lM84x^.
1!06. A large nnmber of crjetals, inclnding thoae belonff^c
to the Priamatic, Oblique, and AnoTthic s^stema (24; III., IV^
VI.), poB£cu lao optic aiet, and are hence denominated biaxial
eryttali. These axes da not usuallj^ correspond with anj promi-
nent lines in ihe ci^slal, and form various angles with each other ;
from a few degrees, to one of 80° 30*, as in carbonate of potasa, and
to a right aii^e, aa in sulphate of iron. Tbe following list taa-
lains toe namee of soine of the more important (uunol <:i7«taU,
es to each other, taken fmm
Sulphate of nickel . 3° to 42'
Biboraleofsoda . . 28 42'
Sulphate of bar^tes . 37 42
Heulandite . . . . 41 40
ign. 46 49
■" lo 60*
Soda^ulphateofmi
Brazilian topaz .
Sulphate ofatrontia
Sulphate of lime .
Nitrate of silver .
jhate of potai
Kitnte of polsah . . b'W
Carb. atroutia . . . 6 56 1
Talc 7 i4 (
Ciirbonate of lead . . 10 3fi I
Mica, some varietiei .14 0 {
Sulph. magnesia . . 37 34 ^
. 43 S4
Sulphate of nnc .
Pbasphale of soda
Tartrate potass ,
Tarlaiio add . .
UHDULATI0K8 IH 8PACE. 687
1207. In crystals with two optic axes, neither raj corresponds
with the ordinary ray in uniaxial crystals (1200), as neither oheys
the law of sines ^1054^; so that the two sets of undulations, into
which ordinary hght is resolved hy a hiaxial crystal, are hoth to
be considered as producing extraordinary rays : this obserration
is due to M. Fresnel. Crystals are occasionally met with pos-
sessing two axes of double refraction for light of one colour, and
but one axis for light of another tint : thus, Sir David Brewster
found that glauberite possessed two axes mutually inclined at an
angle of 5** for red lignt, and but one axis for violet light. Sir
John Herschel found that the axes occasionally vary in inclina-
tion, according to the kind of light ; thas, in the potassio-tartrate
of soda, the inclination of the axes for violet li^ht is 66"*, and for
red light 76"*. In nitrate of potass, the inclination of the axes for
violet light is greater than for red. Temperature also affects the
relative position of the optic axes ; in some crystals, when heated,
not only the inclination of the optic axes, but also the plane in
which they lie, is subject to change.
1208. When glass is unequally heated or cooled, it assumes a
doubly refracting structure, the axes being variously situated,
according to the shape of the substance. A solid cylinder of
glass, heated by being plunged into hot oil, acquires a doubly
refractive power, having one positive axis in the position of its
geometric axis : and if previously heated and plunged into cold
oil, it acquires a similar property, but its axis becomes negative
(1203). In both these cases the doubly refracting power is tran*
sient, and vanishes as soon as all the parts of the cylinder have
acquired the same temperature. A sphere of glass similarly
treated, becomes doublj refractive, but with innumerable axes, as
IB naturally the case in analcime, in which the axes are almost
infinite. The crystalline lenses of all animals j)ossess one or two
optic axes. Pieces of unannealed glass, and in fact almost any
other transparent substance the molecules of which are subjected
to mechanical strains unequal in different directions, as from pres-
sure, contraction during desiccation, or otherwise, more or less
exhibit similar properties.
1 209. The more palpable phenomena of double refraction having
been explained, it remains to examine the changes which a ray
of light has undergone during its separation into two distinct
colourless pencils, as in its passage through a rhombohedron of
calcite : and in order more fully to comprehend the remarkable
phenomena about to be detailed, it is necessary, in the first place,
to extend to space our conceptions of undulatory movements,
which have hitherto been confined to lines, as in the vibrations of
chords and rods (371-376), or to surfaces, as in the undulations of
fluids (463), and of plates and membranes (575-578); and
secondly, to consider tne influence of unequal elasticities of the
medium in different directions on the transmission of undulations.
688
POLARIZED LIGHT.
It has already been stated (1021) that the vibrations constitnfing
a ray of light may be conceived to take place in an indefinite
number of planes passing through the path of the raj, which maj
be exemplified by the various positions of the leaves of a book,
when the covers are bent back against each other. It has been
remarked (463), that in the spreading of undulations from tbe
point of disturbance over the quiescent surface of a fluid, tbe
situation or locus of all particles simultaneouslv disturbed, or, «a
it mav called, the wave-jront^ is a circlOi of wliich the point of
disturDanoe is the centre ; similarly when the undulations of a
uniform medium take place in an indefinite variety of planes, tbe
wave-front will be the surface of a sphere. If, however, from any
cause, the waves shonld meet with more resistance, and conse-
quently travel more slowly in one direction— east and west, for
example — ^than they do in the contrary direction, namely, north
and south, then the outline of the advancing wave would no
longer be a circle, but an oval or ellipse ; and similarly, when a
pencil of light is transmitted throagh a transparent medium, if the
rays meet with greater or less resistance, and consequently travel
more slowly, or more rapidly, in one certain direction than in any
direction perpendicular to the former, then the wave-surface would
no loneer be a sphere, but a spheroid ; ObkUtf or flattened in the
case ot greater, and prolate^ or lengthened, in that of less rem»-
tance. If the elasticity of the medium be supposed unequal in
three directions |)erpendicu]ar to each other, then the wave-suifaoe
will be, not a spneroid, but an ellipsoid.
1210. A ray of light is said to m polarized in any given plane,
when all the undulations of which it consists take place in planes
parallel to the given plane. A ray of common light, incident upon
the surface of any doubly refracting cr^'stal, becomes polariwd ;
and the act of polarization, which takes place at, or indefinitely
near to, the surface, consists in the resolution of all tbe unduIationB
of which the ray consists, in the direction of two planes perpendi-
Fig. 643.
cular to each other ; and in uni-
axial crystals (1203), one of these
planes is the principal section
Sassing through the point of inci-
ence. A ray of liRut, therefore,
after passing the surface of adoubly
refracting crystal, as calcite, con-
sists of two rays polarized in planes
perpendicular to each other ; let a
section of these rays be represented
by 1, Fij^. 643, in which ks is in
the principal section, and c d per-
penoicnlar to it. If these rays tra-
verse the crystal in a direction parallel to its optic axis, they ooo-
tinue their course undivided ; but if they pass in any other diiec-
8INOLB- AND DOCBLE-IMAOB PSISIfB. 689
tton, they are separated from each other, as 2, 3, and of these, 3
represents the ordinary ray, polarized in the direction n s, in the
plane of the principal section, and 2, the extraordinary ray, pola-
rized in a plane passing throngh o d, perpendicular to the prin-
cipal section.
1 2 1 1 . A ray of light, which has traversed a doubly refracting crys-
tal in a direction parallel to an optic axis, is not distinguishable in
its properties from common light, but cannot be considered identical
witn it, since the position of its planes of polarization depends
only on the position of the crystal with regard to the ray. It is
not improbable that a ray trayersing a crystal belonging to the
Cubic system (24, I.) ma^ have been thus modified, although the
raTs polarized in perpendicular planes are not separated from each
other, in whatever direction they traverse the crystal.
There are several modes of obtaining plane polarized light : —
1, by artificially^ stopping one of the polarized rays separated
by double refraction ; — 2, oy transmitting a ray of light through
some substance capable of absorbing one of the polarized rays;—
3, by permitting light to be incident oi^ a medium capable of re-
fracting one polarized ray, and reflecting the other : these will now
be separately considered.
1212. A ray of plane polarized light may be obtained from
ordinary light incident on a doubly refracting crystal, as calcite,
when it will be divided into two l>eams polarized at right angles
to each other ; by sticking a wafer or a piece of black paper over
the point of emergence of one polarized ray, the other may be ob-
tained in a state of isolation.
A more convenient mode of procuring a single pencil of pola-
rized light from this crystal, is to divide the rnombohedron into
two wedge-shaped portions, by a plane intersecting the axis at o,
Fig. 641, and perpendicular to one of the principal planes, and in-
clined at such an angle to the faces of the crystal which it intersects,
as may be between the limiting angles of incidence (1060) for the
two rays ; and then to cement them together with Canada balsam.
The layer of balsam thus included allows one of the doubly-reiraoted
rays to be readily transmitted, whilst it causes the other to be
totally reflected from its sdrface, and to be thrown altogether out
of the field of vision. This arrangement is known by tiie name ti
the $ingle-imagef or Nicol's prism.
1213. If it be required to retain both the ordinary and extra-
ordinary rays, but at the same time to separate them more widely
from each other than the simple rhombohedron of calcite is capable
of separating them, this may be effected bv cutting firom that
crystal, F^. 580, a wedge-shaped piece by a plane passing through
the solid angle h, and two P<nnts in the edges c o, d f, that are
equidistant from o and d. This is cemented by Canada balsam
in a reversed position, to an equiangular wedge of glass. Hie
extraordinary ray ?rill now be found to be more widely separated
Y y
690 TOhAMIEKD LIOHT.
by refraction, while the glass prism prevents chramatie dn
(1082) ; this arrangement is called the dtmbMmage prisnL
' 1214. When a ray of light is incident on a thin and traomrent
{)1ate of agate, cat in a direction perpendicular to its siuoeoQs
ajem, one of its constituent polarized rajs, as c d (Fig. 643* 1>,
becomes dispersed in a nebulous manner, whilst the other, v s, u
transmitted ; Uie transmitted rs^ being lUwrnys polariaed in a plane
parallel to the direction of the nliceous layers of the nuneraL A
thin plate of agate may also transmit a ray polarised in a directioB
perpendicular to its laprers, and disperse one polariaed in an oppo>
site direction. The direction of these layers may be readily made
out, even in thin sections of agate, by the lines visible in their
substance. A plate of agate may thus be employed to furnish
polarized light ; it does not, however, oompletoly separate a be^n
of light into two polarised rays, and is the least satislactofy of the
oidinary means of effecting polarization.
1215. The tourmaline, a siliceous mineral, and especially the
varieties of a yellow or hai^brown colour, when cut into thin plates
parallel to the axis of the« crystal, separate a ray, incident perpen-
diculwly on their surface, into two nys polarized at right angles
to each other ; one of which, the ordinary ny, is transmitted, and
the extraordinary rav is absorbed by the mineraL This actioa of
the tourmaUne on lignt is very remarkable, as no stratified structure
can be detected in this mineral, which, as in the case of agate,
would even help to explain its powerful polarising influence.
The action of a plate of tourmaline, or asate, on oommaD U^ht,
may be familiarly illnstrated by fixing two slips of pasteboard u a
direction at right angles to each other, as n s, o d, fV. 644^ repre-
senting respectively the two planes in which the undulations aie
resolved at their incidence on the polarizing substance ; let a l k t
be a*Bmall frame of wood, having a number of wires fixed across
it, as shown by the lines in the figure. Let a k be supposed to
represent a plate of tourmaline, the bars being parallel to the axis,
and the ^per figure, m o sd, a ray of lieht ; when the latter is
brought into contact with the former, the slip of pi^r v s will
Pjg 044^ readily pass between the wires, but
0 D will be checked ; k s wiB here
represent a transmitted ray of li^ht
polarized in the direction of a pnn-
cipal plane (1201). Let t ▲ be now
tamed round, until the <UrectioD of
the wires becomes vertical instead
of horizontal, then trv to posh the
paper figure through it; the vertical slij> cd will then psas
throagh, and irs will be stopped. By this little apparatus, the
action of polarizing plates of tourmaline or other minerals, actinic
in the same manner, is readily impressed upon the memory : but it
must not be forgotten that the comparison of a set of parallel bars
POLAKIZATIOH BT ▲BSOBPTIOV. 691
to a tourmaline plate is strictly hypothetical, and although avail-
able as pointing out the effects of this substance in different posi-
tions on a beam of light, yet it must not be considered as present-
ing a correct view of the real modus agendi of such polarizing
plates on common li^ht.
1216. Dr. Bird Herapath discovered a new salt,* which ap-
pears to be much superior to the toarmaline as a polarizing agent,
on account of its giving scarcely any tint to the transmitted ray,
as well as from its caosing so little loss of light, on account of the
extreme thinness of the lamion employed : a specimen of this salt,
the sulphate of iodo-quina, suflScient for microscopic examination,
may be very readily prepared. For this purpose a drop of a solu-
tion of disulphate of quinine, in acetic acid, is placed on a slip of
glass, and a very minute portion of tincture of iodine is added :
the whole becomes immediately tnrbid, but soon afterwards minute
crystals, of almost metallic lustre, are formed in the fluid. After
the fluid has eva^rated, a drop of Canada balsam should be added,
and a piece of thin glass placed over it as a cover. The specimen
thus prepared, when examined by the microscope with an object-
glass of |-inch focus, is seen to be made up of minute laminie with
acuminated ends, nearly colourless, except when they cross each
other : at these points light is more or less intercepted ; and if the
crystalline plates lie on each side at rieht angles, the point of
intersection is very deep violet, or perfecUy black. All the light
transmitted through one of these crystals is polarized in one plane,
and is of course incapable of being completely transmitted by a
superposed crystal, unless they happen to lie on the glass com-
pletely parallel to each other.
If sucn a specimen be placed on a plate of selenite, iUaminated
with polarized light, and then examined with the microscope, a
veiy beautiful result occurs ; the little crystals analyse the light
passing through the parts of the selenite on which they are placed,
and present a beautiful display of the complementary colours.
To prepare this salt for the formation ot polarizing laminae, the
following plan may be adopted : dissolve 50 ^ains of disulphate
of quinine in two fluid ounces of acetic acid, and two of proof
spirit, warmed to ISO** in a very wide-mouthed flask, or glass
beaker ; then slowly add 50 drops of a solution of 40 grains of
iodine in an ounce of rectified spirit ; agitate the mixture, and
then set it carefully aside for six hours, in an apartment main-
tained at a temperature of about 50** F. The utmost care must
be taken to avoid any motion of the vessel ; indeed all accidental
vibrations should be guarded against by suspending the vessel by
a string, or by allowing it to rest on a mass of cotton wool. If in
six hours the large laminae of the salt have not formed, warm the
* It has bera proposed to name tbii moet valuable addition to our pola-
rizing media, Herapathiie, after iu ini^uioaa diacoTerer.
Y y 2
6d2
FOLARIZBD LIGHT.
Fig. 646.
fluid with a spirit-lamp, and when it has become clear, add a few
drops of the solution of iodine in spirit.
The large laminae form on the top of the fluid, and should bo
remoTed carefully by glidine under one of them a circular piece of
thin glass. The specimen snould be drained by resting the edge
of the glass on a piece of bibulous paper, but it must not be touched,
on account of its extreme fragihty : if any small crystals adhere
to its surface, they must be washed off by pouring oyer it a few
drops of watery solution of iodine. When dry, the specimen
should be placed for a few minutes under a bell-glasa, by the side
of a watch-glass, containing a few drops of tincture of ioaine ; and
lastly, a little very fluid Canada balsam should be dropped on it,
and a thin glass cover applied without heat. Specimens may thus
be obtained of extreme thinness, and half an inch in diameter, or
even larger, possessing scarcely the slightest colour, and yet oom*
pletely polarizing transmitted light
1217. The mode of obtaining polarized liffht by reflection waa
first discovered in 1810, by the celebrated philosopher Mains, an
officer in the French engineers. M. Malus, whilst examining the
light reflected from the windows of the Luxembourg throusb a
rhombobedron of calcite, observed that light, when reflected mnn
the suriace of ^lass at an angle of about
56°, acquired the very same properties as
one of the beams obtained by submitting
light to double refraction in calcite : haT-
ing, in fact, become polarised in the plane
of reflection. This discovery was so quickly
followed up by others, and so snccessfolly
studied by the iUustrious philosophers of
the age, Uiat it has led to the development
of some of the most beautiful and impor-
tant physical facts that have ever been
discovered.
The most convenient mode of repeating
the experiments of Malus, is by means of
the^ apparatus represented in Flf. 643.
This consists of two uprights of wood,
supporting a mirror and frame, c d, ood-
structed like a common looking-glaas. A
circular plate of wood, e f, rests on the pillars, and has a circular
aperture in the middle about three inches in diameter ; a ring of
wood, M N, moveable round a circular projection on b p, suppoita
two pillars, m o, h h, between which rests, by means of screws, a
frame, k l, like c d, but somewhat smaller. A slip of paper gra-
duated into 360^ is fixed on that portion of b f which projects be-
vond M H, a black line being marked on the latter, to serve as an
index, and to point to zero on the graduated paper, when the ptllari
M a, s H, are exactly over the supports of the lower frvne, c o. A
FR0PEBTIS8 OF F0LA2UZBD LIGHT.
693
plate of glass rests over the aperture in the centre of e p, to serve
MS a stage on which objects to be submitted to the action of pola-
rized light may be placed.
A plate of glass, covered at the back with a black varnish, is
fixed m the lower frame, c d, and a similar one in k l. The for*
mer is termed the polarizing^ and the latter the analysina^ plate.
As, however, it is of ^ importance to obtain as bright a beam of
polarized light as possible, it is better to make the lower frame,
o D, deeper, and to place in it a dozen or more thin plates of sheet
glass pressed together bj means of a piece of wood at the back ;
le hindmost plate being blackened, ny this contrivance a very
bright beam of reflected polarized light may be obtained, since
many of the rays transmitted by the first plate will be polarized by
reflection from the second, . and so on. This instrument, which
was first suggested by M. Biot,* is the most convenient that can
be used for experiments on polarized light \ it may be conveniently
termed a Polariscope.
1218. Place a lighted candle at a short distance from the j^ola-
rizing plate, p, and adjust the latter so that the light may be inci-
dent upon it at an angle of 66* 45' ; then bv means of the side-
screws fix the upper^or analysing nlate, so that the ray reflected
from p may be incident upon it at the same angle of 56* 45' : the
section of the plates showing their relative position is shown in
Fig. 646, 1. light, on being incident on p, in the direction a p,
is resolved into two portions, j- one being polarized in a plane per-
pendicular to the plane of refleo-
tion, and mixed with much com-
mon light, passes through the
flass, p, and is abaorbedj the un-
ulations beiue checked by the
black paint with which its liack is
covered. The other portion, po-
larized in the plane of incidence
and reflection, is reflected to the
upper plate, a, and thence to the
eye of the spectator at b, who
sees an image of the candle in a.
Then turn round the plate, b,
still keeping it at the same
angle, by moving the collar mh
* Pr^fl de Physique, torn. H. p. 476. Paris, 1824.
t It is, perhaps, hardly necesssry to remind the reader, that the angle at
whieh Hght ahouid be inoident on reflecting sarfiMes for complete polarisation,
is mettsored from • normal to the reflecting sur&oe. On the continent, light
is QSoaUy directed to be incident upon the reflector at an an|{le measored
from its sorfsoe; which will be oomplemenUiy to the true polarising angle.
Thus the angle at which light is polarised by reflection from glass is 68° 46'
measured from a line pernendioolar to its sonace, and SS° 16' measored from
the reflecting snr£soe itself.
^.648.
-f
\
694 POULBIZED UOHT.
on the wooden collar ef, Fig. 646, and when r ib at right
angles to p, as in Fig. 646, 2, the image of the candle will atmoat
eiitirelj disappear. This might be indeed anticipated, for the rav
reflected from p is polarized in the plane of reflection ; bat this
plane is now perpendicular to the plane of incidence on the npper
plate, B, and as there is therefore no motion remaining to be re-
solved in that plane of incidence, no rajs are reflected, but the
polarized ray passes through the glass, and is absorbed hj the
black paint at its back; so that, in looking at b in this fKwitioii,
scarcelj a vestige of light is to be seen reflected from it. On
moving r round through another anffle of 90"*, as at Fig. 646» 3,
the light and figure of the candle wiU reappear in r, as the planes
of polarization (1210) and reflection coincide, being both identical
with a plane passing through a p r b. At the intermediate ares
of rotation, the light in r will increase or decrease in intensitj,
according as it approaches to, or recedes from, the positions diown
in 1 and 3. In tnese three figures, aba shows the position of
the plane of primitive polarisation, and ban the position of that
of reflection, in passing from the position, 1, of the apparatus,
to 2, the light reflected from r decreases in the ratio of the square
of the cosine of the angle formed hy the pjanes of polariEation
and reflection.
1219. The intensitj of the polarized light reflected in variona
positions of the upper mirror (1027), corresponding t« the di^rent
angles contained oj the planes of reflection and primitive polari-
zation, maj be illustrated bj Fig. 647. The lines in the inner
circle point out the different positions of the plane of reflection ;
and the radial distance between the circumference of the circle,
and the two curved outlines a, b, represents the amount of reflected
fig. e«7. greatest amount of light is reflected,
as the lines e b, a d^ are the longest
which can be drawn from the circum-
ference of the inner circle to the limits
of the curved figures. At 45**, the in-
tensitj of reflected light will be leas
than at 0^ as the line e p is shorter
than e b, whilst at 90* and 270* it will attain a minimum, as the
diameter connecting these numbers, if produced, will not be in-
cluded in an J portion of the outer curves.
The eflfect of extinguishine light, bj altering the relative po-
sition of the reflectors, is analogous to that produced bj crossing
two tourmaline plates. If two similar plates of that nuneral be
placed together in the same direction, so that light polariied bj
one maj be transmitted bj the other, oljects maj be distinctly
seen through them; but on turning one at right angles to the
other, nearij absolute darbiess ensues, as the second plate absorbs
most of the light transmitted bj the first. This efiect may be
^
JUa
AKALT8I8 OF FOLJLBIZBD LIGHT. 695
readilj nnderatood bj placing two gratings, i.b,cd, Fig. 648,
opposite each other, bo that
the bars of one may be ver- ^^' ^^'
tical« and those of the other
horizontal, and attempting
to tbrost the paper iiRnre,
K 8 c D (966), through tnem. __
Although, when the bars of -• ^ v|ff ■
the two gratings were in the >
same position, one or other
of the paper slips m s, c d, passes through, yet, when crossed, they
effectually prevent the pMsage of either ; for one, as c d, although
it may ^ass the first grating, is stopped by the second, whilst £e
slip N 8 IS checked by the nrst.
Two plates of tourmaline thus placed may be conveniently em-
ployed m examining the action on a pencil of polarized light of
any substance placed between them ; the plate nearer to the eye
transmitting those rays that have been depolarized by the inter-
vening object. Whenever any two polarizmg arrangements are
thus employed, that nearer to the eye is termed the analyter, A
very compact and portable polariscope was oonstmcted on this
principle oy the late Mr. H. J. Brooke ; this will be found a conve-
nient instrument for the examination of the polarizing properties
of crystals.
1220. If one of the beams of polarized light obtained by double
reiraction (1200) be used instead of the light reflected from p
(1217), it will present the very same phenomena on turning round
the plate b, as the b'ght polarized by reflection from p ; and if light
be polarized by the absorption of one of its component beams by a
tourmaline (1215), or bj Herapathite (1216), the same effects will
be observed; so that m whatever manner light is polarized, it
possesses the same properties, jprovided only that its planes of
polarization be in the same position. Thus, plates of tourmaline
or Herapathite may be used for the purpose ofanalysing polarized
fight, in place of the analysing ^ate of the polariscope ; the
analysis being performed by transmission instead of by reflection.
1221. It has been already stated (1029) that a portion of the
light refracted through glass, when incident at toe polarizing
angle, is polarized, and in a plane at right angles to the reflected
beam. This refracted polarized light may be obtained very ft«e
from common light b^ placing a dozen or more plates of thin sheet
l^ass toother, and nzmg them obliquely in a tube, so that they
may be inclined at an angle of about 25" to its axis. On allowing
a beam of liffht to traverse this tube, it will emerge polarized in a
plane at right angles to that at which the reflected light is under
similar circumstances polarized ; a system of plates thus arranged
in a tube constitutes an excellent mooe of analysing light polarized
by reflection, and developing the colours of donb^ refracting
696
POLARIZED UOBT,
crystalB (1202^. Sir Da^d Brewster has found, that bj increasbe
the number of glass plates, the refracted light becomes pcilarixea
at a much sm^ler angle of incidence : thus, light is coroj^letely
polarized by refraction through one plate of glass at an incidence
of 81"* 38'; through two at 77'' 16'; through six at 71*" 50'; and
through forty-one at 45^. The best glass for polarising light b^
refraction is that which is used to cover microscopic objects ; it
may be obtained of extreme thinness, and is peculiarly valuable
for this purpose.
1222. If the reflecting plate p (1217) be placed at any other
angle except that for complete polarization, a certain portion only
of reflectea light will be polanzed. Very different opinions hare
been offered on the nature of this partially polarized light ; it has
been generally considered as made up of common lights mixed
with a small quantity of completely polarized light. The follow-
ine table, given by Sir David Brewster,* shows the number of
reflections required to completely polarize light at certain anglesi
above or below the polarizing angle, 56" 45'.
No.
ABglM.
No.
Angles.
1
2
3
4
56* 45'
50 26
46 30
43 51
62» 30'
65 33
67 33
5
6
7
8
41* 43'
40 0
38 33
37 20
69* 1'
70 9
71 6
71 51
1223. In the preceding observations, light polarized by refleo-
tion from glass alone has been considered ; the same physical
characters may, however, be communicated to light by reflection
from the surface of almost any non-metallic substance ; that re-
flected from metallic surfaces (1275) differs in its propertiea from
plane polarized light, now under consideration. All bodies have
their peculiar polarizing angle, in the same manner as they have
their index of refraction ; thus, the angle for glass is 56^ 45', and
for water 58" 11'. The effects of the difference of the polarudng
angles of two transparent media upon polarized light may be
shown by an experiment described by Sir D. Brewster. Having
fixed the plates p, a, Fig. 646, 2, at the angles of 56" 45', and with
the planes of reflection and polarization perpendicular to each
other, the image of the candle will be invisiole in a : breathe upon
the latter, so as to cover it with a film of water, and immediately
the candle will become visible, from a portion of the polariaea
beam undergoing reflection from the vapour condensed on r.
1224. The angle of complete nolariaatum for any substance
may be readily determined by the law discovered by Sir D»
• Optios, p. 173^ sad Phfl. Trans. 1830.
AHOIiB or OOXPLBTS FOLABIZATIOV. 697
Brewster, namelji that the rayi polartzed by refection, and by
refraction, are perpendicular to each other; or, in other words,
that the index of refraction i$ the tangent of the an^ of polari-
zation. Thus, if the polariziDg angle of water, of which the index
of refraction is 1*336, he, Teqnire<^ on looking for that number in
a table of natural tangents, the corresponding angle of 53° 1 1'
will be found. The polarizing angle of crown-glass is 56° 45^ ;
for, as its index of refraction is 1*525, that numoer in the table
of tangents corresponds very nearly with the angle mentioned.
1225. Light may be polarissed by reflection from the second
surface of bodies, or by internal reflection ; and the angle for com-
plete polarization has its cotangent equal to the index of refrac-
tion of Uie substance, and may he found by looking for the latter
number in a table of cotaoffents. This, in the case of water will
be 36° W, and of crown-^ass, 83° 15' : so that the polarizing
angle at the second surface is equal to the complement of that for
the first surface of a medium ; whence it follows, that a conside*
rabie portion of the tight transmitted by the first surface of a
plate, will be polarized by reflection at the second or internal
surface.
1226. If, instead of using white light, any one of the coloured
rays of the spectrum be incident on a reflecting medium, it will
undeigo polarization in the same manner as common light, but at
a different angle for each ray. The yalue of the polarizing angle
for each, may be found from its index of refraction, by means
of the law of taugents (1224) : thus, the polarizing an^Ie, when
water is used, is ^° 4' for the red, and 53 Id' for tne violet ray ;
and when plate-glass is employed, 56° 34' for the red, and 56° 55'
for the violet ray. From the data contained in Fraunhofer's table
(1101), the polarizing angle for the rays corresponding with each
of his seven lines may be readily computed. The polarizing an^Ie
of the mean rays of the spectrum is taken to be the polarizing
angle for colourless light; but b^ careful observation, the ex-
tinction of the coloured rays at their appropriate angles may be
observed.
1227. When the sky is tolerably free from clouds, a certain
portion of the light becomes more or less polarized in its passage
to the earth, 'rhe maximum of polarization takes place in a
plane passing through the earth's axis at 90° from the sun, con-
sequently the amount of polarization varies in difierent parts of
the sky according to the position of the sun. According to Araeo,
the rays reflected from the moon also contain a considerable
qnantityof polarized light.
1228. On the above relation between the hour-angle and the
plane of polarization of light from the sky, Prof. Wheatstone has
founded an ingenious device called the polar clo^, Fig. 649. In
this instrument, a tube, a, the axis of which must be placed
parallel to that of the earth, passes moveably through the aial| b,
nulled with the honra, &nd canieB Ml index. Tha tube oaa-
n. UD tdna A peculiar smDgement of poluicing
media, which iadiote the podtion of the
plane of polarizadon bj the diBappearaoce
of crdonred raja (1230) : and as the plus
ofpdaTiiatioii U alwaya 90° from the bdd,
thft index once set right, will alwaja ptant
to the hoDi when the colonre diaappcar.
Tha polaiiiing airangement coniigta of a
doMU'magepritm (1213),a«Bjieje-piece,
and a small hole covered hj a plate g/
setenite at the fdither end of the tube, A-
The upright to which the dial ii at-
tached, is conneoled with the baae <tf tha
iDstrameDt bj means of a hinge joint at d ;
and a ([nadrant, witb a olamp at o, enables it to he a<ijiiBt«d to
the latitude of the place of obaervatioQ.
1229. Having described aome of the more important ^roperiiM
of rectilinearlv polarized light, we hare next to inTeetigste the
phenomena of colour prodnced bj interference. To appreciate
these, the following laws, diacovered b; MM. Arago and FioRtel,
must bepreTionslj well undentood.
A. Two rayt of light derived from tht lamt loyree, aid
polariMd in tXs tame plane <1210j, are eapiMa of inUwftrimg
vnih each olhtr likt cortmon light; they oonseqoently prodnce
fringeg of tbe same character, and all the ex^riments im diffnc-
tioa(1125), if repeated with polariied lieht, wilt produce tbe aam*
' ■" '" ;ht were used.
J irOerfemnce, mhelher the;/ be deriaed /rtim Ihi
name ttruTce, OT not : but if the; be from the same aouroe, then
when polarized at angles intermediate between 0* and 90*, thej
pmduee fringes of intermediate hrigbtneai, the tints disappearing
at 90°, and recorering tbeir lividitj at 1B0°.
C. Too rayt origHudly polarised w planet at right cmgUt l«
each other, map be brought into the sobu j>Iai«« of polartsatiim.
E. I» the phenomena ef intetferenee prodtieed bw ray* that
hoM undergone dovhU refraction, a d^ffertmee of hidf an MaAt
loCim fltutt in tome eatti be aSoaad, a* one of the rayt of light
it retarded to that amount.
The truth of the first of these laws. A, maj be readilj verified
by employing a plane polarized raj in any of " ""*
" ia already detailed (112fr— 1131).
DTTERFEKSRCB OF POLABUED LIGHT. 699
The second, B, may be tested by transmittiiig two small pencils,
deriyed from the sun s image in the ibcus of a small lens, either
throueh two parallel narrow slits in a piece of thin sheet copper,
placed perpendicularly to the path of the pencils, or through cor-
respondinff portions of the prism represented in Fig. 600 ; if two
portions m a plate cut very truly irom a homogeneous crystal of
tourmaline, be placed over the apertures so as to intercept the two
pencilB, it will be found that the usual phenomena of interference
will be produced when the optic axes of the two pieces are pa>
rallel, but will wholly disappear when they are at right angles to
each other ; that is, when tne planes of polarization of the two rays
are perpendicular to each other.
Tnis ascertained fact might have been inferred, 2k priori^ from
the dynamical theory ; for since no portion of the motion in either
set of undulations can be resolved in the direction of the other
set, it is clear that no concurrence or opposition of similar motions,
or in other words, no interference, can take place.
A very simple addition to the arrangements just described suf-
fices to demonstrate the truth of the third law, C ; the optic axes
of the tourmalines being placed perpendicularly to each other, let
a rhombohedron of calcite be placed in front of them, so as to tran»>
mit both pencils, its principal plane being inclined 45° to each of
the planes of polarization. The two polarized rays will now each
be resolved into two others, of which the pairs that describe eoual
fatha, being similarly polarized, might be expected to interwre,
ut no interference is observed to take place.
The experimental proof of the fourth and fifth laws is more
complicated ; it will be found in the treatise already quoted.*
The retarded half undulation mentioned in (£) will be again
referred to (1257V
1230. To exhioit the tints produced by polarized light, place
on the glass stage of the polariscope (1217),^ a thin lamina of
aelenite, of uniform thickness, and allow a pencil of light, polarized
by reflection from the lower plate, p, to pass through it to x. The
source of light ma^ be the 8un*s rays, or diffused daylight, or still
better, the ught of a lamp or candle provided with a ground-glass
shade. Let the index on mn be placed at 0 on the graduated
circle ef, when the plates b,p, will be placed as in Fig. 646, 1.
Let B and p be fixed at the polarizing angle (1218), then, on looking
into the analysing plate, b, the image of the selenite will be seen,
not colourless, but possessing a tint varying with the thickness of
the plate. Let us suppose the film of selenite to be of such a
thickness as to appear red when its imag^ is viewed in the ana-
lysing Phite ; slowly turn round the selenite, and the colour will
gradually disappear and ultimately vanish; at this point the
plane of primitive polarization will pass through one of the optic
• BucydopMlia HetropoUtana, art. Lights § 800-871.
700 POLABUEO UORT.
ftzefl of the selenite, and no prodnction of colour can enmie.
On oontinuinff to turn roond the selenite, the red
'v- <KK). colour graduiul^ reappears, attaining eyentuaOj ita
primitive hrilkancy; on continuing the rotation
the colour again lessens, and disappean when the
plane of pdarization passes through the second
optic axis of the crystal. The greatest intensity
of colour will be obsenred when one of two lines
bisecting the angles contained by the optic axes,
lies in the plane of primitive poiariaation ; these
lines are termed d^olarizing axes, because they
alter the polarization of the transmitted light.
The position of these lines is shown in Fig. So ;
A B D o, is a plate or film of selenite, b r, o h, optic
axes inclined about 75^ to each other, and ▲ d, c b, depolarising
axes, perpendicular to each other.
1281. Having again placed the plates of the polariscope as at
the commencement of the last experiment, let the film of selenite
remain fixed, and when its red image is visible in the analysing
plate, slowly rotate the latter, noticing the arcs of rotation on the
graduated circle : the red colour of the reflected image will gra-
dually lessen, and when a rotation through 45^ has been per-
formed, it will disappear ; after 45*, the film will gradually assume
a green colour complementary (1091) to the red ; and will attain
its greatest brightness at 90^. From 90° to 185* the green
vanishes, and after 185*^ the red reappears, attaining its moat
vivid state at 180^ after which it again vanishes ; at 270* acquir-
ing its green colour, which, on continuing to turn the plate,
vanishes at 825* ; ultimatelv becoming red at 860* or 0*, from
which point we set out. If the plate of selenite had been of such
a thickness as to afford other tints, the complementary colours
would have appeared, as in Newton's experiment, with the colours
of thin plates (1133) ; the colours seen at 0* and at 90*, or at 180*
and 27U*, being invariably such as, when blended together, would
constitute white light.
1232. If a thin plate of mica be placed on the stage of the
polariscope, instead of the selenite, colours disappearing and re-
appearing in the same manner will be seen : and on inchning the
mica, so that the polarized ray may pass through different Sick-
nesses of it, a variety of exquisitely beautiful tints will be de-
veloped. If the mica or selenite be not of uniform thickness, the
analysed image will appear richly tinted with various hues de-
pending for their variety and intensity upon the varying thickness
of the plates.
1283. If the analysing plate of the polariscope be removed, and
the polarized rays passing through the selenite be received by the
eye, no colours will be seen; hence the analysing plate must
have aided in rendering them visible. To unaerstand this^ the
F0L1.RIZJLTI0H OF CONYEBOERT FEKCIL8.
70t
^.651.
course of a polaxised ray, passing through a plate of selenite at
or near one of its depolanzing axes, most be considered. The
selenite being a doubly refracting crystal will cause the incident
polarised ray to be (uyided into two, both of which are extras-
ordinary rays (1200) polarized in perpendicular planes, which
reach the eye together, and a colourless image is perceived. But
if the analrsing plate be used, and the light which has traversed
the crystal be thus reflected to the eye, an important change
occurs, the colourless image is broken up into two coloured ones
complementary to each otner ; one of them, as the green one, is
polarized in the i>lane of reflection, and therefore reaches the eve,
giving a green ima^e of the seleuite; the other image bemg
polarized in an opposite plane, passes through the analysing plate,
and by looking through the latter whilst inclined at a consi-
derable angle to the ray, a red image of the selenite will be visible.
The same tint is produced by reflection if the analysing plate be
rotated through 90°, as the plane of reflection will then coincide
with the plane of polarisation of the red, which is contrary to that
of the g^en rays.
1234. A very instructive mode of analysing the polarized pencil
after it has passed the film of selenite, is to transmit it through a
rhombohedron of calcite ; the transmitted ray will be divided into
two coloured images which will be both
visible at the same time. The red and green
images are complementary to each other,
and, if superposed, would constitute white
light; this may be proved by holding the
calcite at a proper distance, when the two
images will partly overlap each other, pro-
ducing white light, as in Fig. 651. On this
account no colours were seen when the selenite was viewed with-
out the analyser, as both pencils then reached the eye together,
and produced a colourless image.
1235. In the above experiments with selenite or mica, the rays
of incident polarized light were considered as nearly parallel ; if^
however, a convergent pencil enter a crystal so as to traverse its
optic axis (1202), a new and splendid series of phenomena
becomes visible. Let a pencil of light be incident, at tlie pola-
rizing angle, on a bundle
of plates of glass, ab,
Fig. 652, placed on a
black surface, so that a
bright beam of polarized
light may be renected to
the ere at s, which thus
is placed at the apex
of a cone of rays. If
a plate of a doubly refracting crystal, as calcite, cut at right
JV«063.
702 FOULRIZED LIGHT.
angles to its axis, be placed at d, it is obTioiu that the rajrs of
polarized light will traverse it with yarions degrees of obliqoitj,
and thus Yirtnallj permeate different thicknesses of the sectioii.
The central rays which pass through the optic axis do not suffer
double refraction, and therefore will appear to the eje at b, the
same as if no crystal had been present, but the other rays which
pass a little obliquely through the crystal wiU undergo donhle
refraction, each bein^ resoWea into an ordinary and extraordinary
ray, as is the case with ordinary li^ht (1200). These rays, how-
ever, reaching the eye together, will not produce any ooloor, and
cannot be distinguished firom common light. To render the pheno-
mena of coloured polarization obvious, an analysing eye-pieoe
must be placed between the plate of doubly refracting crystal and
the eye. Let this be a NicoVs prism, or a plate of tuunnaline, so
placed as not to transmit the polarized light reflected from a b, iif
the crystal, d, were absent. It will then be found that the liefat
reflected from ab has undergone some physical change whilst
traversing d, as some of it has acquired the power of passing
through the analysing plate of tourmaline, and a beautiful sym-
metrical image, paintedT with the most gorgeous colours, becomes
visible ; this image is composed of a senes of concentric coloured
carves, and traversed by a black cross. Let the analyser be
then turned round 90^, and an image complementary to the first
will be visible, its black cross being replaced by a white one.
1286. The origin of these beautiful coloured rings may be thus
explained : the rays which do not pass through the optic axis are
divided into two pencils, an ordinary and extraordinary, polarized
in contrary planes, and hence one series is absorbed and the other
transmitted by an analyser, according as it is placed so as to
transmit or a1>sorb the originally polarized ray ; but this, although
8u£Bcient to explain the production of two images, is not suflScient
to explain the phenomena of coloured rings. It must be ieool>
lected that the rays pass through the plate of the crystal, d, with
various degrees of ouiquity, and hence some suffer more retarda-
tion of their motion than others : the rays are thus placed in the
yery condition required for the phenomena of interference, and the
consequent production of coloured fringes, as in the case of common
light. The ordinary rays being polarized in the same plane,
mutually interfere to produce one of the coloured images, and the
extraordinary interfere to produce the other: the two images being
complementary to each other, and, if superposed, produce white
light. The figure of the rings results from the rays which pene-
trate the crystal at equal distances from the optic axis, pasdng
through similar thicknesses of the plate, and consequently under-
going the same amount of retardation, and producing siimlar tints
at equal distances from the centre. The smgular appeaniDce of
the black cross is owing to tlie ra^s which traverse uie ciystel in
the direction of the planes of primitive polarization emerging un-
maoa n calcitb. 703
cbwgBd, tnd in these two directiong the dark blue or black »pp«Br-
ance presented by th« reflector, ab, Fig. 652, when vieved throagh
the anul jKT nlone, will be lidble as the arms of a black crtwa ; tSe
■nns of the cross end in bnuhea, and attpBar to eitenc'
siderable distance: Fig. 603, a, ahowa tma bcautilitl a;
Let the analysar ba then tDtatod through 90°, so as to irannnii
th« light reflected from A B. the Bgme B will then be Tiaible, all
the coumri in the rings of which are complementaij to then in a,
■nd a white croaa takes ths plaoe of the black oae.
1237. If the plate of calcite b« rotated on its axis, no change
whatsTer occnra in the rings, and if a portion be covered np with
a piece of black paper, the uncovered portion of the plate will
exhibit the rings as pericctl; as the whole plale. This mar be
reaJilr nnJentood bj lecollecling that the optic axis in these
crystals is not a Bied line, but merelj a fixed direction, and exists
as completal; in the smalleiit fragment as in a large plate of a
iSiystal. If the plate of calcilo be thinner, the rings will appear
wider, and less closely packed together : the diameters of the nogs
beinR invfrsel; as the squara-roet of the thickness of the plate.
1338. Ifa similar plate be cnt from anTOtberaDiaxiafcr]-'-'
will exhibit tbe same beantifol rings, when placed ir
of the pencil of polarized light. If a crystal with a positire optic
axis (1012), as dnwn or ice, ba examined, the nn^ will be
identical with those of calcite, which has a negative axis: but if n
plate of zircon be placed on one of calcite, and the combination be
examined, they will be fonnd to interfere with each other's tints, so
that, if of proper relative tbit^ness, no colound image will be visible.
1239. If| instead of allowing ordinary colourless light to be inci-
On the polarizinc plate, ab (1235), Lomog«neo(u light lie
' ' ' ' . mU be observed as when white
70*
depend on tlie colonr of the ligbt, the riaga being Ui^eat id &t
meet refrangible or violet, and BmalleaC in red light.
1240. In cryBlaU posgeBBiag two optio &iei, inclndinff bj ttt
the gre&ter praporlion of natural and artificial crjrBtalliied bodies
somewhat different pbenomena are obeerred of which the tendenej
to ellipticitf in the rinei, and the presence of a black bar acroM
UwiD, ooDBtitnte the chief. In hiaiial ciyatali, in which the axel
are at a Terj amall aagalar diiUoce from each other, both ajMemi
of rii^ niaj be observed at once, one around each axis, a« in
Ultra, arragoaite, and some ipecimeng of fertocjanate of potaab.
In the ^at majorit;, however, the aiea are so ur wpai»tBd, that
bat a single i;atem can be seen at a lime.
inclined about 65° to each other. Let a
ray of polarixod light, p z, incident ob-
j \ )"' 1 liquel; on this ciyatal, be viewed by
I 'Y I means of an analysing eye-piece in the
j'--'''''^^-, direction OD. An elliptio system of
rinei traversed by a black bar, will b«
visible, provided the aye-piece be so
placed as lo absorb the onginal ray, when
not traversing the clTBtat! If the plate
be then altered in position so that the
poUriied ray may pass in the direction
o ■ F, a second system of rings precisely
similar to thoee seen in the Erection
c D will beoome visible : thiu,rKaaiid
cor represent the directions of the two
optic axes of the plate of topai. If Ibe
eyepiece be rotated through 90*, a &gan
complementary to the last will be ob-
served, all the red rings being replaced
« readily discoverei] by
a piece of the ordic
taie oTthe shop*, about J i
holding a piece of the ordinaiy
taie of the shop*, about J inch
thick, in an indined porition ai
near to the eye-fnece as pcanUe,
and allowing a ray of polariied
light to pais throngh it : but
one system is visible at a dme,
as the axn an so mocfa in-
clined to each other. IT dn
rings be not at fint viaible, thej
reuilv become to by moving
the nuca. The figure is gene-
BlXaS a BIAXIAL GBTVTAia. 706
rally nearly circular, as in Fig. 654, and traversed by a black bar,
wbicb is replaced by a white one, when the complementanr figure
is obtained by rotating the eye-piece through. 90*. rlatee of
borax, or sugar-cand^, cut perpendicularly to one of their optic
axes, may be conreniently used to exhibit these rings.
1243. In most biaxial crvstals in which the angular distance
of the axes is considerable, the system of rings is always elongated
into an elliptical figure (1241^, and the tints are not arranged with
the symmetry we meet with m crystals with one axis: this is beaa-
tifully shown in sections of the Ifochelle salt, the potassio-tartrate
of soda. If a plate cut transversely to one of the axes of this salt,
in the manner described in the case of nitre (1244), be examinea
by polarized light, a splendid elliptic system of rinss, traversed as
usual by a black or ratner a deep-blue bar, will be observed. These
rings are most gorgeously tinted, but the colours are not equally
arranged, the red predominating at one end of the long axis of the
ellipse, and green or blue at the other, adding indeed much to the
beauty of the figure. In some crystals presenting these pheno-
mena, the red ends of the rings are within the resultant axes,
whilst in others the blue ends are thus placed. To the former
belong phosphate of soda, sugar, carbonate of lead, &c., whilst
the Rochelle salt, sulphate of magivesia, and topaz, affi>rd examples
of the latter.
1244. When the inclination of the axes is small, both systems
of rings can be seen at once. To show these, take a crystal of
nitre, and by means of a fine saw cut off a thick plate, at riffht
angles to the axis of the prism. The best mode of rendering this
sufficiently thin, is to rub it on a fine file moistened with water ;
and it ma^ be polished on a plate of glass or wood,
moistened with water. A plate one-sixth of an inch in thickness
catf thus readily be procured, and should be preserved between
plates of glass vrith Canada balsam. In general these sections
of nitre are perfectly transparent- only at their margins, being im-
perfectly crystallized in the centre. This is, however, of no con-
seouenc^, as the transparent edge shows the rin^ very beauti-
fully. For this purpose, the plate should be held in the course of
the polarized ray, as near as possible to the eye, armed with a
tourmaline, or a plate of Herapathite. which answer best for this
purpose, as they allow the crvstal to be brought nearer to the eye
than a Nicol's prism, or other analyser. Ihe beautiful figure
shown in Fig. 656 will be visible, or will readily become so,
bv slightly altering the position of the plate. The two systems
01 rinffl are distinct and splendidly coloured, and the outer rings
of bo£ systems coalesce, and surround the whole figure with a
sort of elliptic border. H whilst the eye-piece is fixed, the plate
of nitre is slowly rotated, the black arms of the cross will open,
and when the line connecting the two axes of the crystal is in-
clined 45° to the plane of pnmitive polarization, the appearance
z z
n«Mnt«<
Been ; the black crossed liDei beiav repUced hj w
Kd rii^ t>v green, the 7ellow by mdigu, &a.
IS46, Ttie double eyetem of ringe may often b« Guelv awo in
ferrocyanats of potaah ; this salt is laminalsd, and nuMlj *pfitt
in the direction of its la^ra. A plate aliaiild be ^it off about a
quarter of an inch thrck, and if not qnite imwith, slxmld be
polished hy friction o^net a piece of wood Dxneteiied with water.
Bj bolding Boch a piece in Uie direction of a poUiued ray, uid
analysing it with a tounaalioe, a fine doable sfBtem of rings wiU
be often soon, No ult, however, has bean observed lo Tai;
in the direction of its aies so mnch aa tUs: in aome Epeciment
tbey are nearly merged into one, en as to be Tirtnaltr uniaxial,
whilst in others they are conaidenblf aeparated : this is probsUr
owing to tome Dndiuoveted complenlf m the pbjncal itractare
of the crystals mbinitted to '
oryslallino plates, may be e&cted by e
Tionsly described ; it may also be GonTenicu«i_^ lutkuQ u, iquduuuu
from a elass pUte, which should be ae small ae ptoseible; a picca
of blaci glass, one-fonrth of an inch in diameter, fixed to a little
arm of bnss, so as to allow of its being inclined at any angle, and
rotated on its axis, constitntes a conrenient form of an^yver,
which, indeed, wae the one used by Sir David Brewster in hii
elaborate researches on the riogs in crystslB. But a much motv
convenient inetrument for theae parposes is the iiolari-«icrotoo«
of Prof Descloiseaui, varioDsly modified by M. Hoffinann, of Faiv^
Mid other opticians. This instmment is analogous to the polaii.
seope, F!g. 64S, of H. ffiot ; but the pa1ariE«d rkjB are brought
to a focmi at the atage by a combiniituin of Isnsei ; and the raja,
tranamittfld by the e^eot their' ^i--**^ -« aT.vrtir^arj Kir an
anaiigement of leniei uialD^m
■e produced bj thU apparatus,
■ enabled to receive cooTergiog peacils which
«re inclined at conaideTable angles to oach other.
1S4T. Bj means of the propertj poaaeBsed by polarized light of
deyeloping these coloured rin^s, which alwajs, in titit and
sirangeaieDt, bear a constant relation to the physical structure
of the crystal producing them, we are enabled frequently to make
out theexiatence of certain peculiaritiea of ntolecufar aggr^atioa ;
and thus acquire a new ana powerful mode of iureatigsling the
intomal arrangement of some of those simple but wonderful btruc-
tores, presented to us so copionel;^ in both the crsanic and in-
oi^auic world. This may be beautifully illuBtrated b^ aubjecting
nnannealed glaas to the action of polarised light : it has been
afaowu that glass, by anequally heating or cooling, acquire! the
property of doable refraction (ISOS). If the glass be properly
prepared, by healing it red hot, and rapidly cooling it, the strain
npon its iatenial molecules is permanent. 8uch a piece of glass
appears, when viewed by ordinary light, liiie any other ; nor can
•ny peculiar feature be detectecl In it, in wbicn it difTera from
r specimens of that anbetance. But if a piece of this
' ' ' ' ' ... - •■ nolariacope (12
^. lie in the analyaii
plate; whilst, under eimilar circumstances, the glass would ni
nealed glass be placed On the stage of the polariacope (121T), a
beautiful coloured image »'" ' '
before heating, exhibit the sligbteet colour.
1246. A solid cylinder of glass carefully heated and cooled
qoickly is generally found to be uniaxial, and whea examined by
polsriied Bght, by placing a trans-
Terse section of it on the stage ^'
of the polariscope, the planes of
reflection and poUrizatioa being
at right angles, llie system of rings
shown in Fig. 658, much resem-
bling those seen in caldte, will be j
Tiaible. The apparent optic axis ia, j
however, generally somewhat eccen- I
trie, so that, on rolating the cylinder, \
• slight tendency to distortion in the
armsof the croasis observed. There
is this essential distiuctioa between
therings visible in nuatinealed Elass,
and tboae in natural ur artificial
cr^itali, that in the latter they may be detected in the minutest
particle, so that if any port of the crystal be covered np, the ODCOfered
708
POLARIZED LIGHT.
portion (1237) will show these rings as perfect! j as the whole
crystal : on the other hand, if any part of a piece of onannealed
glass be covered with black paper, the^ corresponding portioiL of
the rings and cross developea by polarized light will cease to he
visible.
1249. Let the planes of the polarizing and analysing plates be
at right angles (Fig. 646, 2), the index being at 90°, then if the
glass be shaped into a square plate, the beautifol figare, i.. Fig.
659, will appear. The circular curves in the angles possess the
most vivid liues, in which red and green predominate \ the centre
being occupied by a black cross.
On turning the analysing plate round 90^ so that the planes of
reflection and polarization may coincide, the colours, whicn almost
entirely vanish at 45**, will undergo a remarkable change, and the
figure B will appear, all the colours of which are complementary
to those of ▲, and the black cross will be replaced hy white
spaces.
If the plate of unannealed glass be square, and about one-third
as thick as it is long, the elegant figure shown at ▲, Fig. 660, wiU
be visible when the analysing plate is set at 90°, so that the plane
of reflection may be perpenmcular to the plane of polarizatiofn:
the complementery figure, b, replacing it, when the analysinft
plate is placed at 0* or 180°, so that the planes of reflecUon and
polarization coincide with each other.
LECOD>t's POIilBUCOFE. 709
1250. The dork lioeB formia^; the bl&ck crou teen when thcM
pktea are submitted to poUnied light, mast be cnaddered u
pointing oat the poaition of the directians in which the polariied
nj paageg through onchaneed, an^ are henee convenienu; called
Una of no polarisation. If the Kg.Kl.
•nalysing pUte be Gied, and the
Duaunealed glass b« slowly turned
round, the black cross wiU begin
to open, and ila arms to separate
in elegant currea, until lis re-
•pltant axes (1202) are incHoed
45° to the planes of polarization
and reflection, when a beautiful
BjninietriGal figure vill he Ttsi-
ble, aa io Tig. 661. On con-
tinuing to turn the plate of glass
the dark crons gnidu&Uy reap-
pears, and attains its greatest
intansitji when two of ita arms correspond with the plane of pola-
rization, aud the others with that of refleolion.
1351. The beaulifhl Ggores thoe visible in uuannealed glass are
rendered more bnlliant bj allowing the polarized nf to pass twice
through the piece submitted to experiment. For this purpose, the
very simple apparatus Tor polarizing light proposed Bj LecouDt,
can he conveniently employed ; it coOBistBof a small looking glass,
i, ^g. 662, placed on the table, aud a frame, D, fastened to the
toirror by a hinge at c, containing about j^.oaa.
ten plates of common nlale window-glass, ,^^
which is fixed in an inclined position to the
mirror bj means of a support, d. The piece
of unannealed glass is placed ou the mirror,
audit is riflwed in the direction s r, when
the figures become beantifully distinct, ihe
rings being much more numereus than when
examined in the ordinary manner. Com-
mon light is incident on a in the direction
ar, and is divided into two oppoaitely pol
which is transmitted and the other is reUecCed towards the mirror,
pasno^ in its course through the unannealed glasa plate; trom
the mirror it is reflected back again, passing througb the plate,
and being partly depolarixed, paasos iu part through the inclined
glass plates, rendering the Genre visible from e.
1252. When a mass of animal jelly is placed oo the stageaftli»
polariscope, no colours are visible in the analysing plate, so long
as the jelly is not submitted to pressure ; but as soon as it is com-
pressed with sufficient forcp, it assumes a doubly refracting stmc-
tore, and a series of tints traiersed by a black cross become vi-
rible, provided the analysing plat« be so placed that the planes of
reflection and polarizalion are at right anglts to each other.
710 POLAB1ZBD UGBT.
Jelly, solutions of gnm, and albuminous fluids, allowed to ^^AP^*
rate spontaneously, so as to leave an indurated mass, also exhibit
the four coloured sectors, traversed by a black cross.
A slip of glass, pTevionslv without action on polarized light,
develops a series of tints, by bending it, or submitting it to preaaare
by means of an iron frame and a screw.
Fragments of ordinary quills, and other indurated animal struc-
tures, also exhibit these tints, when submitted to the action of
polarized light, in an extremely beautiful manner.
1253. No series of objects exhibits the tints of polarized liefat
more beautifully than the crystalline lenses of animals, enpeciaily
of fishes : to examine these, they should, to prevent their bringing
the incident rays to a focus, l>e immersed in a glass vessel con-
taining oil, or some fluid possessing nearlv the same refnctivo
power as the lens. The crystalline lens of the cod-fish exhibits
twelve beautiful coloured sectors, separated by two darkconoentric
circles of no polarization, and traversed by a black cross.
1254. Many interesting results maj be obtained by examining
sections of organized structures, or minute crystals, in a polarizing
microscope : all that is required for this punpose is to place under
the stage of an achromatic microscope a KicoPs prism, or aome
other of those means bv which the lignt transmitted through any
object on the stage will be rectilinearly polarized. The analyser
should be a short Nicol's prism, fixed over the diaphragm in the
body of the microscope, or as this must slightly interfere with the
achromatism of the instrument, the same, or a thin nlateof brown
or grey tourmaline, may be placed over the eyeglass. In this
manner the molecular arrangement of quills, horns, hoofs, teeth,
and other animal structures, is most beautifully developed.
1255. A magnificent class of objects for the polarizing micro-
scope ^ is found in crystals of different doubly refracting bodies
deposited on glass plates by allowing their solutions to evaponte
spontaneously. To preserve them thev should be coverea with
a second plate of glass, some Canada oalsam being allowed to
run between them. Chlorate of potaas, nitre, salicine, acetate of
lead, sulphate of copper, camphor, and ferrocyanate of potaas, are
objects of really gorgeous beauty when thus examined. Scnne
bodies exhibit toe coloured rings traversed by a black cross, like
calcite, and are peculiarlv beautiful. The spherical crystals of
carbonate of lime, which the late author found to be spontaneonaly
deposited in abundance from the urine of the horse, nnely exhibit
these figures. A rare salt, the oxalurate of ammonia, exhibits
the same phenomena. All the varieties of starch, as that of the
potato, tous-les-mois, plantain, cassava, &c., show the black oroBS
well defined. In some of these varieties the granules are more
or less regularly oval in their form, as in those of arrow-root and
tous-les-mois, but the centre of the black cross is always veiy
near to one end of the granule, indicating an eooentric atmctura ;
1256. Orcvlar iUartmfton.— When t<
It thna generated will not be raetiliBear, ■■
in tlie TArietfofpoUriicd Win-ta.
lit;ht juat examined, but
ciicular. Tbe nature of
the wn*e nauliiDg from
the coinpOBitioQ of two
plftDo wBTei in perpendi-
cular directioni will be
more full? andentood b;
a reference to the appa-
raloB represented in Kg.
663. An open reotan-
gnlar frame, a. b, bai ila
oppodte vertical lidea,
AC, BD, filled Bp with a
■erieB of parallel alipa,
which leave narrow equal
and equiiiislant spaces
between them. A series
of nxU of equal length,
each having a bead of
enamel >t one end, paae
through the correaponding oppwite vertical spaces; and when
tbeia rods rest on the bottom of the frame, and the beads
against the face ofsD, thej form a atraight line. The individual
ceptible of an^ reqnired vertieal diapUcement, hr
raising ttie nvla to which thej are attached parallel to their
original position ; and of an; required horizontal displacement by
tbmating the rods out from the face of the frame, and these two
dienUcementj may take place quile independently of each other,
while the poution of the wads will indicate the joint, or resultant
eSect of both. Let two aeries of equal wavea be accurately cut
traniTemely on two rectangular bars of maho^njr (as being least
likely to warp), and let one of theae, e, banog its wave ana-face
horiiontal, be placed under the rods in the frame A D, and the
other, r, with ita wave surface veKicsl, agunat the ends of the
lodi projeoling through the back of Ihe frame A c. It is evident
that the row of beads, repreaenting a row of vibrating molecules,
will be dieptaced in a vertical wave correaponding with the mi^
(see of K, and also in a horizontal wave equivalent to f, and the
form of the resultant wave will depend on the relative poaitioni
13ST. Tha tthct predaeed on the rwiiIUiit wvn, hy tPwWnJw
differant phases (369) of its components, hu next to b« tEaMdaoi.
For thii purpose, four diSennt combiaktioni of the
irBma ftre lepnsented bf a, h, c, d, Fig. 661, and the
ing resultant waves hy a,b,e,d: in eaufa of thesa oombiiimliau
the horitontal vbts, which liea to the right, maj be lepnasBted
by a, and the vortiol w»vb by L.
If R and I. be combined as in jt, either of them being balf aa
ondulation in adrance of the other, that ii. the cmt of one wan
cnrreapanding with the trough of the other, the tcsoltani, n, i« a
plane wave, hoI inttrmedialt in direction between the plana oC
the two componenta. And cooveracly, if the ware, a, be reaoind
into two, in the direction of Ihe perpendicular planet a and i, one
of them will, ai in a, he half an nndulation in adrance of the
other : this will probably account for tha diflerence of half an aa-
dntation in the paths of two rays simnltaneaualj polaiiied in
perpendicnlar planes, mentioned in 1 1!9, E.
If E iajof an nndulation in aJraiuw q/'L.ai in B, the ivraltaM
Ifa and L coincide, as in c, the resnlt will be a plane w ., _,
in a position intermiduitt between > and h. And converaelr, a
plane wave, e, in an intermediate position, may be reaolred nrlo
two perpendicular coincident waves, c a, c u
If a is I of an undulation bekind L, at in n, the reanltant van,
d, will be circniar. and the locos of the distothed paiticlaa ^riglo-
ktmded cylindrical spiral.
1SS8. It may be readily shown by the tame apparato^ that
when the coincident phases of the component wares ■ and l are
inlennediate between a and b, or a and a, the resnllaat wan wiU
be a lefl-handed elliptical spiral, the mi^or aiis of tbe elliptic
base leaning towanta the direction of o in the former caae:, and
towards that of e in the Utter. When the coneapondinK phasra
of the compouenU are intermediate between o and d, or between
o and A (for it is precisely the same thing, whether a i* half aa
andolation hefi>rt or UkiTtd l), the retollanta will aiinilariy be
right-handed elliptio spirals.
It wiU readily be seen fivm fig. 668, that in drcDlarij and
ellipticalli palBriz«<] Ugbt, tbe path of each displaced particle in
the HTBTe-ntint is a circle, or &□ ellipea respectiTelj, jnat u the path
mait be a Btnight liue in plane, or rectilinearly polarized ligDt.
1259. Circulartf polarised light amy ibe produced in several
waja; perhape tlie readiest ia that propoaed b/ Mr. Air;. Ha
•Uowa a ray of plane polarized light Iii be transmitted ihroDgh a
lamina of mica nr Mlenite of lulEcient thickneag to retard the
orfinaij ray (1200), an odd nnmber of qnarter undulalioni more
than tbe eitraurdinaiy raj ; ander these circumatancea, tbe emer-
gent light will he circuhirlj polarized.
Another proceea is that of M. Fremel, hf allowing a ra? of
plane polarized light, Jlb, Fig. 665, to anfler two reflecliona from
the internal Burfaces of a parallelopiped of crown-glasa, tbe snrAlcei
of which meet at k, l, at an angle of Gt° SC ; the emergent ray,
or>, will be drculaii]' polarized. The plane of reSection, ahcd,
■bonld form an angle of 45° with the plane of polarization of tbe
raj, xn. Bj each of these internal reSectiona, a retardation of
coe^ighlh of an nndulalinn is produced in c ' '
into which tbe incident light ia resAlved on
raflec^oo at the internal anrface, k h. If
the nuus of glass be of snfficient length,
the ra; will emerge polarized circularly aftfr
two, BIX, ten, &c., '2(2n— 1] reSectione, and
rectilinearly, after fonr, eight, (welTe, ail-
t«eD, &c., i n reflectiona. CircnlaKv polarized
Hght may be readily diiCingnisbed from tbe
rectilinear form by eiamining it with an *
analyaing eye-pieoa (1246): for it will
merely gradnallj- decrease in iatenaity aa the latter ia rotated to
tba right or to tbe left. ocTer diaappearing and reappearing twice
in each TCTolation (1216).
1260. Let a plate of re^arly cryslallized qnartz be cnt in a
direction perpendicular to its axis, and placed on a alagBof the
polariacope ; on looking into the analysing plate, no black cross
will be visible, as in calcite, unless tbe plate be aufficientlj thick,
and then if held near tbe ej;e in the manner already described for
examining cryalala, a binish ill-defined cross will be seen. Coloured
rings are not generaLy risible ualess the plate be beld near
the eye, that it may receive as wide a I^. MS.
cone of nys aa poMible. Wheneinmined —
on tbe stage of the polariscope, or at some
distance from tbe eye, the whole plate
presents an uniform tint, as in Fig. 666 ; ,
and DO rings will he seen at tbe circum- I
ference of tbe crystnl, the whole being I
filled np by an uniform tint, providpd the '
plate he of the same thiclcnesa throagboat ;
otherwise it will vary, as tbe iotensit; of
714
FOLARISBD LIORT.
colour depends on the thickness of the plate. If the cdoar be
red, slowljr rotate the analysing plate, and the tint will be
changed to orange, yellow, greon, and ultimately to Tio^ : at
though the analysing plate had, during its rotaUoUi acquired the
power of reflecting these different coloun.
In some specimens of quartz, and other crystals possessing this
power of circular polarization, the colours are chan^ped from red
to violet when the analysing plate is turned from right to left,
and in others, when it is moved from left to right Hence these
crystals are termed right-handed, or left-handed, according as
they possess the propertv of causing the planes of polarization
to revolve spirally in a direction from right to left, or from left
to right (1257). The succession of cMours is represented in
Fig. 667.
^.667.
RICKT-HAHDEO
RED
IHDICO^ NYELLOW
BLUE# \cREEN
CREENi JBUJE
RED
LEFT-iMRDED
RED
YEUOVV^ XiNDJCO
OREEN \aLUE
BLUE. JCREEN
INOIOo\ JrVASm
VI OLZrSv^^^^^^^/^RAHCt
1261. A plate of left-handed quartz, 0*3 inch thick, when placed
on the stage of the polariscope, so that a polarized ray may pass
through it, appears of a 6ne olue when viewed through an ana-
Ivser held m such manner as to receive the ray transmitted
through the crystal. On turning the quartz round on its axis, no
change of colour ensues : bnt on rotating the analyser, the follow-
ing changes of colour are observed at dSferent azimuths: —
AfimtttlM.
Ooloor.
Asimaths.
1
Ooloor.
0' or 180*
28 „ 208
73 „ 253
Fine blue.
Pea green.
Greenish yellow
98* or 278*
115 „ 295
145 „ 325
Tawny orange.
Vivid red.
Violet,
The phenomena thus observed are the same as would necessarily
occur if the polarized light had been, by passing through theqi
resolved into a series of homogeneous rays, and become dii
in different planes radiating from the centre of a circle, as i
in Fig. 668, representing Newton*s chromatio circle, in one half
of which the colours of tne spectrum are arranged. The thicker
the plate of quarts employed, the greater is tne aro required to
BOTATXON OF THB FLAXB OF FOLARIZATIOH.
715
Iig.9n.
effect the convenioii of the image into one of s different tint ; to
that, although in the ahoTO experiment a rotation of the analyiing
e^e-piece through an arc of 180** was soffi- ^,en,
cient to develop the series of coloured
images, jet, on increasing the thickness of
the plate, a mnch larger arc is required to
proauce the same effect.
1262. In plane polarized light it was shown I
that the maximum of light is reflected bj 1
the analysing plate (12 ItJ, when the plane of '
reflection coincides with tnat of polarization ;
and the minimum, when the plane of reflec-
tion is perpendicular to that of polarization ;
this, howcTer, is not the case with circnlarlj polarized light. To
make this intelligible, place on the stage of a polariscope a plate of
right-handed quartz 0*04 inch thick, and illuminate it with homo-
geneous light, as b^r that transmitted through a piece of red glass.
The greatest intensity of the light will
not be any longer at 0° and 180 , but at
lO"* and 199, and the least at 109"* and
289^ instead of 90' and 270^ asifthe
plane of polarization had been tnmed '
round 19 towards the right. This
may be illustrated by Fig. 669, simi-
lar t^ that employed in 1219. It is
evident that the line produced from 0*
and 180** is not now the longest that
can be drawn within the external curves, bnt that the longest line
must now be drawn 19' from its former position, or in the direo-
tion of the dotted line, A b.
1263. If homogeneous liffht of other tints had been employed, a
still frreater alteration in the position of the plane of polarization
would have been observed : thus, for the mean coloured rays with
a similar plate of quartz, the deviations of the plane amounted,
from Biots experiments, to the following: — ^Bed, 19**; Orange,
21" ; Yellow, 23' ; Green, 28* ; Blue, 32* ; Indigo, 36-; Violet, 41\
This alteration in the position of the planes increases with the
thickness of the plate of quartz. Thus, if a deviation of 19" is
produced by a plate of quartz 0*04 inch thick in red light, one of
88' is produced by a plate 006 inch thick, and of 95° by one 0*2
inch thick.
1264. The colours visible by polarized light in quartz are never
simple when white light is usea ; for as the different coloured rays
arethusshownto be unequally dispersed, it follows that although
an excess of one tint may he visible at a time, so as to give a
well-marked colour to the transmitted rays ; yet it most in every
case be a mixture of several. To comprehend this, let the series
of curves in ▲ o b b, Fig. 670, represent the intensities of b s l^e
716
POLABIZBD LIQRT.
red, TT the yellow, BBtbebloe, and yt the violet nije renpec-
tively. Let a plate of right-haDded quartz 0'2 inch thick be tnen
^ ..Q examined by polarized light,
^' * the analyser being bo placed
as not to reflect the polarised
ray, if the quartz were absent.
If homogeneons light be em-
ployed, the red ray will ob-
tain its greatest intensity at
95" and 275^ and iU least at
5** and 185^ the depth of the
cnrres on the line b b being
the greatest at the former an*
gle, and least at the latter.
In the same manner the curves on the lines r t, bb, and vt, re-
present the intensity of the yellow, blue, and violet rays at difib-
lent angles.
Let homogeneous be replaced by white polarized light, and the
tints produced by its passage through the qaartz may be obeerved
at 0** ; on referring to the figure, the blue and violet rays will predo-
minate, the yellow and red oeing sparingly reflected ; at 5° the red
attains its minimum, and the image will be the darkest from the
presence of excess of violet light. At 95^ red will predominate in
the image, but mixed with much yellow light ; at 1 1 5*, the yellow
will attain its greatest intensity, at 160" the blue, and at 205* the
violet will be at their maximum. Thos, in no case can a pure
homogeneous tint be obtained when white polarized light traverses
quartz, all the colours being mixtures of several, of which, however^
one predominates over the rest.
1265. One of the most interesting contributions to science, for
which we are indebted to Prof. Faraday, is the discovery of the ex>
citcment of a molecular change in various snbstances, as glassy
water, alcohol, oil, when under the influence of the magnetic force,
sufficient to cause the rotation of a polarized ray. To show this
«^^ Q^^ with the magnet, a piece of
flint glass, ▲, Fig. 671, or
^^ much better, a slip of heavr
^^,,^jC^^ glass, the fused borate of leaa,
n 2 inoties square and 0*5 inch
thick, is placed between the
poles, H, B, of a powerful eiec-
tro-magnet (877), so that tiie
lines of force (592) may pass
through the len^h of the
glass. A beam of Ti^ht, b d, ia
Jolarized in a vertical plane
glass, D, and passing thnnuHi
t£e glass, A, is examined at d through a Nicol's pnam (1212). So
BOTATIOM UVDBB UAONBTIO IHFLUBKCZ. 717
long as the bars n and s are not magnetic, the rays are transmitted
or extinguisheil as usual during the revolution of the prism. Let
this be then turned ho that light is eztiDruished, then on con>
necting the wires c, z, with the battery, the bar instantly becomes
magnetic, and some rays are transmitted. It will be necessary to
rotate the prism to the right to extinguish the rays which have,
•under the influence of the developed magnetism, been made to
rotate. If the north pole be next the observer, as in the figure,
the plane of polarization will be rotated to the right, but il the
poles be reversed, the rotation will be to the left.
1266. When a glass tube is filled with water, and placed in the
axis of a long helix of wire traversed by a current from a Grove's
battery of ten pairs of plates, the water assumes a similar rotatory
power over a rectiliuearly polarized ray, turning it to the riflpht or
the left according to the direction of the current, the ray always
rotating in the direction in which the positive current traverses
the wire of the helix. When a wide tube of glass is filled with
water, and the helix traversed by the current immersed in it, the
water in the centre of the helix will alone exert any action on a
transmitted polarized ray, that lying between the exterior of the
coil and the side of the tube having no rotatory power. A piece
of borate of lead glass placed in the helix acquires a similar power.
Thus by the inductioo of magnetic force Prof. Faraday communi-
cated temporarily to glass the rotatory power naturally possessed
by quartz (1260j, and to water and other fluids the power proper
to syrup, and oil of turpentine.* The intensity of this acquired
power IS shown in the following table \ that in water being taken
as unity : —
Oil of turpentine 11*8 examined naturally.
Heavy glass 6'0\
Flint glass 2-8 . , j x.
Rock ilt 2-2 I ?"»>ned under the
y/g^XQj j^.^ Vintluence of electno
Alcohol! ; ; iesi than'witer currents.
Ether . . . less than alcohol /
1267. Solutions of su^r, camphor, and a large number of
organic fluids, naturally develop the phenomena of circular pola-
rization. If a tube, closed at the lower end with a plate of
glass, and about six or eight inches in length, be filled with oil of
turpentine, and placed on the stage of the polariscope, the richly,
coloured images (1261), and a rotation of the plane of polarization
finom right to left, will be observed. A test-tube, the bottom of
which rests in a drop of water (to diminish the refraction}, will
answer the same purpose.
It is far better to examine the circularly polarizing power of
fluids by means of a polariscope constructed for that piu|K>8e. The
* Phil. Truu.l846,p.l4.
718
FOLARIZED LIGHT.
Fig, 972*
following IB ft T617 BiiDDle one, which the late aaUior need for eome
yeani, consisting of a oondle of plates of sheet-p^laes, a, Fig. 679,
as a polarizing mirror, fixed to an arm, so
as to admit of ready motion, and supported
bj a screw from a common retort-stand,
and a tube of brass, b, an inch in diameter,
and ei^ht inches lonff, closed at its lower
end with a plate ofglass holding the floM
to be examined, l^e transmitted raj is
analysed by an analyser, o, oonsistinr of a
single-image prism (1212), or bnmue of
thin glass plates (i221)^capable of being
placed at any azim nth. The action of oil <?
turpentine is much less intense than that
of quartz, in the proportion of 1 : 68*5 ;
hence the necessity of using a tube full
of the oil, so as to form a fluid oolnmn
about six or eight inches high. For some
purposes it is desirable to use a tube <^
glass in place of the brass tube, b ; and
where the rotating power of the fluid is yery feeble, a much
greater length of tube than 8 inches is necessary. In M. Biot's
apparatus, the analysing eye-piece is proyided with a graduated
circle, for the purpose of measuring the angle of rotation.
1268. Some organic products turn the plane of polarisation
from left to right, others from right to left (1260); this is best
seen by using homogeneous light, which for practical purposes
may be effected with sufficient accuracy by obserying tne rota-
Fluid.
Oil of turpentine
Oil of citron . .
Oil of bergamot .
Oil of anise . .
Oil of caraway .
Oil of spearmint
Oil of rue . . .
Naphtha . . .
SoL of cane-snear in water
Sol. of sugar of milk „
Syrup of grape sugar . .
Grape juice
Apple juice
Sol . of tartaric acid in equal
weight of water . . .
Boiatiao.
Aro. Direotton.
45''
84
29
(?)
lOO*
(?)
(?).
12*40'
23 5
10 8
(?)
6*
8 33
8 6
Hiichtof SpeoiAo
6*0 in.
60
6*0
6-4
60
60
60
6-4
60
60
60
6*3
6-3
6*3
M05
1*054
BOTATIOS ni OBOAXIG BODIES, 719
tion through a piece of g^MB coloured red bj protoxide of copper,
and which tranBmits scarcely any except the extreme red rays. By
operating in this manner, M. Biot* succeeded in detecting the
property of circular polanzation in a large number of fluids, and
he has even applied this property to organic chemistnr as a mode
of distinguishing between closely allied organic pro<uicts, as the
differont varieties of gums and sugars. In the preceding table
are the results of some of the more interesting of Biot*s experi-
ments; the direction of the points of the daggers m the third column
]n<Hcates the direction of the rotation of the planes of polarization
observed through red glo9$.
1269. A solution of one part of common white sugar in four
parts of water was placed in the tube so as to form a column seyen
inches long ; on transmitting a polarized pencil throueh it, and
analysing the emergent rajrs by a KicoPs prism, placed so as to
reflect or disperse the rays, if the syrup haa been absent, the late
author found the following to be the tints of the images transmit-
ted at difierent azimuths : —
0*" Pea-green I 80** Purplish violet I 152** Fine orange
65 Rich blue | 95 Beddish violet | 200 Deep blue.
1270. In order to applv the property of ciroular polarization in
establishing distinctions between closely allied organic products,
and to the detection of •differences of molecular arrangement in
bodies composed of the same elements in nearly similar proportions,
it is necessaiy to determine what M. Biot has termed the force of
their molecular rotation. This force is nothing more than a com-
parative expression of the ciroularly polarizing powers of bodies
when reduced to an unity of density and thickness ; the unity of
thickness assumed by M. Biot is the millimetre, equal to 0*03937,
or nearly 0*04 inch. The formula deduced from these interesting
researohes is of great value, as affording^ a simple mode of dis-
covering the molecular circularly polarizing, or rotating force, of
different organic bodies ; the following is its simplest expression :
Quantity of organic matter in an unit of the solution . ■■ p,
Specific gravity or density of the solution . . • . » d^
Length of the column of fluid emploved as 2,
Are of rotation observed through red glass . . . • = a.
Molecular force of ciroular polarization ■■ m;
The following is an example of the application of this formula :
MM. Biot and Persoz digested 400 parts of potato staroh in a
mixture of 160 parts of sulphuric acid and 1000 of water, and dis-
solved the sugar thus generated in water, when the following data
were obtained:
* M^m. ds 1' Acad, rojile dot Boianoes de I Iiwtit«t« ziii, pp. S9— 176, pauim .
720 POLA.BUED UOBT.
i)=0-2107, d=1084, Z-162™*, and a=60', then
50
•" = 162 X 0-2107 X 1084"^'^'
which is the rotating force of sugar of starch at a onit of densitj
and thickness.
1271. The most delicate test of the circular polarizing power of
fluids, when this happens to he too weak to produce an^ marked
deviation of the planes of polarization, consisiks in examining the
raj after it has traversed a column of fluid, hy means of a double-
image prism (1213). If, at any period of its revolution, the two
images should appear differently coloured, it is certain that a rota-
tory power Is exerted by the fluid under examination.
1272. It has been observed by Arago,Biot,Marbach,De8cloi8eanx,
Seebeck, Bouchardat, Chantard, Pasteur, Amdtzen, Wilhelmy,*
Brewster, Herschel, Leeson, and others, that there are several
substances of which different varieties, possessing either chemical
or physical differences, exhibit also opposite rotating poweta ; of
these the following may be enumerated : —
Quartz
Cinnabar
Bromate of soda
Chlorate of soda
Camphoric
Tartaric acid
Tartramide
Tartramic acid.
Acetate of uranium
and soda
Oil of turpentine
Camphor
From the French oil of turpentine (the produce of the Finns
maritima), and the American oil (obtained from P. Austndis),
which rotate oppositely. Prof. Jellettf sought to obtain a compen-
sated mixture, iree from rotating power : but the dispersive powers
of these two oils are unequal, and hence complete compensation
could not be obtained for white or heteroeeneous light Bat he
found that, in a mixture of 67 A. + 38 F., the red rays were rotated
to the nght, and the blue rays to the left ; compensation can in
foot be obtained for homogeneous rays only.
It has been observed by Wilhelmyi that a solution of saotonin
in alcohol rotates white ught to the left, and the red raja alone
to the right.
The ool^ substance possessed of rotating power that haa been
examined m all the three different states of aggregation, vis^
solid, liauid^ aud gaseous, is oil of turpentine ; and it has been
observea by .Biot to exhibit this power in each of these statea.
1272a. JeUett^a Seuxharometer. — This is by fiu* the most deK-
cate instrument for ascertaining the presence of small quantities
of sugar, its peculiarity is in the construction of the analysing
•eye-piece. A crystal of calcite, elongated in the direction of four
parallel edges, is bisected by a plane very nearly coinciding with
a principal plane passing through two of these, and the halves
* Fofff., Ann., sad Ami. Ch. Pbya.. patrim,
t Beport Brit. Amoo.. 1866. % PoggenduiA; Aiiii.» IzxzL 617.
ISFLUENOB OP BOLCTIOH OV BOTATIOS. 721
are cemented in a reTersed position ; tliis is equivalent to taking
oat a tkin wedge of about 5' or 6'. Sections are then made perpen-
dicular to the length of the crystal, and the small difference in the
directions of the optic axes in the two halves will, in some positions
of the analyser, produce a difference of tint in the fluid examined,
if it contain but a very small quantity of sugar.
1273. Infiuenee of TeiRpera^ure.---Ju8t as the refrangibility of
various substances changes with the temperature (1102), but not
pari JUU8U with the change of density, so also the amount of
rotatory power varies under similar circumstances ; there are, how-
ever, great differences in the amount of this change. In quartz
the rotating power is increased by more than 100° for an increase
of temperature of 70** C. Tartaric acid is similarly affected, and to
snch an extent that Biot found, on reducing the temperature, that
the right-handed or positive rotation was reduced to zero, and
then became negative, before the ireezing point was reached.
Grape-sugar in solution is much affected by change of tempera-
ture ; and this is important in the application of polarized lignt to
saccharimetry. Clerget* has found that the power of rotation
decreases with increase of temperature. Wilhelmy has determined,
between wide limits, the rate of change by the formula
r'=r [1-0-012 (t'-<)],
in which r and t^ are the rotations at the temperatures t and tf.
Salts of quinine, and many other substances, are also known to
vary their rotating power with their temperature ; but, on the
other hand, it is stated that oil of turpentine does not change its
power from 55** C. down to the freezing point.
1274. Influence of Solution. — Bodies exhibiting circular polari-
zation are affected by solution in three different ways: — 1. Some
crystallized bodies, as chlorate of soda, bromate of soda, and the
double acetate of soda and uranium, possess rotating power ; but
they are perfectly inactive in solution :t here, therefore, the
power is due to crystallization. 2. Some organic substances ex-
nibit the same power, whether in a solid eoUotd state or in solution
in an optically neutral fluid, as water or alcohol ; thus, " barley-
sugar has the same power as when in solution, but in sugar-
candy the rotating power is masked by double refraction. In
these bodies the rotating power must evidentlv be molecular.
8. There are other substances partaking of both the preceding
characters, in which the molecular power seems to be modified by
the conditions of ageregation. Thus, a solution of sulphate of
strychnine evaporated at a temperature between 10° and 20° C.
yields crystals belonging to the pyramidal system (24, II.), which
rotate the plane of polarization to about half the extent of left-
handed quartz (1260), but the solution effects only 0*04 of the
rotation produced by the crystals.
• Aon. Ch. Fhyi., [3] xxtL 176. f Marbach, ib. zliii., xUv.
3a
TS3 rOI-lBIUD LtOBT.
Tartaric acid, radac«d to a traniparent ooUoid state bj miz-
tnre with borscic acid, wsi found to eiliibit circnjar polarintion ;
but different proporlionB of water cange Tei7 remarkable diflerenoea
ID the optical properties of Ibe loluttoa. ArDdtxen* foood thkt
IrDin right-handed tartaric acid dinsolTed in alcohol, he could ob-
tain a lefl-handed rotation for the bine raj's. The same obwrrer
found, in reference to camphor dissolved in alcohol, that the rota-
torj power IncreBgeBwithtba refrangibilitj' of the rays more rapidlj
than m the case of the meet active Godies, and also tliat that power
deoreases regularly with tbe concentration of the Bolntion.
1275. EUtptie Polariialum.^ It has alreadj been ahowo
{1357] thatiriliedifliirenceofthepBthBoftwoByBtemBofwaTM,
instead of amounting to one or three-fourths, is any other fraction
of an ondalation having a larger denominator, the movement
which ensnea will not be circular, but elliptical, producing ellipti-
cal polarization. This variety of polarized light is obtained bj a
seriea of reflections f^m metallio Hurfaces, Offering in angle of
incidence according to the metal employed.
8ir David Brewster discovered, in iS15, the pivpert; poaaesaed
bj polished plates of gold and silver of dividing polarized raja by
tDccessivfl reflections into their coniplc me ntory colours. Beflectioiu
from metallic Eurfaces bat imperinctly polarize light : thus ei^t
reflections from plates of steel, and about 36 from thoee of silver,
are required to polarine the light of a wax-candle ten feel distantf
If polarized light be reflected from metallic plates in a piano
coinciding with, or perpendicular to, the plane of primitive polari-
lation, no parlicnlar phenomena occur : when, however, tlie plane
of reflection is inclined to that of polarization, complementary
colours are aeon in the images, when the reflected light is analysed
by any of tbe means already mentioned. These colours are peculiarly
beautifnl, when the reflecting plate is composed of sQver or gold-
1276. Let a ray of light polarised in a plane Inclined 45° to the
plane of incidence, be reflected from
Fig- >73. a polished steel plate at on au^e
of 75°, then tlie reflected ray will
be found to diOer materially fhon the
taj before reflection, as it does trat
vanish when viewed tbiougb a tourma-
line or other analysing eye-piece under
• the same circnmstances as it did before
reflection from the steel plate : it has;
in fact, been converted into elliplieally
polarized Ueht. _ The best test of thig
kind of light is the modification it
produces in the rings of calcite (123GX
Dio^iha
Tniu, ISW, ud Fret Powd, IB Pkil-
ZLIiIPTIC OPTICAL COMBTA.RT8. 723
the transmitted light being analysed as nsaal. Under these cir«
cumstances the appearance shown in Fig. 673 will be seen, which
differs from that seen by ordinary polarized light, in the distortion
of the black cross and diulocation of the rings, as if a film of selenite
capable of producing a blue tint had been placed across the plate
of calcite.
The conversion of a plane into an elHptically-polarized ray
may be effected b^ replacing the reflecting steel plate by^ a thin
film of mica, previousK beated red-hot, so as to split it into in-
numerable buninse, and communicate to it a silvery lustre. This
discovery is due to Prof. Forbes of Edinburgh.
1277. The angle at which a ray of plane polarized light becomes
elliptic, by a single reflection from a metallic surface, differs with
different metallic substances. The following are some among a
series given by Sir D. Brewster : —
Mercuiy 78" 27' j Bismuth 74" OS' J Zinc . . 72" SQf
Steel . 75 00 ^ Silver . 73 40 ! Gfold alloy 70 45
The late Prof. Powell observed that in ^neral the elliptically
polarizing power of metals is greatly dimmished by oxidation.
1278. Elliptically polarized light is produced by any odd
number of reflections from surfaces of steel, and is restored to a
state of plane polarization by an even number : thus, a plane
polarized ray becomes elliptic with 1, 3, 5, 7, &c., reflections from
steel at 75°, and is restored to its primitive state by 2, 4, 6, 8, &c.,
similar reflections.
1279. Several extensive series of observations have been made
by M. Jamin* and the Rev. S. Hanffhton,t ^^ ^^^ subject of
elliptic polarization ; and from these the following optical con-
stants 01 various substances have been determined.
The prineipcd incidence (I) is defined to be that angle of inci-
dence at which rays polarized in any plane inclined to the plane
of incidence have the major axis of the elliptic path resulting
from reflection situated in the plane of incidence and reflection :
and the coefficient of refraction (p) is the tangent of the principal
incidence.
The coefficient of reflection (R), is the cotangent of the angle
contained between the plane of incidence and the plane of
polarization of those rays that, after reflection at the principal
incidence, become circularlv polarized.
In a large number of allocs of the last two metals, the values
of I and p were all intermediate, but in no certain ratio.
1280. Many varieties of crystals present different colours, ac-
cording to the direction in which hght is transmitted through
them ; this property is called dichroism. An excellent example of
this is met with in the chloride of palladium, which is deep red,
♦ Ann, Ch. Pbys., [3] xix. to xxix. f Plul. Tr»M. 1888.
3 a2
724
POLIBISBD LIGHT.
when viewed in tbe direction of its axis, and yivid green, wlien
examined transverselj. Similar phenomena are observed in the
Subetance.
I
P
B
Munich glass ....
66»
or
1-4287
00780
Fluor-spar
54
46
1-4158
00053
Glass of antimony . .
58
49
1-6519
00199
Quartz
56
47
1-5274
0-0144
Speculum metal . . .
76
33
4-1901
0-6865
Silver, rolled ....
72
07
3-1016
0-8901
78
07
4-7522
0-9255
Gold
75
37
3-8994
0-9073
Mercury
81
04
6-3616
07315
Platinum
76
37
4-2030
0-7265
Palladium
77
37
4-5546
0-7058
Lead
69
37
2-6918
03265
Bismuth
73
37
3-4013
0-6993
Tin
75
07
3-7627
0-7341
Iron
76
07
4*0458
0-5163
Steel
77
87
45621
0-5197
Aluminium ....
77
07
4-3721
0-6457
Copper
71
53
30662
0-8656
Zinc
77
22
4-4723
07281
iolite or dichroitc, and some other natural and artificial nub-
stances. When such crystals are placed on the stage of the pdari-
Uniaxial.
Sapphire .
Emerald .
Blue beryl
Quartz. .
Amethyst
Tourmaline
Tdocrase .
Mellite .
Lilac apatite
Biaxial,
Topaz, blue .
green
pink .
Cyanite . .
Dichroite . .
Epidote, olive-gr
whitish-gr.
Yellowish green
Yellowish green
Bluish white. .
Wbite. . . .
Blue ....
Greenish white .
Yellow . . .
Yellow . . .
Bluish ....
White .
White .
Pink .
White .
Blue .
Brown.
Pinkish white
Blue.
Bluish green.
Blue.
Faint brown.
Pink.
Bluish green.
Green.
Bluisb men.
Reddish white.
Blue.
Green.
White.
Blue.
Y'ellowish white.
Sap-green.
Yellowish white.
BEFEBBNCB8. 725
scope, their colours will be found to vary with the inclination of
the principal section (1201) to the plane of polarization. The
preceding list contains some of the results of Sir David Brewster'tf
researches on this sahject, showing the colours of the two images,
when crystals possessing the property of dichroism are submitted
to polarized light. The colours in the first column are those seen
when an optic axis is situate in the plane of polarization ; those in
the second column are seen when the same axis lies in a plane
perpendicular to that of polarization.
Beferencbs.
In the able and comprehensive Treatise on Light in the Encyclo-
paedia Metropolitana, by Sir J. Herschel, the student will find a
most valuable source of reference on all points connected with
physical optics. The Essav on Optics by Sir D. Brewster, in
Laraner^B Cyclopsedia, will be found an excellent guide for the
less advanced student.
For further information on the subjects treated of in the last
four chapters, in addition to the general treatises on physics before
referred to, the reader should consult Dr. Young's Lectures on
Natural Philosophy. The subject is geometrically treated bv
Newton, and more or less analytically in the treatises bv Wood,
Codington, and Griffin; and, more recently, in that by Prof.
Potter.
In addition to the Treatise on Light in the Enc^clopiedia Me-
tropolitana, and to Sir D. Brewster's work on Optics, and to his
papers diffused through the Philosophical Transactions, the student
may with advantage be directed for further information on pola-
rized light to the General View of the Undulatory Theory, by the
late Bev. Baden Powell, 1841 ; to the Lectures on Polarized Light,
by the late Dr. Pereira, in the second and third volumes of the
I^armaceutical Journal, and subsequently published in a senarate
form ; and to apaper by Dr. Leeson in the Journal of the (Jhemi-
cal Society. Tne more advanced student will consult with great
advantage the Undulatory Theory of Optics, in a volume of
mathematical tracts by the present Astronomer Royal, and some
papers on Physical Optics by the same author, in the Cambridge
Phil. Trans., vol. iv. ; also a concise mathematical investigation
of the laws of double refraction, by Mr. Griffin. The works of
Biot will also yield much information ; and several able papers
by Jamin, on the subject of elliptic ])olarization, and of many other
observers on circular polarization, will be found in PoggendorfTs
Annalen, and in the Annales de Chimie et de la Physique.
LLl-i.~J! .'.U^^- ■ J"-* UJ-^T f ,
726
CHAPTER XXn.
CHEMICAL ACTIOH OF UGHT; PHOTOOBAPHT.
1281. The chemical iniiaence exerted by the solar rays upon
Baits of silver has been already referred to (1106). The earliest
investigation of the action of light on silver compounds appears
to have been made by Scheele, in the year 1777 : he discovered
that the different coloured rays were not equally active in pro-
ducing the observed chemical changes. These phenomena haTe
much more recently been made the subject of carefiil stndy,
and with so much success, tbat a property, long sunposed to be
peculiar to a few argentine combinations, bas been snown to be of
a much more general character. The labours of Sir John
Herschel have been among the most interesting and important in
this inquiry, and this ^at philosopher has shown tnat there
scarcely exists a combination, whether of organic or minenl
origin, the molecular constitution of which is not more or less
i^ected by the solar rays. The study of these extraoidinazy
effects constitutes the science of Photography.
1282. If a piece of naper be moistened with a solution of com-
mon salt, and then with one of nitrate of silver, a thin covering of
chloride of silver will be formed on its surface, and it is then sensi-
tive to the action of light. If a piece of such paper, commonly
ddled senaiiive^ be exposed to the sun*s rays, or to the diffined
light of day, it becomes darkened in colour, and assumes a brown,
bluish, or black hue, according to the length of exposnre, or to
the proportion of silver present. The chemical change thus ex-
perienced by the chloride of silver is not yet satisfactorily luder-
stood ; it, however, appears probable that a partial conversion
into oxide, or even reduction to the metallic state with evolution
of chlorine, occurs. It is certain that some important molecular
change does take place ; for if a piece of paper thus blackened
by exposure to light, be digested in a solution of hyposulphite of
soda (in which chloride of silver is readily soluble), it giyes np
but a small proportion of the silver, all the chloride which has
been changed b;^ the action of the sun's rays being insoloble in
that saline solution.
1283. Metuurement of Solar Chemiad Action. — ^A careful series
of investigations on this subject has been made by Profs. Bnnsen
and Roscoe.* The method pursued was that of exposing paper
• PhiL Trtts. 1863, put I.
TSTESeaY OF OHBMICAL ACTIOH. 727
coated with a definite layer of cUoride of silver to the inflnenoe
of either direct solar rays, or difihsed daylight, for known hnt
gradually varying periods of time ; and then comparing the action
on the paper with a definite grey tint.
The paper was first salted hy immersion in a solution con-
taining 3 per cent, of pure chloride of sodium, and dried : this
strength was chosen because it was found that the paper ab-
stracted equivalent quantities of the salt and water, and therefore
left the strength of the solution unchanged. The paper was then
floated for two minutes on a 12 per cent, solution of crystallized
nitrate of silver, and dried in the dark. This strength was
adopted because it was found that two-thirds of the silver-bath
mignt be used up before the strength of the solution fell below 8
per cent., which was found still adequate to saturate the chlorine.
The paper was subjected to the action of light by placing it
beneath a parallel slit in a horizontal plate of metal, which was
covered by a thin lamina of mica blackened : this was withdrawn
and replaced by the swing of a pendulum, from the time of oscil-
lation of which the time of exposure of each portion of the paper
was readily calculated ; and a table is given of the times of eX'
poBure of each successive millimetre of tne paper.
The standard tint was obtained by triturating together 1000
parts of oxide of zinc with one part of pure carbon, obtained
from the smoke of a turpentine lamp. It was found that by drving
ihe mixture and re-grinding on a glass plate, the tint was deep-
ened; but after several repetitions of this process, the tint was
not susceptible of further change : it is therefore at all times
definitely reproducible.
It was also tbe result of observation that equal products ofths
ifUenaities and the times of exposure produce equal shades of
blackness: hence, adopting as the unit of measurement the in-
tensity of light which produces in one second tbe standard tint
upon the standard sensitive paper, it became easy to institute a
comparison of the chemical action of daylight, at different times
or places, and under different circumstances.
Many interesting observations have been made by Prof. Roscoe :
in regard to the chemical brightness of different parts of the sun^s
disc he has ascertained that, takinfl; the luminosity of the centre
of the disc to be 100, at 15*^ from its edge the luminosities were
at the— N. Pole, 38-8 ; Equator, 484 ; S. Pole, 68-1 :
and at the edge of the disc, the corresponding intensities were
18-7; 80-2; 282.
The total solar action was found to be generally proportional
to the sun's zenith distance : at the summer solstice, the equi-
noxes, and the winter solstice, the intensities observed at Owen's
College, Manchester, were in the ratio of the numbers 1 1 3 : 33 : 5.*
* Proceedings of the Bojal InstitutioD, toL It. p. 652.
728 .. CHKUICAL ACnOK OF LIGHT.
1284. The relative powers of the atmosphere to tnuisniit and
reflect solar rays have also been observed by Prof. Boecoe, bj in-
tercepting the direct rays of the sun by means of an opaque
disc, and observing the difference of action on the standard sensi-
tive paper of totu daylight, and of the same minus the direct
solar rays. From these experiments it appeared that the chemical
action of direct sunlight was much less than would have been
anticipated from its visual intensity. Thas it was found at Man-
chester that, at an altitude of the sun of 12° 3', not more than 5
per cent, of the chemical rays falling on a horizontal surface were
due to direct sunlight ; while at the same time the direct beam
contained 60 per cent, of the visual rays. And on some occa-
sions it was observed by Dr. Wolkoff, at Heidelberg, that, at ele-
Tations not exceeding 13^ the bright sunbeam seemed totaDj
devoid of chemical rays, as no difference of action resulted from
its obscuration. At an altitude of about 25** the chemical in-
tensity of the direct rays was half that of diffused d^ight, and
at about 40° the two intensities were equal. But at Para it was
observed that the action of direct rays was only one-half that of
diffused light at the sun*s altitude of from 60° to 77°.
This remarkable power of the atmosphere to transmit the lower
rays of the spectrum, and to disperse or reflect the rays of higher
lefrangibility, is identical with the property known as opalesceneef
and is due to the suspension in the atmosphere of particles of
aqueous vapour or other matter. The absorption or dispersion of
the chemical rays may be thus illustrated. If a minute quantity
of finely-dividea sulphur be suspended in water (0*1 gram to a
gallon) it will be rendered very slightly turbid, but nevertheless
sufficiently so to intercept nearly all the chemical rays: this
may^ be shown by the following experiment. If equivalent pro-
portions of chlorine and hydrogen be introduced into a glass bulb^
on exposure to a bright hght, those elements will combine with
explosive force, provided the chemical rays be not intercepted.
If the rays of an electric lamp (810) transmitted through yellow
^lasB fall on the bulb, it will remain intact ; but if blue glass be
interposed, the bulb will explode in a few seconds. The same re-
sult will ensue if the rays pass through clear water ; but the
slight opalescence in the sulphur-water above mentioned wiU en-
tirely prevent the explosion.
The rich blue tint of the summer sky, as well as the varied and
mellow tints of evening, are probably alike doe to the same pro-
perty of opalescence in the atmosphere.
It appears also from the observations of M. Janssen* that
many of the chemical rays of higher refrangibilify are inter-
cepted by- aqueous vapour, which is always prevalent in the at-
mosphere in clear weather during the aflemoon hours. The
effect of this will evidently be to lower the position in the
• Beport Brit. Aaioo. 1860.
OPPOBTTB BPrBCIS OP 80LAS BATS. 729
spectraiii of the mean chemical rajs, and hence to lengthen the
chemical focus by a small quantity : this is a fact well known to
photogrnphers.
1285. When a slip of senBitive paper is exposed to the^ solar
spectrum (1082), it is most darkened in the violet raj, and in the
space bejond it, occupied by the lavender band (1087). In the
locality of the less refrangible rays, the paper is scarcely affected,
except that occasionally it is observed to assume a very faint
tint, bearing some resemblance in hue to the coloured bands of
the spectrum, which thus, within certain limits, imprint their own
tints upon the paper. Tbe following table shows the results of an
experiment in which the paper was rendered sensitive with
chloride of silver : —
Ooloimd bftad of speotnon. Tint impretMd on the p«p«r.
Bed None.
Orange . .
Orange-yellow
Yellow , .
Tellowish-green
Green . . .
Bluish-green .
Blae . . .
Violet . . .
Lavender . .
To the agency in the
Faint brick-red.
Brick-red.
Bed passing into green.
Dull bottle-green.
Ditto passing into bluish.
Sombre blue.
Black passing into metallic yellow.
Ditto.
Violet or purplish black.
^ sunbeam which is capable of exciting
chemical influence, as distinguished from heat and light, the term
Actiniam, or ray-force, has been applied ; but the term is not dis-
tinctive, as light and heat are e<j[nally the results 9f ray-force.
1286. The opposite extremities of the solar spectrum exert
Yerv different eflects on sensitive paper, appearing to neutralize,
witnin certain limits, each other's effects : thus, a piece of that
paper, blackened by violet light, may be bleached by subsequent
exposure to the red ray. Sir J. Herschel found that when violet
and red rays were allowed to fall simultaneously on a piece of the
paper impregnated with chloride of silver, they nearly neutralized
each other's effects. And Mr. Hunt observed that if a ray of
sunlight, having undergone jprismatic refraction be allowed
to fall upon a sheet of sensitive paper, after traversing a
plate of yellow glass, even after some length of time, the sensi-
tive paper will remain unaffected, from the chemical rays having
been absorbed by the yellow glass. Then let a second beam of
sunlight be reflected from the surface of a mirror, so as to illumi-
nate the whole paper; in a short time its surface will be
entirelv blackened, except on the part where the spectrum falls :
there the paper will remain unaffected. This experiment proves
onlj that the altered rays of the spectrum interfere with the
action of the unaltered rays subsequently thrown upon the paper,
730 CHEMICAL ACnOH OF LiaHT.
bat does not prove, as Mr. Hunt imaged, that there is any
tial difference in the nature of lominous and chemical r^a.
1287. The e£fect of the antagonistic powers of the dififerent rayi
in the spectrum is remarkably shown by the varying sensibility
to light of photographic paper in different regiona of the earth.
Thus, advancing from England towards the equator, the difficulty
of obtaining good pictures is considerably increased, and more
time is required to produce an effect on sensitive paper, under the
full blaze of a tropical sun, than in the gloomier atmosphere of
London. Prof. Draper observed the same fact in travelling firum
Kew York towards tne Southern States. These curious facta may
probably be explained by the preponderance of yellow rays in the
more tropical countries. Even in England it is found that photo-
graphs are more readily obtained in March than in June.
These statementa, however, appear to be at variance with some
observations made by Prof. Roscoe ; he found that the ratio of the
mean chemical intensity of light observed on three days in April,
1866, at Kew, to that ooservea on the same days at FaA (long. 48"
SC W., lat. 1" 28' S.), was : : 1 : 18*2.
1288. The chemical action of the more refrangible and invisible
rays of the spectrum on the iodine compounds has been investi-
gated by Dr. Miller* by means of photographs of the solar and
electric spectra ; taken by means of a prism and lens of auaitz.
In the solar spectrum the chemical rays thus transmittea were
found to be most intense at about the position of the line h ; and
to subside gradually towards o, and to a point about the distance
of G above h ; beyond these points they are very feeble in both
directions.
In the chemical spectrum of cadmium the most jpowerfnl rays
are distant about four times the length of the visinle spectrum
above the line h; but these are entirely intercepted by glass.
The chemical spectrum of magnesium is the most intense of all
the substances submitted to experiment, and is also one of the
most compact spectra, occupying a space, about equal to the
visible spectrum, above h : the mngnesium light is therefore pe-
culiarly suitable for the pufposes of photography.
The general results oi Dr. Miller s experiments on the absoip-
tion of the chemical rays are the following:—
1. Colourless bodies which possess equal powera of trans-
mitting the luminous rays, vary greatly in their permeability to
the chemical rays.
2. Diaetinic solids (permeable to chemical rays) preserve this
power both when liquefied, and when converted into vapour.
3. Colourless transparent solids, which absorb considerably the
chemical rays, preserve more or less of their absorptive power,
both in the uquid and in the gaseous state.
1289. Photography. — The firat attempts to render the chemical
• Phil. Tnna. 1862.
agBDC7 of ligbt avulttble id the arts were toade b; Wedgwond
uid Dary, in 1B02. Thsj produced tbe thadoa of an alyect laid
on a ibeet of paper covered with s toluuon of silTeT, and expoaed
tuj mode of remoTing the vnehaHged Hilver salt rrom the paper,
or, aa it ia technically termed, Jixing the impreaBion. the picture
Iras e&ced by subsequent exposure to light. This difficulty nas
first sncceRBfully orercome by the discorery ol Sir J, Herecnel of
the solubility of the salts of silver in a solution of the hyposulphite
of soda, already alluded to ( 1 382).
1Z90. Ae the darkening of the chloride of silver ia confined to
those portions which are exposed to light, it is easy to apply this
property to the copying of patterns of lace, leaven, engravings, &c.
For this purpose, a piece of paper properly prepared should be
Shiced upon a amooth BurTace, and the object laid upon it,
1e. GT4 ; a plate of glass should tben be placed on the whole,
and pressed dovn witb a moderate weight. A pleasure fraine,
rimitar to those conlaining the plates of ground glass used by
..kiM—.. .n t—^ drswings, is very conveiueDt for this purpose,
fw- (71.
children ti
Bj a sbort exposure to the aun, or a longer one to diffused day-
Bght, all that part of the paper uncovered by the object will be
darkened in colour, or even blackened ; the remainder being pro-
tected trom the action of light, retains its primitive whiteness.
On removing the paper, an exact copy of the olyect placed upon
it will be found. This drawing will, however, soon vanish by the
blackening of the whole iinpresBion, unless it be preserred by the
removal of the unchanged chloride from the paper. For tliis
{urpme, after soaking for a few aeconds in water, the |»per should
9 washed in a solution ofbyposiilphile of soda, containing two or
three drachms cf the salt in a fluid-ounce of pure water, which,
by disaclving the UQchanged chloride, renders the image of the
otject permanent; this is called ^"nj the impreaaion. Washing
with a solution of CerrocTanide, or iodide, of^ potaasinm will also
Doriially fix the picture ; but a photograph thus fixed is extremely
733 PHOTOOKAPHT.
The class of objects best adapted to tbis mode of trcntmeiit ii
the fern tribe ; the delicacy and artistic beauty, with which the
fronds of ferns have been photographed, is truly surprising.
1291. In the process just descnoed, it may be remarked that
the portion of surface representing the object is left white, while
the surrounding portion or " ground" of the picture is darkened :
and the same remark may be applied to the pictures ordinarily
taken in the camera (1293), in which the portions acted on by the
stroneest light are most darkened, and vice versA: The hghts
and shadows are consequently all reversed, and the result is called
a negative picture ; on the contrary, a picture is termed /KMt/ioe,
when the object directly copied by superposition is represented by
dark marks on a white ground, or when the lights and shadowa,
presented by the image in the camera, are correctly depicted
in the photograph. In order to obtain this result, the tie^tioe pho-
tograph should oe placed, face downwards, on a piece of sensitive
paper, and kept in close apposition with it by a plate of gUsa
pressed down by means of screws or weights : and in proportion
to the amount of pressure exerted, the paper becomes more trans-
lucent, and the photographic impression consequently sharper.
When the pressure is exerted in a strong frame, by a powerful
screw, and between plates of glass i an inch or more in thickneaa,
the increased transparency of the negative is evident from the im-
pression to be copied becoming visible through the paper. After
exposure to direct sunshine for a sufficient time, the paper should
be removed, and the positive picture fixed by the means deacribed.
In order that the gradations of light and shade may be cor-
rectly transferred to the positive photogmph, it is obvious that if
the negative be taken on paper, it should be as thin and trans-
parent as possible ; and its transparency may be considerably
augmented by saturation with white wax. But the processes on
glass plates, hereafter mentioned (1323-1330), are much better
suited for negative photographs than those on paper.
1292. The greatest triumph of the photographic art is un-
doubtedly the rendering permanent the beautiliil but fleetii^
imagesof a camera-obscora(1147). The first step in photomphy
by the aid of this instrument was made by M. Ni^pce, of Chalons,
in the year 1814. He discovered that a resinous substance, kiiown
as the " Bitumen of Judea," was so altered in its physical pro>
perties by the action of light, that it became insoluble in certain
essential oils. A metallic plate, covered with this substance, was
placed to receive the image of any proposed object in the camera,
and the portions of bitumen unchanged by light were subsequently
removed by solution, the image remaining depicted in bitumen.
This, however, was found to be a very slow and unsatiafiMitofy
process, and was soon abandoned.
1293. The Fkotographie Oamerti, — This invaluable appamtus
should be provided witn a good achromatic (1120) lens, or com-
binstion of lenKB, and be capablo of wljmtmeiil by meuu of a
jlidinK tube at a, Fig. 676, furnished with a r»ck and pinion ; ths
farther end of the boi ehonld be made to slide within (he anterinr
ertioQ, in order to obUia a rongh mljustment to focna, acd shonld
prondfld withgrooTes, ioaa to admit either a wooden frame o
or a pane of ground glass, at will : in either caaa these mnst m
fit the grooves, as to prevent tbe admission of eitraneous light.
To use this instrnment, the end n should be closed bymeaiiBof
the plate of ground class, for
the purpose of Teceiving the 'V- WS.
image produced bj the lene.
On phtcing the camera on a
convenient support opposite
the landscape or object to be
oojiied, its image will be
visible on the ground glass,
which sbould he adjusted
until the image becomes as I
perfect and nell-defiued as
possible.
In cameras desigaed for
taking landscaMs a ringle achromnlio meniscns (1068) ij some-
times employed, the concave side being tomed towards the ob-
ject. Ii. front of the lens a diaphra-m is placed, the aperture of
which may be large, if the parts of the object are nearly in the
■ame plane, as the elevation of a building ; bat in a landscape the
apertore must be diminished in proportion to the space between
the foreKTound and the distance. As both fore and background
Mnnot be in Socaa at the game time, the eonfnsiou of the image
fiom the oToriappmg of contignous pencils is diminished by re-
ducing their angular aperture, aa in myopic vision (1 187).
in the best constructed landscape cameras, the frame is capable
of a little inclination in any required direction: this is often
roond convenient for the duo preserratioa of perspective in
wchifectural photographs, ^
1294. In cameras for taking portraits (for which shortness of
time IB an object), a larger amount of inddent light, aod conse-
quently a larger angle of aperture of the ill mnina ting penoil,beeomeg
necessary ; and then in order to correct the increased amoiuit of
aberration, a double achromatic com-
bination is employed: of these the Fv,.«7ti.
front 18 plano-convex (Fig. 598), the
coQToi surface being turned towards
the object, as at a, Fig. 676. Tte pos-
terior combination, b, which is Mpa-
rated from the anterior by a space
about equal to the diameter of the
latter, coDBists of a conoavo^Muvei
736 PHOTOGKAPHT.
bein^ exposed to the yaponr of iodine^ which is best efifected br
placing at the bottom of a wooden box a piece of thin deal,
saturated with a solution of iodine in alcohol, and resting the plate,
with the silver face downwards, on two little projecting ledges
in the interior of the box, so that it may be a couple of inches
above the iodized wood. In this manner a delicate yellow coating
of iodide of silver is formed over the plate : this is extremelj sen-
sitive to light, and therefore must not be exposed to its influence
nntil placed in the camera. The surface may be rendered still
more sensitive by suspending it for a short time over a solution
of chloride of bromine, by which the yellow colour of the iodized
plate is converted into a pale rose-red. These operations should
be, like all preparatory photographic processes, performed in a
room illuminated only by the rays transmitted through yeUbw
or red glass. The prepared plate oeing placed in its frame, and
the lid closed to shield it from light, it should then be placed in
the camera, previously carefully adjusted to focus, and the sliding
cover being withdrawn, the sensitive surface of the plate is ex-
posed to the action of the rays forming the image.
In the course of a few seconds the full effect is generally ob-
tained, and the closed frame, with the plate, should be transferred
to the photographic room. The plate being then removed, will
not appear to have undergone any visible change, the picture being
present, but latent. To render it visible, the plate is suspended
in a dark box over a capsule of mercury, gently neated by a spirit-
lamp to a temperature of about 140° F. The ledges whicn support
the plate should be so placed that the latter may rest at an angle
of 45 degrees to the side of the box. The mercury will slowly
rise in vapour, and will adhere in the form of extremely minute
grey globules to those parts of the plate where the light has
nJlen, leaving the parts corresponding to the shadows of the
picture untouched. In this way the picture may be seen gndnally
unfolding itself, and this beautiful part of the process may be
watched by the light of a taper through a little yellow window
in the box. As soon as the picture has obtained its maximum
of distinctness, the plate must oe removed, and being placed in a
yessel, should be covered with a weak solution of hyposulphite <if
soda (12 grains to one fluid ounce of water), to dissolve all the
bromide, iodide, and chloride of silver left unaltered by lieht
After washing with water, the plate may be allowed to dry, when
the picture remamBjixedf and the plate insensible to any farther
action of light.
1299. The rationale of this curious process of exposing a highly
sensitive surface of iodide or bromo-iodide of silver to bgfat,
consiste in some molecular change effected in the parte on which
the light has acted, in consequence of which the vapour of mercniy
is condensed upon those portions of the prepared plate oa^.
Mr. Hunt has ascertainea by actual experiment that all the
PROCE88K8 OX PAPER. 737
rays of light decompoBe iodide of silver in a longer or shorter
time. They have also the power of producing such a chauge
in the constitution of the coating of oromo-iodide of silver as
to render it capable of retaining the minute globules of metallic
mercury by a direct attractive, or cohesive force. The darks
and shadows of the Daguerreotype pictures are formed by the
naked surface of the silver shining with its full "black " lustre,
whilst the lijghts are formed by the grey globules of mercury.
Hence the slightest touch with the finger is sufficient to rub off
Bome of those adhering globules, and thus to spoil the picture.
The state of surface produced by the light, by which the vapour
of mercury is enabled to adhere to particular portions only, is
probably analogous to that which induces the formation of Muser^s
ngurea with the vapour of wator f Ch. XXIV).
It may be remarked that a visible image may be produced upon
the Daguerreotype plate, by the prolooged action of light alooe.
In this case a white powdery deposit is formed upon the plate,
the particles of which appear crystalline when highly magnified,
but their nature is not exactly known. This property is not,
however, of any practical utility, as 3000 times more light is
required to produce an image, tnan when it is developed by the
yapour of mercury.
M. £. Becquerel has observed that pure yellow light is capable
of continuing the action initiated by ordinary light, if the plate be
prepared wim the vapour of iodine only : but if prepared with the
yapour of bromine, tne yellow rays not only do not promote the
action on the plate, but actually efface it.
1300. The image on the Daguerreotype plate may be con-
siderably strengthened, by immerbin^ the plate in a hot solution
of one part of hyposulphite of gold in 500 of water. The term
"toning " the picture has been applied to the subsequent addition
of some chemical substance for the purpose of altenng the colour
or deepening the effect of the image produced.
1301. Processes on Paper. — In giving a brief sketch of the
different modes which have been employed to render paper and
other surfaces sensitive to the action of the chemical rays of the
Spectrum (1106) as they exist in ordinary light, it is important to
istingnish between two classes ; one including those which re-
ceive a direct and visible impression from an image thrown upon
them ; the other containing those which undergo a certain
molecular change, but in which the picture is UUentf and requires
the application of some reagent to render it visible. Almost all
metafile salts, and vegetable pigments, belong to one or other of
these classes, some of the most remarkable of which will now
engage our attention : of these the compounds of silver, consti-
tuting the different species of argentotype^ are among tne most
sensitive.
1302. In selecting paper for photographic purposes it is i^b-
3 B
738 PHOTOGBAPHT.
Bolutely necessary to be very caatioas in choosing specimens free
from inorganic matter, especiallj sulphate of lime, alum, and oxide
of iron. Causon's paper in France, and Whatman's and Tniner's
in England, are well suited for photographic purposes ; and the
demand for suitable paper having become considerable, there is
now no difficulty in obtaining it.
1303. It is essentia], for obtaining a good pictare, that all
photographic preparations should be uniformly and evenly distri>
buted over the sensitive surface ; and as exceedingly minute
quantities of organic or other foreign matter, will frequently be
found to mar the beauty of the effect, it is desirable to avoid as
much as possible the contact of brushes, sponges, or other bibu-
lous bodies, with the sensitive surface. The best mode of applying
the requisite solutions is generally either by immersion, or by
floating the paper on the surface of the solution placed in a
shallow glass or earthenware dish, and then holding it edgewise
to drain on a piece of blotting-paper. In those cases in which
the preparation would be rapidly deteriorated by oiganic matter
derived from the paper, it is desirable to lay the paper on a piece
of plate-glass a little BmulUr than the paper, so as to leave a
narrow projecting margin, and having poured a small quantity
of the solution on the paper, to distribute it over the surface by a
straight glass rod, which, for the convenience of holding, may be
bent twice at right angles, thus : —
\
The object of the projecting margin of paper is to prevent any
portion of the fluid, tiiat reaches the edge, from passing behind
the paper by capillary attraction. This is most important in
negative (1291) photographs, as any stains on the back will be re-
produced in the positive copies.
1304. As nitrate of silver is an important ingredient in atmcst
all practical photography, a solution of 50 erains of crystallized,
or colourless fused nitrate of silver in one nuid ounce of diadlled
water may be understood to be the solution subsequently
mentioned.
1305. Argentotype. — The employment of chloride of silver has
already been mentioned (1282); a paper msy be prepared with
this salt of sufficient sensibility to receive the impression of an
image in the camera (1293), after an exposure of from ten minutes
to half an hour : but the picture will be far inferior in beauty and
distinctness to those obtained by other processes. For this pur-
pose the paper should be first covered witn a solution of 20 grains
of chloriao of sodium (common salt), drained and dried ; and then
with the solution of nitrate of silver, after which it may be again
dried, but it is more eficctive if used at once, ¥rithout diying. A
leas sensitive paper, for copying botanical specimens, or other ob-
POSITIVE PH0T0OBAPH8. 739
jects, hy superpositioii, may be prepared with a solution contain-
ing one half the quantity of nitrate of silver.
More sensitive papers may be prepared by applying first a
20-grain solution of iodide or bromide of potassium, and subse-
quently the solution of nitrate of silver.
It appears that these three preparations, although closely re-
Bemblmg each other, are acted upon differently by the chemical
rays of me spectrum: thus, if slips of these different papers be
exposed to the spectrum, the paper prepared with the chloride of
silver will be most darkened in the blue ray ; that with the iodide,
beyond the violet : and that with the bromide will be blackened
for nearly the whole length of the spectrum, but most intensely in
the indigo ray.
The pictures impressed on these papers may be fixed by wash-
ing them first in warm distilled water, to remove any portion of
unchanged nitrate of silver remaining in the paper, and then in a
solution of hyposulphite of soda containing two drachms in the
fluid ounce : which will require to be warmed in cold weather,
in order to facilitate the solution of the iodide or bromide of silver
not acted on by light. The hyposulphite of silver may then be
removed by washing in plain water, until it no longer presents a
sweet flavour.
It may be remarked that the neutral iodide and bromide of
silver are scarcely sensitive to light ; but they acquire this pro-
perty by the presence of an excess of nitrate of silver.
1306. The photographs obtained by the processes hitherto de-
scribed are all negative | it is, however, possible to prepare a
poBitive paper t or one which will at once give a picture with its
lights and shades in their proper position ; the following is the
process contrived by Mr. Hunt for this purpose. The photogra-
phic paper is soaked for a few minutes m a solution of ten grains
of chloride of sodium in an ounce of water: it is then drained,
dried, and washed over with the solution of nitrate of silver, to
one ounce of which are added two fluid drachms of alcohol : the
paper is then exposed to the sun, which darkens it immediately ;
the solution of silver is again to be applied, and the paper once
more exposed to the sun, until it assumes an uniform chocolate
colour ; it is then to be dried in the dark. To use this paper, it
should be covered with a solution of thirty grains of iodide of
potassium in an ounce of water: and if placed whilst moist in the
camera it will be impressed with a beautiful sepia-coloured posi-
tive picture in half an hour. These pictures are best fixed by
washing them with water, to remove the excess of iodide ; but
unfortunately they are liable to fade.
1307. If any of the sensitive papers (1305) be exposed in the
camera for too short a time, either no picture at all, or at least a
barely visible one, will be noticed ; still the absence of a visible
impression is no proof that a sufficient efiect is not produced. This
3 B 2
740 PHOTOOSAFHT.
maj indeed be demonstrated hj the application of some reducing
agent, as gallic, pyrogallic, or succinic acid, proto^ulphate of
iron, &o., which would act either more slowly, or not at all upon
the paper before exposure : we have, founded on these principiea,
some processes for preparing paper of extreme sensibility ; and of
these the first whicn merits attention is the eaJtotype,
1308. The first communication made to the Koyal Society by
Mr. Fox Talbot, in January, 1839, comprised only the applicatioD
of chloride of silver to the copying of objects superposed on the
prepared paper, and subsequently obtaining a potitive (1291)
cop^ of the negative photograph thus produced : tne repetition of
positive copies, from the same negative, forms an important fea-
ture of this contribution to science.
The ealotype process was patented by Mr. Fox Talbot at the
beginning of the year 1841 ; and as at present practised, diflfers
little from that originally proposed. The paper is firvt coated
evenly with a 20-grain solution* of iodide of potassium, and
dried ; it is then covered with a 30-grain solution of nitrate of
silver, washed in pure water, and again dried. The paper thns
prepared may be kept for a considerable time. When prepared
for use, in order to g^ve the iodized paper its maximum of send*
bilit^, it must be rapidly covered with equal parts of the solution
of nitrate of silver, to which one-sixth part of strong acetic acid
has been added, and of a cold saturated solution of gallic add ;
then immerse it for a moment in distilled water, and drain it on
blotting-paper. Tn this state it is remarkably sensitive, and when
nearly dry, may be exposed in the camera ; but a short time,
often a few seconds, is sufficient to obtain a beautiful picture.
The paper should then be removed, and the picture impressed
upon it, although as yet invisible, must be developed bv again
washing it with the mixed solution of silver and gaUic acid, keep-
ing the paper warm, especially in cold weather, by placing it in a
dish over boiling water. The picture thusproducea may be fixed
by washing first with warm water, and then with a warm solution
of hyposulphite of soda ^1298) for a few minutes; after which, it
should be again soaked m warm water, and dried between folds of
blotting paper. These pictures, which are among the most beau-
tiful in the photographic art, are negatives.
1309. Some photographers prefer preparing the iodized paper
bjr a process called the " single waui : for this the silver con-
tained in the solntion is precipitated in the form of iodide, by
the addition of iodide of potassium: the precipitate is then
washed, and dissolved in a strong solution of iodide of potassinm.
The paper is floated on this solution, drained, and immersed in a
bath of^pure water ; when the iodide of silver is thrown down on
ita surface in a finely divided state, being almost insoluble in a
dUute solution of iodide of potassium.
* Twentj gr»ina to one fluid ounce of dlstiUrd wat«r.
PR0CBB8 BUITABLB FOB ABTIFICIAL UOHT. 741
1310. It has been found that the sensitiveneBS of the calotjpe
paper is considerably increased by adding some chloride of sodium,
or ammonium, to the solution ot* iodide of potassium. The fluid
recommended by Mr. Cundell contains twenty grains of iodide of
potassium, and^ five grains of chloride of sodium, in an ounce of
water. A similar quantity of chloride of ammonium is preferred
by some photographers.
1311. The original project of Mr. Fox Talbot has received some
modifications and iinprovements ; but its general details have re-
mained unaltered. It will not be necessary to allude to more
than the process sug^sted by Mr. Channing of Boston, which has
the great merit of simplicity. He directs the paper to be first
carefully washed over with the solution of nitrate of silver, and
when dry, with a solution of ten grains of iodide of potassium, and
five of chloride of sodium, in an ounce of water : aher which it is
to be washed with water, and dried between folds of blotting
paper. The paper is then fit for use, and may be placed in the
camera without any further preparation. After exposure, the
latent picture is brought out by washing with tbe mixture of
gallic acid and nitrate of silver, as in Mr. Talbot's process. A
mere aqueous solution of gallic acid is, however, quite capable of
developmg the picture, if the dish, containing the paper, be placed
over a vessel of boiling water, after adding the gaUic acid. The
picture may then be fixed as usual, by means of the solution of
nyposulphite of soda.
1312. Availing himself of the greater susceptibility of the
bromide of silver to the less refrangible rays of the spectrum, the
present editor of this treatise devised a preparation of considerable
sensibility, especially to the feeble chemical rays of a lamp- or gaa-
light, wblch has been for some years past suocessfally employed
at the Royal Observatory, Greenwich, and elsewhere, in the
automatic registration of magnetic (624), barometric (495), and
thermometric (1234) variations. His process is the foUowme : — one
surface of well glazed paper is covered with a solution of twelve
grains of bromide of potassium, eight of iodide of potassium, and
four of isinglass, in an ounce of distilled water ; care must be
taken to apply the solution equally over the paper, which is then
to be dried. When required for use, it is washed over with the
solution of nitrate of silver ; the only light used in this process
must be that transmitted through yellow glass. ^ This j)aper
should be used whilst damp ; a few minutes are sufficient for its ex-
posnre, when employed in the camera, and on removal, it should be
covered with the aceto-gallo-nitrate solution (1308), when in a few
seconds the picture will hecome visible. As soon as it is sufficiently
developed, tne process should be stopped by immersing the paper
in water, and fixing the picture in tne usual manner. With ttds
paper a sharp impression has been obtained in a few seconds from
the image of a narrow slit illuminated by gas-light formed at a
742 PBOTOORAPRT.
distaooe of seyeral feet in the conjngate focus of the concaTe
mirror of the registering apparatus.
1313. Waxed Paper JProeeta. — Very beantiinl results maj he ob-
tained by this process. The paper is first waxed, by placing it on
a plate of hot iron, and rubbing a piece of white wax smoothW
and eyenlv over its surface ; it is thus rendered transparent It
is desirable to procure pure white wax for this purpose, as the
commercial white wax, sold in cakes, is usually aaulterated with
some fatty substance. Pieces of paper of the proper size should
be allowed to soak for some hours m the solution of chloride of
sodium, or, still better, in that suggested by Mr. Cundell (1310) ;
they are then removed and carefully dried. Some photographers
prefer iodizing the waxed paper with a mixed solution of iodide and
oromide of potassium, to which a little free iodine has been added.
Immediately before exposure in the camera, the paper is passed
over the surface of the solution of nitrate of silver, to which one-
sixth part of acetic acid has been added. After draining for a
minute or two in a darkened room, the paper may be placed in the
camera in the usual manner. After a safficient exposure, the latent
picture is brought out b^ means of a saturated solution of galKc
acid, or by the gallic acid mixture before mentioned, and fixed in
the ordinary manner by means of the hyposulphite of soda.
1314. Atbuminixed ^per has been frequently employed ; it is
thus prepared : 40 grams of chloride of sodium or ammonium ars
dissolved in one fluid ounce of water, and placed with three fluid
ounces of albumen (white of egg) in a bottle capable of containing
nearly double the quantity ; the mixture is shaken, until it
to be ropy, and the sediment allowed to subside. The clear
portion of the mixture is poured into a shallow vessel, and the
paper floated on its surface, until it is thoroughly wetted, and then
orainedj and dried ; when required for use, it is rendered sensitive
by the acetonitrate bath, as in the waxed paper process, and ex-
posed whilst moist in the camera. A very brief exposure is saf-
ficient to form a latent picture, which is developed by means of the
gallic acid solution, ana fixed by the hyposulphite. It has been
stated by Mr. Cousins that the paper is rendered more sensitive by
albuminizing it by means of a mixture of twenty-two grains of
iodide of potassium, six grains of bromide of potassium, and two
grains of chloride of sodium, with one fluid ounce of white of egg.
1315. Many varieties oi ferrotype haye been described ; these
are all indebted for their sensibility to light to the per-«alta of
iron, which become wholly or partially reduced by the solar rays.
The most convenient salt for this purpose is the ammonio-citrate ;
if paper be washed over with a solution of this salt in ten or eleven
parts of water, and carefully dried in the dark, it is remarkaUy
sensitive to light ; if any object be superposed on a piece of this
Saner, and exposed for half an hour to the sun, a sharp and well-
enned picture will be obtained. If, however, the buu'b rays be
CTANOTTPES. 743
not verj bright, or the expoBure to their influence be mucb eborter,
a Bcarcelj visible impression will be formed ; but as all the points
exposed to the light have undergone a change, from the partial
conyersion of the sesqui-citrate into a proto-salt, on washing the
paper with a solution of the red pnissiate of potass (ferro-sesqui*
cyanide of potassium], a pretty picture will appear in yellow on a
blue ground, the prussiate acting only on the reduced portion of
iron salt. In the same way a solution of nitrate of silver will pro-
duce a fawn-coloured picture on a blackish-brown ^und, and a
neutral solution of chloride of gold will produce a picture with a
splendid purple g^und, which is the chrysotype of Sir J. Herschel.
1316. Many other salts of silver may be employed with more
or less success as photographic agents, their range of sensibility
differing very considerably; among these compounds, one pro-
duced by the action of ferrocvanate of potass or iodide of silver,
discovered by Mr. Hunt, is the most emcacious ; this, the eyano-
type^ is thus prepared, and is wurtlw of notice from its presenting
a close approach to the beauty of the calotype pictures. The
paper is washed over on one side with the solution of nitrate of
silver (1304), qm'ckly dried, and a second time washed with the
same solution ; it is then immersed for a minute in a solution of
20 grains of iodide of potassium, in one ounce of distilled water,
removed, and gently washed with pure water, and, lastly dried in
the dark. The paper, so far prepared, may be kept for any length
of time, and to render it highly sensitive, it is only necessary to
moisten it with a solution of one part of ferrocyanate of potass in
eight of water. These papers are only sensitive whilst moist, for
if allowed to dry in the dark, they become nearly insensible to
tiie influence of light; immediately, however, acquiring their
susceptibiUty to light by merely moistening them with pure
water. The pictures taken in the camera oy this process are
readily fixed, by bein^ washed first with water, and then with the
solution of hyposulphite of soda, and then washed sgain, as before.
It was on this kind of paper that the curious result of an impressed
coloured spectrum was first obtained.
1317. llie eyainotyp€9 are very beautiful, and some of them have
peculiar claims to our attention. One of the best is made by
washing paper with a solution of the red prussiate of potash, and
when dry, .exposing it to the sun, with tne object to be copied
pressed in close contact with it by a plate of glass. The effect of
the light is to evolve prussian blue, which remains deposited on
the paper. On soaking the picture thus produced in water, the
unchanged prussiate is removed, and a white picture on a beauti-
ful blue ground results. This easily prepared paper affi>rds a ready
and excellent means for copying ferns||pr other objects by super-
position, but it is not sufficiently sensitive for the camera.
1318. The most sensitive cyanotypes hitherto obtained are
those prepared by the following processes, devised by Sir J.
744 PHOTOGRAPBT.
Herschel. Mix together equal parts of a cold saturated Bolatkn
of bichloride of mercury, with one of the ammonib-citrate of iron
(1 part of the citrate to 11 of water) ; and before any precipitatMO
occurs, wash over the paper with this fluid, and diy it ; pa]>er
thus prepared may be kept for some time without being deterio-
rated. A piece of this should be placed in the camera, until a
decided, although faint impression is just visible ; it should^ then
be removed, and rapidly washed over with a saturated solution of
ferrocyanide of potassium, diluted with three times its bulk of
strong gnm -water; the picture then gradually unfolds itself in an
extremely beautiful manner, and should be allowed to dry in the
dark, where it should remain for some days ; after whicn it will
bear a strong light with impunity. If a picture thus obtained be
heated, it is converted from a blue positive, into a brown negative
one : by keeping, however, it recovers its positive character and
its original colour. The term amphUype has, on that accoont,
been applied to this process.
1319. The diromoiifpe affords a sensitive paper of sufficient
delicacy fur copying superposed objects, and may be made by
simply washing thin paper with a solution of bichromate of potass,
and drying it quickly before a fire. Let an object be placed in
the pressure-frame (1290) on this paper (which possesses a fine
yellow colour), and exposed to the direct rays of the sun until
the yellow colour of tne paper is changed to a rich brown, on re-
moving the object, its outline will be found beautifully defined on
the paper, the drawing being yellow on 'a brown ground. To
secure this from further change, all that is necessarv is to soak
the paper in water, to remove the unchanged bichromate, by
whicn process the picture is left nearly white on a brownish
yellow ground, and may be preserved without further chan^
The rationale of this process is found in the reduction of a portion
of the chromic acid to the state of oxide.
1320. From the elaborate researches of Sir J. Herschel, it has
been proved that scarcely any coloured fluid from the vegetable
kii\gdom, or any compound with which chemistry has made va
acquainted, exists, which is not more or less sensitive to the
chemical influence of light. He succeeded in obtaining well-
defined photographs, by merely using paper impregnated with
the coloured juices of flowers and other parts of vegetables ; and
to these the generic term of arUhotype has been applied. For an
account of these interesting researohes, the reader must be referred
to the original papers in the Philosophical Transactions.
1321. It is worthy of remark, that all the photogranhio pro-
cesses are considerably expedited by the apphcationof a gentle
heat. Sir J. Herschel found that in many instances of photo-
graphic action produced by exposing a strip of prepared paper
to tne solar spectrum, the cnanges which would have reqnired a
considerable time to be effected, were rapidly produced by hold-
POSIflVE PRIKTIKO. 745
ing behind the paper, whilst thas exposed, an iron heated below
redness. In the account of the calotjpe process (1308) of Mr.
Fox Talbot, it has been mentioned that the paper must be
heated to bring out the picture properly. In the cyanotype
pictures (1318) we have seen that an actual alteration of colour,
and the conversion of a positive into a negative picture, takes
place by the mere application of heat.
The agent in producing this curious change is considered by
Sir J. Herschel to consist of rays existing in, and below, the red
and orange region of the solar spectrum, and emitted by bodies
heated just below redness. These rays, for which the term of
parathermie has been proposed, appear to bear the same relation
to the true calorific rays, as those active in producing chemical
and photographic phenomena do to the luminous rays.
1322. Positive Frinting. — The paper selected for printing
positive photographs shoufd have as fine a texture, and as smooth
a surface as poesibie ; and the right side of the sheet, which can
generally be detected by reflected light, should be chosen. The
albuminized paper already described (1314), is very effective.
Another solution frequently employea contains 8 gp'ains of
chloride of ammonium, 1 of gelatine, and 3 of Iceland moss in
each fluid ounce. Either of these pre^red papers roa^ be
rendered sensitive by floating on the solution of nitrate of silver,
then draining, and drying in the dark. The impressions may be
fixed in the usual manner bv the hyposulphite solution, but a
picture on the latter paper will be much strengthened if it be fixed
in a toning bath, containing half a grain of chloride of gold, 4
grains of nitrate of silver, and half an ounce of hyposulphite of
soda, in each fluid ounce. In preparing this bath, the three salts
most be separately dissolved, and first, the solution of gold, and
subsequently that of silver, stirred gradually into the solution of
the hyposulphite, in order to effect a complete solution.
The ammonio-nitrate of silver is preferred by some photo-
graphers as a sensitizing solution : this is prepared by precipi-
tating the oxide of silver, by adding gradually caustic ammonia
to the nitrate solution, and then dissolving the precipitate in an
excess of ammonia. This process is more difficult than the former,
but when successful, the results are very satisfactory; it is not
applicable to the albuminized paper.
In ordec to avoid subsequent fading, it is desirable that every
trace of hyposulphite be removed by washing in pure water.
1323. Collodion Frocessea. — ^The most important improvement
in the photographic art has been the employment of collodion, for
which science is almost entirely indebtea to the researches of the
late Mr. Archer; no process has yet been contrived which
famishes such satisfactory results, even in the hands of an inex-
perienced operator. The basis of collodion \% pyroxyline^ a sub-
stance formed by the action of a mixture of nitric and sulphuric
746 PHOTOGSAPHT.
acids on cotton- wool, or some other form of vegetable fibre : the
change produced in the fibre probably consisting of the subetitn-
tion of one equivalent of the peroxide of nitrogen (N O^) for an
equivalent of hydrogen. Pjroxvline maj be convenientlj pre-
pared in a small quantity by immersing one drachm of cotton-
wool for 10 minutes in a mixture of 6 fluid ounces of sulphurio
acid, and one of water, to which 3 ounces (troy) of dry
powdered nitre have been gradually added. When removed from
the acid mixture, the pyroxyline should be rapidly washed in a
large quantity of cold water. The temperature of the mixture
should be 140'' F. Collodion is pre{)ared by dissolving 80 grains
of pyroxyline in a mixture of 5 fluid ounces of sulphuric ether,
ana 3 of highly rectified alcohol, so as to furnish a solntion suffi-
ciently viscid to leave a transparent coherent pellicle when a lew
drops are allowed to evaporate on a plate of glass : if too little
alcohol be emplo^red, the film dries too rapidly, if too much be
used, or not sufficiently rectified, the film loses its cohesion, and
the surface is broken up in washing. The collodion is then
iodized in the following manner: prepare a saturated solution of
iodide of potassium in rectified spirits of wine, and add to it aa
much frasnly precipitated iodide of silver as it will dissolve :
after repose, decant the clear fluid, and add about five drops of it
to an ounce of collodion. After agitation aud careful subaidenoe
for a few days, this iodized collodion will be ready for use. This
preparation is not veir durable, as the iodide of potassium be-
comes decomposed, and free iodine is liberated, wnich may be
recognised by the gradual darkening of the fluid. Collodion
iodized by the addition of four or five grains of the iodide of
cadmium to each fluid ounce is a more stHblei and not less efieotive
compound.
1324. A piece of colourless thin plate-glass, fitting into the
sliding frame of the camera, should be carefnlly washed, and
polished with a silk handkerchief; holding this plate bv one end,
pour over it enough of the iodized collodion to flow freely over it :
when thus covered, allow the excess to drain ofl^ by inclining one
comer of the plate over the bottle of collodion : some care is re-
quired, and a certain oscillating movement of the plate in the
hand, to produce a film of perfect uniformity. After a minute's
repose, let the plate be earned into a dark room, and by the aid
of a feeble yellow light carefully plunged into the nitrate bath.
This consists of a solution of 30 grains of nitrate of silver to the
fluid oimce of distilled water, which must be saturated with iodide
of silver, to prevent its taking up any iodine from the ooUodion
film. This is effected by dissolving the whole of the nitrate of
silver to be employed in a small quantity of water, and adding
a few grains of iodide of silver, which will be dissolved after a
little agitation. More water is then added, to make up the re-
quired quantity, when a portion of the dissolved iodide of silver
DBYELOPMBRT OF HEGATXYE8. 747
will be a^ain precipitated, but the flaid will remain saturated,
fifteen minims of alcohol should be added to each fluid ouncei to
prepare it for use. A narrow flat vessel, placed obliquely, in
which the plate may be completely immerBed, is the most conve-
nient form of bath. The plate should remain in the bath about
one minute, and having been carefully lifted out and drained by
resting the lower ed^ on blotting-paper, should be placed in the
sliding frame; it is then ready for tne camera. Having carefully
ac^'usted the camera to the object in the usual manner, the plate
of ground-glass is removed, and replaced by the frame containing
the prepared plate ; the screen is tnen drawn up, and the picture
allowea to fail on the plate. It is not easy to state with any
accuracy the time required to produce an impression on the sen-
sitive sur&ce, as it varies from a few seconds to a minute or two,
according to the intensity of light, and other circumstances.
1325. After a sufficient exposure, the frame is taken into the
dark room, and the plate removed from it Nothing will be visi-
ble on its surface, until after the application of the developinjg
agent. For this purpose a solution of three grains of pyrogallio
acid, and one fluid drachm of strong acetic acid, in one fluid ounce
of water, is employed. The plate being placed on a leveUing
stand, or on a porcelain dish, with the &ce upwards, enough of
this solution is poured on its surface to cover it completelv. The
picture will rapidly appear, and when its details are distinctly
visible, further action must be stopped by gently pouring some
pure water over it : then a small quantity of a strong solution of
hyposulphite of soda should be poured over its surface, and
allowed to remain for a minute or two. The plate should then be
repeatedly but gently drawn through a basin of water, or placed
under a eentle stream of running water, then drained, and slowly
dried. By this plan, which is exceedingly simple in practice,
pictures are produced possessing a sharpness of outline, and an
artistic beauty, which can scarcely be equalled bv any other known
process. Photo^phers are likewise indebted to Mr. Archer for
the first application of that most powerful reducing agent, pyro-
gallic acid.
1326. The pictures obtained by this process are negative, and
will yield an unlimited number of positive pictures by placing the
plate with the picture downwards on a piece of positive paper
(1322) in the pressure frame (1291). A few minutes' exposure
to the light of tne sun, or if that be too powerful, to bright diflused
daylight, will print off 9k beautiful picture on paper, which may be
fixed in the usual manner. Before the collodion negative is
made use of for printing positives, it is desirable that the film be
protected from abrasion oy a varnish. For this purpose, a solu-
tion of one ounce of white stick lac, and one drachm of picked
gum-sandrac, in 12 fluid ounces of alcohol (sp.gr. 0*815) is recom-
mended by Mr. Hardwich. A good varnish tor this purpose is
748 PHOTOORAPHT.
made by digesting piecee of amber (or animd ?) in chloroform :
perfectly clear white mastic spirit varnish will, however, answer
the purpose exceedingly well.
1327. CoUodion Positive*. — Any collodion negative may be
converted into a poeitive, by coating the film with an opaque
black varnish, and viewing the picture through the glass, by n-
flected light. This process, which in fact constitutes the staple
of cheap photography, requires some variations from the pre-
viously described negative processes. Opacity and depth of tint,
BO essential for negatives, are not here so much needed as bril-
liancy under reflected lignt, from the deposition of metallic silver
in a state of minute subdivision. For iodizing, Mr. Hardwich re-
commends a solution of 6 grains of iodide of ammonium, 4 of
bromide of ammonium, and 8 of iodide of cadmium in one fluid
ounce of alcohol (sp. gr. 0*815), to be added to 3 fluid onnces of
the plain collodion ; and the nitrate bath to be very dighUy acidu-
lated with nitric acid. The required tone of development is more
readily effected by a proto-salt of iron, than by pyrogallic acid ;
good results may be obtained by a solution of 15 grains of proto-
sulphate of iron in one fluid ounce of water, to which is added half
a fluid drachm of alcohol, and of strong acetic acid, to which some
photographers add 10 grains of nitre.
The best fixing solution for positives is one containing from 5
to 10 grains of cyanide of potassium in each fluid ounce, to which
some add one grain of nitrate of silver. This solution must be
cautiously applied, especially the stronger, as otherwise it wiQ
efface the half-tints of the picture.
1328. When washed and dried, the positive picture may be first
protected with one of the transparent varnishes already mentioned
(1326), and then coated with a solution of 3 grains of caoutchouc,
and 25 grains of asphaltum, in one fluid ounce of mineral naphtha.
It has been recommended by Mr. Fav to place a piece of thin
gutta-percha in the bottle of prepared collodion ; although scarcely
any visibly dissolves, yet the viscidity of the fluid and the tenacity
of the resulting film are said to be increased.
1329. Dry CoUodion Processea. — ^Various processes have been
devised for the preservation of the prepared film of collodion in
.a moist state, in which it is very much more sensitive, than
when dry. A solution of a highly deliquescent salt, the nitrate
of magnesia, was employed by Messrs. Spiller and Crookes;
but it was found to impair the sensibility of the film. Honey
was emploved by Mr. Shadbolt, and a mixture of honey and acetic
acid by Mr. Llewelyn, with the same view ; but in both cases
the film was found to become gradually less and less sensitive ;
the attention of photographers was consequently directed to
some process by which the collodion surface might remain sen-
sitive when perfectly dry. One of the most successful of manj
plans that have been proposed is that of Mr. Fotheigill. This
RBPBODUOTXOH OF HATUBAL C0L0UB8. .749
consists in slightlj washing the plate when removed from the
silver-bath, so as not entirely to remove the solution of silver from
its ^ surface, then pouring over it a dilute solution of albumen,
which, being gently rinsed off, the plate is left to drv. Another
effective dry process is that of Major Bussell. In this the glass
plate is first covered with a solution of 20 grains of gelatine in 8
ounces of distilled water and 5 drops of strong acetic acid. The
gelatine should be allowed to swell thoroughly in the water, be-
fore it is heated for solution. When cold, add to this three grains
each of iodide and bromide of cadmium and a small piece of iodine,
all first dissolved in a small quantity of water. The plate is then
dried slowly, and coated as usual with collodion : it is then im-
mersed for five minutes in a thirty-grain silver-bath, and the free
nitrate of silver being carefully removed by washine, the plate is
covered with a filtered solution of 6 or 7 g^us of tannin to an
ounce of water : this solution is then washed ofij and the plate
is drained and dried in the dark. The developing solution is
14 grains of pvrogallic acid to the ounce of water, to which at
the moment of use is added a few drops of a solution of 20 grains
of nitrate of silver, and 40 grains of citrio acid in an ounce of
water. In all dry collodion processes the addition of a small
quantity of a solution of nitrate of silver to the developing solu-
tion is indispensable.
1330. Betore the discovery of the value of collodion as a photo-
graphic agent, starch, casein, and albumen were employed; of
these the latter has, in the processes of Mr. Mayall and Mr. Martin,
produced very pleasing results. To use cUbuminized gkuSj add
ten drops of a saturated solution of iodide of potassium to the
white of a fresh eg^. Beat this into a froth and allow it to repose
for a few hours, ana strain through muslin. Then having polished
the surface of a glass plate, pour over it a quantity of the pre-
pared albumen, aUow the excess to run off and the plate to dry on
a levelling stand. Then, in a dark room, immerse its prepared
face into a bath of 50 grains of nitrate of silver to 1 ounce of
distilled water with 1 fluid drachm of acetic acid for about ten
seconds, and dip it into a vessel of distilled water : the plate is
then allowed to dry, and will remain fit for the camera for some
dajrs. About five minutes' exposure is required for a eood picture,
which is developed by means of a saturated solution of gaUic acid,
and fixed in the usual manner.
1331. Beprodudion of Natural Colours^ — It has been already
stated (1285) that when a solar spectrum falls on paper coated
with chloride of silver, various tints, more or less resembling
those of the rajB producing them, are produced on the paper:
and it has ever been a problem to photographers to reproduce
more perfectly the natural colours. This object has to a certain
extent been attained by M. Poitevin, by a comj^licated process.
A sheet of paper is first covered with a layer of violet subchlorido
^■^■cz^
750 PHOTOORAPHT.
of Bilver, obtained by reduction in the light of the white chloride,
in the presence of a reducing salt. Equal Tolnmea of saturated
solutions of bichromate of potash, and of sulphate of copper, and
of a sohition containing fiye per cent, of chloride of potauium, are
then applied to the paper. The image of the solar spectrum im-
pressed on this paper is developed by applying, firat, a dilute
solution of chromic acid, secondly, a solution of bichloride of mer-
curj, and thirdly, a solution of nitrate of lead. This is not, how>
ever, a process of any practical utility, since the coloured picture
thus obtained will not bear exposure to light, and the paper is
not sufficiently sensitire for use m the camera.
1332. A very interesting extension of photography to micro-
Bcopic drawings has been made ; Mr. Delves, Mr. ShadboH, Mr.
8. Highley, and others, have successfully pursued this subject :
they have all adopted some collodion process. For this purpose
the body of the microscope is placed horizontally, its eye-piece
removed, and a tube of pasteboard lined with blacc velvet, to cut
off all lateral reflections, is placed in the body : the upper end is
then inserted into the brass tube of a camera from which the
lens has been removed, a collar of black velvet being placed over
the juncture to exclude light. The object is then placed as usual
on the sta^, and a strong light being thrown on it by the con-
denser, a snarp well-defined image of the object, when carefiiDy
adjusted to focus, will be formecT on the ground-glass plate of the
camera ; the prepared collodion plate is then introduced into the
camera, and a picture is obtained in the manner already described.
Bright daylight, or direct sunlight, always affords the best pic-
tures, although by artificial light, especially by that of a camphine
or belmontine lamp, or still better, an electric lamp, very satisfac-
tory fesults have been obtained. With a half-inch object-glass
good pictures have been obtained in a few seconds.
There is some difficulty in obtaining an accurate focus in con-
sequence of the intentional over-eorr^xtion of the achromatic ob-
ject-glass, by which the violet rays are projected beyond the lower
rays of the spectrum. Hence the collodion plate should be placed
a Mule further from the lens than is required for the sharp defini-
tion of the picture. This difficulty has been overcome in the
large achromatic combinations for the camera, Mr. Ross and others
having constructed lenses, in which the actinic and visual foci are
made to coincide accurately with each other.
1333. Micrthphotographs. — This interesting class of micro-
scopic objects na9 lieen for some years in vogue : these photo-
graphs are of such small dimensions as to be almost inappreciable
to tne naked eye, but when seen under a low power in a compound
microscope, they come out with marvellous distinctness. Many
admirable portraits are thus produced in oval spaces not exceed-
ing -rV*^ of an inch in width. The mode of obtaining these is
precisely the converse of the preceding, the sensitive soriace
CEIA6TIAL PHOTOOBAPHT. 751
now occnpjing the place of the object, and vice verad. Unusual
care is neceesary in the preparation of the collodion for this kind
of work, as any stroctural imperfections in the film become so
magnified as to mar the effect of the photograph. The late Mr.
T. Jackson, Mr. Shadbolt, Mr. Dancer, and others have been very
Bucoessful in producing ihese curious objects.
1334. Of all the stereoscopic effects, none are so grateful to the
eye as those resulting from two well-a(^usted photographs ; from
tae unerring truthfulness of each delineation, the most perfect
impression of relief is produced. For inanimate objects the two
pictures may be successively taken by the same camera, its position
being suitably altered ; the great depth of relief thus produced is
truly surprismg. For portraits, it is desirable to take the two
pictures simultaneously, by a double camera.
1 335. Oelestud Photography. — The first attempts were made by
the late Prof. Bond in America, who placed some daguerreotypes
of the moon in the Great Exhibition of 1851. Since that period the
subject has been successfully pursued by Mr. Warren de la Rue,*
who has succeeded in obtaining most beautiful and instractiye
lunar photographs on collodion, by a reflecting telescope f 1092).
The variations in the angular position of the moon's visiole disc,
termed by astronomers her libration, afford opportunities of ob-
taining lunar photographs at various angular distances from her
mean position. By an appropriate combination of these, very
striking stereoscopic effects oave been produced : the variations of
surface on our satellite have thus been brought out in as strong re-
lief, as they would be, if an exact model of the moon were placed
at a moderate distance from the eye. The moon's greatest libra-
tion measured diagonally amounts to nearly 21°, and as the maxi-
mum stereoscopic angle does not exceed 15^°, it is evident that
any amount of *' relief" may be thus obtained: the view, in fact,
as Sir John Herschel has remarked, is such as would be seen by
a giant with eyes thousands of miles apart I so greatly may our
visual perceptions be aided by the " giant eyes of science."
It appears that the visual and chemical rays are not proper-
tionabfy reflected from the lunar suriace, for the brighter portions
of the moon's apparent disc do not always produce the brighter
photographic images. The lines converging to a prominent object
on the moon's surface are shown to be furrows, one of which ex-
tends through a space of 45'', and another over 100" of latitude.
The physical structure of the lunar surface is wonderfully re-
vealed by examining these photographs by the aid of a compound
microscope of low magnifying power.
Various attempts have been made to obtain solar photographs,
by employing surfaces of small sensibility, and by reducing the
intensity of the solar rays by transmission through coloured glass,
but without any satisfactory results. This object has been suo-
• Report Brit. Aasoo. 1859.
752 PROTOOBAPUT.
cessfuUy attained by allowing a very narrow slit in an opaque
screen to pass rapidfy across the primary focns in the telescope ;
each portion of tne sensitive surface is thus successive] j acted on
during only a verj small fraction of a second, and correct delinea-
tions of the sun*s disc have thus been obtained. A highly interesting
series of solar photographs were thus taken by Mr. De la Roe in
Spain, during the total eclipse of the sun in May, 1858. The
distribution of chemical and visual rays at the moment of total
obscuration appears from these to have been by no means identicaL
1336. Carbon-printing. — This and the succeeding processes
depend on the property of chromic acid, discovered by Mungo
Ponton, of coagulating under the influence of light, and thai
rendering insoluble, a thin layer of gum or gelatine. In this pro-
cess of M. Poitevin, finely-divided carbon is suspended in a solu-
tion of bichromate of potash, with g^m or gelatine, and the
mixture is evenly distributed on paper and dried. This prepara>
tion is not suitable for the camera, but a positive copy may be
obtained by the superposition of a negative, in the ordinaiy
manner. After sufficient exposure, the paper is washed by meaos
of a sponge, or still better, by a copious stream of water, when
the still soluble organic matter, protected from light by the
darkened portions of the negative, is washed away, and thoa the
lights of the picture are brought out.
The oxide of chromium, or chromic acid, which is reduced by
organic matter under the influence of light, is a substance deno-
minated "mordant," having an affinity for colouring matters.
Hence various pigments, as well as carbon, so employed, will
remain fixed upon the image, thus producing coloured pictures.
Suitable pigments may thus be deposited on a surface of glaaa
or porcelain, and subsequently burnt in ; by which means many
beautiful and permanent effects have been produced in enameL
1337. Photo-lithography^ and -zincograpny. — ^For the purposes
of transferring an impressed image to a surface of stone or zinc,
for subsequent printing, the defects of M. Poitevin^s process are,
that a negative picture is obtained in the camera ; and a poidtive
picture ootained by the superposition of an ordinary negative
photograph is identical in position with the object, and will there-
tore print revened copies, just as printers* type is set up baek-
tDaraSf in order to be retLdfonoarda,
Mr. Osborne communicated, in 1859, a paper to the Melbonme
Institute (a very flourishing colonial scientific body) on a process
analogous to tnat of M. Poitevin, but differing from it in im-
portant points. In this process the solution pf the bichromate and
gelatine is applied to paper, and the light-impressed film is sahse-
quently transterred to the stone or sine plate. The whole surface
18 then inked by a roller, and the unaltered gelatine washed
away, 'as in carbon-printing. The solution employed is one of
100 grains of gelatine, and 56 of bichromate of potash, in an ounce
PH0TO4CULFTUKE. 753
of wann water; to this, when cooled to ahont 110* F. is added
i of an ounce of fresh albumen. The paper coated with this is
dried in the dark, and glazed by pressure. It receives the
impression through a negatiye in about a minute ; and (accord-
ing to a subsequent process of Mr. Osborne) it is then inked all
over with lithographic ink, and floated on boiling water for about
an hour, when portions of the film not acted on, together with the
ink covering them maj be removed by a sponge. The paper is
then dried, and the impression transferred to the stone or zinc
plate in the usual manner. Woodcuts and engravings may thus
oe copied with wonderful precision and sharpness.
A process independently discovered by M. Asser, of Amsterdam,
and adopted by tne Ordnance Survey Department at Southamp-
ton, is very similar to that of Mr. Osborne. It has been there
observed by Col. Sir H. James, that the development of the half-
tinte is greatly improved by keeping the prepared paper for a con-
aiderable period of time before it is used in the camera.
1338. Motoglyphy. — ^This brief outline of the chemical and
physical actions of hght (or, more correctly speaking, of the lumi-
niiorous undulations) would be incomplete without some notice of
the above ingenious processes. Photoglyphy is a method, due to
Mr. Fox Talbot, of impressing a photograph on a steel plate by
corrosion of ite surface. If a steel plate coated with a solution (n
bichromate of potash and gelatine be submitted to the action of
light in the camera, those portions of theeelatinous film that have
been exposed to light are rendered partiafly insoluble. The plate
is then covered with a solution of perchloride of iron, which, pene-
trating the soluble portions of the film, corrodes the plate, and
thus etelu» the picture.
1339. PhoUHfcUvanography, — This is an analogous process due
to Herr Pl^tsch of Vienna, in which a plate of glass is covered
with the same solution, and submitted to the action of light
The plate is then moistened, and those portions of the surface
whicn have not been impressed by light, oecome raised by the
imbibition of moisture. A mould of the surface is token in gutta-
percha ; an electrotype taken from this mould will be evidently
capable of printing, by the ordinary means, a stereotyped copy of
the original photographic impression.
1340. Fhoto-sculpiure. — ^Photography has been in a remark-
able manner made available to the sculptor, in afifordin^ a series
of exact outlines, from which, if sufficiently numerous, the whole
contour may be very readily filled up. For this purpose a dozen
or more cameras of equal dimensions are placed equidistantly
round a circular room lighted by a skylight, the optical axis of
each being directed to the centre. The intended subject of s por-
trait is placed in the centre of the room, and photographs in as
many different azimuths are simultaneously taken by all the
cameras. The outlines thus produced are cooied at corresponding
3 o
754 PROTOORAPHT.
azimuths iind in any required dimensions, on the block of mar-
ble, by means of an arrangement analogous to the pentagrapii,
the action of which depends on the constrained similarity of two
triangles haying moveable sides. The intervening portions oi the
bust may be so readily filled in by the hand of the sculptor, that
he has succeeded in producing an admirable likenese of a perwn
whom he has never seen.
1341. As the successful practice of photography depends en-
tirely on the accurate adjustment of nicely balanced, but unntaUe,
chemical affinities, an accurate knowledge of the nature and
manipulation of the chemical elements is essential to the soooesa-
ful pursuit of photography as a branch of science ; without that
knowledge, it is but the practice of an empirical art. From the
extremely delicate nature of some of the processes, it is essential
that many minute, and apparently trivial directions, shoold be
carefully observed, since small errors of time, temperature, quan-
tity, or succession of steps, will frequently be founa to mar the re-
sult. For the same reasons, the most scrupulous deofdintn mast
be observed in all the materials and implements employed ; not
only as regards actual dirtj but also any undue admixture of the
minutest portions of the chemical elements themselves : thus, fcv
example, most porcelain dishes that have been used for the fixing
bath of hyposulphite of soda, are permanently disqualified for use
us a nitrate of silver bath, in consequence of a minute quantity of
the former salt having penetrated the substance of the earthen-
ware, which cannot bo removed by washing.
Beferenges.
For further information on the early history and progress of the
interesting subjects of this chapter the student is referred to the
papers of Sir J. Herschel in the Philosophical Transactions, and
to a Treatise on Photogrnphy by Mr. Hunt. Most ample details
of the mutual relations and manipolaiMn of the photographic
elements will be found in Hard^dch's Manual of Photographic
Chemistry (Churchill, 1864). On Micro-photography, the papers
of Mr. Delves, and Mr. Shadbolt in the Microscopical Journal may
be consulted.
756
CHAPTER XXm.
THBBMIG8,
1342. Thb same difference of opinion has existed amons^ philo-
sophers with regard to the nature of heat, as that which nas ex
isted respecting light : some have contended that the evolution of
heat, as that from the son and other sources, depends upon the
emission of indefinitely minute particles of matter, to which the
general term caloric has heen applied. Others have subsequently
applied the undulatory hypothesis to the explanation of the phe-
nomena of heat, as to that of light (1019), and have entertained
the opinion that both are alike the results of wave motion. The
dynamic theoiy of heat is readily adapted to explain the phe-
nomena of roiHation (Chap. XXIV.), but less obviously to those
oimscific (1218), and latent (1223), heat.
The dynamic theory of heat is, however, a doctrine of very long
standing : it was thus pithily enunciated by Locke : " Heat U a
very hmk agitation of the tnaensible parte of the ohjectj tohu^
produces in us that eenstxtion Jrom whence we denominate the
object hot; so what in our sensation is heat, in the object is
nothing but motion.''
1343. The chief proximate cause of heat is the sun, whose rayp
convey to us this important agent in common with light. There
are, however, other exciting causes having a dynamic, electric,
or chemical origin, to which it is necessaiy to allude.
A. JMction. — ^Heat is produced whenever two bodies are rubbed
leather. Thus, when two pieces of ice are rubbed tog[ether, sufficient
heat is generated to melt their surfaces. Among uncivilized nations,
fire is commonly produced bj the friction of pieces of wood against
each other : and the old-fashioned flint-and-steel, now superseaed by
lucifer matches, and percussion caps, is an examnle of the ignition
of minute particles of steel bv the heat developea in scraping them
off. Count Bumford found that in the operation of boring a brass
cannon, 7^ inches in diameter, and the borer making thirhr-two
revolutions in a minute with a pressure of 10,000 pounds, sufficient
heat was generated to boil eighteen pounds of water, in which it
was immersed, in 24 hours. The friction of two iron plates
against each other has been applied in N. America as a practical
source of heat.
The development of heat by friction has been neatly illustrated
by Fhif. Tynaall, by attaching a vertical brass tube containing
do2
756 THBIBIIOB.
water to the mandrel of a wborling teble (321) and pressing its
sides during rotation by two grooved pieces of wood hineed
togetber like a lemon-squeezer. Tbe water may be thna raised to
tbe boiling-point in a few minutes.
B. Percussion. — ^Tbis is anotber dynamic source of beat, and
appears to depend upon molecular displacement, or upon tbe in-
ternal friction arising from tbe condensation of Ae body atmck,
for, as a general rule, wbenever bodies are diminished in bulk,
beat is evolved. This is well illustrated in tbe coining press.
BertboUet submitted a piece of copper to tbe strokes of a press,
and found tbat tbe greatest evolution of beat occurred at the first
blow, and diminished with each succeeding one : tbe quantities
of beat evolved at tbe three first strokes having been 9*6', 4-2% and
1-05 C. respectively. Tbe country blacksmith used to light bis forge
by an iron rod heated by repeated blows on tbe anvil. A leaden
bullet will be fused and scattered in splashes, and a steel or cbiUed
iron shot rendered exceedingly hot by its impact on an iron target.
0. Chemical Action,— A constant source of heat, in all cases in
which a combination of heterogeneous particles takes place, as in
combustion, or in tbe union of hydrogen and some other bodies
with chlorine.
D. Electrical -4cf ion.— Examples of this mode of evolving beat
have been already given (816) ; it appears to be connected with
the resistance afforded by conductors to tbe transference of elec-
tric motion through them.
Tbe beat developed by resisted magnetic induction has been
well exemplified by Prof. Tyndall. If a btdlow cylinder of copper
filled with fusible metal (1409), be placed across tbe lines of mag-
netic force (592), between the poles of a powerful electro-magnet,
and be there made to rotate rapidly on its axis, it will in a few
minutes become sufficiently heated to melt the alloy contained in
its interior.* . ,., , ,
E. Vital Action.— 'hX\ beings possessing life have the property
of evolving heat, and generally maintaining a temperature above
that of the medium in which they live. In tbe case of animaU,
at least, it is beyond doubt tbat tbe evolution of beat depends
upon a slow combustion going on in tbe organism : carbon and
• This fket is mftiiifMtly eoBfirmatoiy of the dvnamio theory of dcctridtT ;
for if eleotrieitr and maimetism be nothing bat ipina WKve-motaoo, like
that of circularly polarised light (1867), thia reaolt woiUd be a dynunioal^
neceeearr oonBeqnence ; for the motion of each diktorbed particle will be cms
of revolution round the line of direction of magnetic force, and the t«eia>
tanoe of the maM to rotation will be the aum of the reaiatanoea of each
molecule to a change of the direction of ita axis of rotation, or aa it is
aometimes expressed, the sum of the inertia of rotation of each molecaK
as exempUaed by the grrasoope (865). The intenial fHetion arisiog from
the oonaiant change ordirection of the plane of the orbit of each particie
will fully account for the heat dcTeloped, which moat eridently be in pro>
Dortion jointly to the molecular energy of rotation, •••., to the intenai^ of
Se magnetic Ibros^ and to the amount of orbital diapkoement^ Cs^io dw
DTHAMIO EQITITALBHT OF HEAT. 757
hydrogen being slowly conyerted into carbonic acid and water,
not only in the Inngs, but in eyeiy portion of the capillary system
(1007) : a theory long since advanced, and to which notice has
been subsequently drawn by the ingenious arguments of the late
Prof. Liebig.
1344. The correlation of heat with dynamic force is a principle
that has arisen in many other intelligent minds than that of
Locke; heat was denominated by Crawford,^ "a force or power
belonging to bodies," but to Count Romford' the first direct ex-
perimental proof is due. Having noticed the great. amount of
heat deyeloped by the boring of cannon, he remarked, " It appears
tome extremely cufficnlt, if not quite impossible, to form any distinct
idea of anything capable of bein^ thus excited and commanicated,
except it be motion." And it is remarkable that the estimated
yalue of the dynamic equivalent of heat, which he roughly
deduced from this process, does not differ unreasonably from this
result of more precise subsequent investigations.
The dynamic origin of heat was confirmed by an experiment of
BUt H. Davy,' in which liquefaction resultedi from rubbing two
pieces of ice together in the yacuum of an air-pump.
The constant relation existing between heat and force is further
confirmed by the experiments of Dulong,^ which show that equal
yolumes of all gaseous fluids, at the same temperature and pres-
sure, on being suddenly compressed or dilated to any equal
volumes, disengage or absorb the same amount of heat.
Mr. Joule confirmed in 1840 the correlation existing between
heat, and other modes of motion resulting from impressed force :—
1st, That the heat evolyed by any yoltaic pair is proportional
eeeteri$ paribtUf to its electromotive force.' 2nd, That the heat
eyolved by the combustion of a body is proportional to the inten-
sity of its affinity for oxygen.* He also showed in 1844 that
the beat absorbed by the rarefaction, or evolved by the con-
densation of air is proportional to the force evolved or absorbed
by those operations respectively,^ and these observations are
confirmed by those of M. ceguin on the dilatation of steam."
uiumnt of work raaployed in rotating the maw. But from this another
qneation arise* : does e&otrio wave^motion take plaoe in palpable matter, or
in impalpable and imponderable ether f clearly not in the latter, beoaoae a
moleooie destitute of attraction towards anj other molecule in the nnlTerae
can baTC neither inertia nor energy, and therefore cannot do " work :" and
if then palpable matter be susceptible of the ascertained velocity of electric
notion, why should it not be equally sut ceptible of the less rapid motion of
light and heat? and if so, to what good purpose does the interstitial— im-
ponderable—impalpable— ether hypotheeis tend P
1 On Animal Heat, p. 15. * Phil Trans. toL zriii. p. 28e.
' Elements of Chemical Philosof'hy, p. 94.
* H^m. de I'Aead. des Sciences, torn. z. p. 188.
9 Fhil. Mag. vol. zix. p. 276. • Ibid. toI. zz. p. 11.
7 Ibid. vol. zzvi. pp. 876, 879.
• Comptes Bendns, torn. zzt. p. 421.
768 THBHMICa.
G. Bebenstein* proposed some Utopian schemes for the practi-
cal production of heat by the employment of dynamical force ;
but his mind does not appear to have reab'zed the true relations
of beat and force.
1345. The Dynamic Equivalent of Heat. — An extensiTe series
of experiments was made oy Mr. J. P. Joole, for the pmpoae of
determining the number of units of work (842) or foot-pountU t^
quired to be done, in order to produce an elevation of tenDperatore
amounting to 1° F. in one pound of water at about 50° F. Thefrree
expefnded was measured by the descent of weights employed
in rotating the apparatus, and the heat etfohed by the iHction of
water, mercury, and cast iron was carefully estimated. The mean
results of these experiments show the dynamic equivalent of heat
to be veiy nearly 722 units of work.f As 1* C. {i.e, in the centi-
grade scale) is now much more frequently employed as the scale-
unit of thermal measurement than l^F. (in Fahrenheit's scale),
it will be convenient to state that the corresponding centigraae
unit is 1890 foot-pounds veiy nearly.
It should here oe remarked that the element of time has been
erroneously introduced into the dynamic equivalent of heat in some
elementary treatises ; this is likely to lead to much misconeeptioo,
since, provided a ^ven amount of work be done, it is abeomtelj
immaterial (mmaking dynamically) what amount of time may be
occupied in doing it
1^6. Several other reciprocal relations between heat and
*' work*' have been observed. Thus air gives out heat on com-
pression, and its temperature is lowered on dilatation. A stretched
piece of vulcanized caoutchouc contracts on the application of heat
(a property common to this substance, and to water between the
tem^ratures of 0° C. and 4°) ; and if suddenl;^ compressed it
manifests loss of heat. Similarly water diminishes m volmne
from 0° to 4*", and it has been observed by Mr. Joule that if water
between these temperatures be suljected to a pressure of 24 atmo-
spheres, it manifests a depression of 0*008** C. If the hoe of a
thermopile be placed in contact with the surface of a flat rod. aad
the rod be suddenly stretched by a weight of half a ton, and there-
fore increased in volume, it shows a loss of beat
1347. It has been demonstrated by Mr. Joule, that If no wmk
be done by the expansion of a gas, there is no loss of heat For
this purpose two strong copper vessels of the same aiie were
made to communicate by a tuoe furnished with a stop-cock. One
was exhausted and the other filled with 20 volumes of air, and
the whole immersed in a water-bath. When the whole had
arrived at a unifonn temperature, the stopcock was opened, and
the air allowed to rush from one vessel into the other. The con-
densed vessel and its contents became cooled ; but by a Ices of
* WMrme-ErregnnfT ohse BrsimiDtttcriaL BTHnberv. 18SS.
t Phil. Traat. 1860, p. 83.
DILATATION OF B0DIB8 BT HXAT. 759
exactly the amount of heat produced by friction and impact in the
tube and exhausted vessel : for on carefully stirring the water, no
change of temperature was obseryed to take place.
1348. The Point of Absolute Negation of Heat.—li has been
obseryed that for each increment of 1** C.| there is a corresponding
increase of elastic force in a gaseous body equal to yfj of the force
at 0* C. If the same law hold good below that point, it is evident
that at —273** there would be no elastic force, and consequently
no heat ; it is, however, probable that the law does not progress
downwards uniformly, and we have no means of determining the
temperature at which all substances would assume a solid form*
1349. When a body has acquired the power of communicating
the sensation of heat it is said to be hot^ and when, on the con-
trary, it takes heat from the hand when brought near it, the body
is said to be oold. But within considerable limits our perceptions
of heat and cold ara rather relative than absolute : uius, if two
basins be filled, one with hot and the other with cold water, and
a third, placed between them, with equal parts of both ; and if
the hands be held for a short time in the outer basins, and then
plunged into the middle one, the same water will feel warm to
one hand, and cool tO the other, by contrast.
It may here be further remarked that the sensihle effects of
heat and cold are very similar ; and both extremes- are equally
destructive of organization — the sensation of touching a globule
of frozen mercury is the same as that of touching a hot iron ; and
the frost-bitten extremities of the inhabitants of the cold regions
evince the destructive power of intense cold.
Kot the slightest difference of weight takes place in bodies by
the abstraction or addition of heat; a mass of matter so cold as
to freeze a little water when placed upon it, weighing the same as
when at the temperature of ooiling water: the volume of the body
alone undergoes a change.
1350. As the heat of Dodies is increased, they, with few excep-
tions, increase in bulk : when this force is impressed on bodies
their molecules become forcibly separated, causing, first, an in*
crease in bulk, next, a change of tne physical condition of the
solid ; it becomes changed into a liquid, and, lastly, it assumes the
gaseous state, if the repulsive action of heat be sufficient (9).
Solids expand less, and gases more, than liquids, for equal incre-
ments of temperature.
Several methods have been pursued for determining the ex-
pansion of solids : the ordinair apparatus consists of a horizontal
water-bath, capable of being heated by a row of gas-jets, through
the ends of which a cylindrical rod of the substance to be ex-
amined passes water-tight, one end of the rod resting against a fixed
point, and the other against a lever near its fulcrum : the long arm
of this rests against another lever, also near its fulcrum, and in
the movement of the extremity of the long arm of this, the expan-
760 THEBHIC8.
Bion of the rod is confiiderablj amplified, and may be approzi-
matelj determined.
A much better result maj, howeyer, be obtained by attaching
a small mirror to the axis of the first lever ; a small pencil of light
from a lamp, or other source, may be reflected from this on to a
scale at any required distance, and thus the indications may be
amplified to any required extent.
The readiest, and probably not the least accurate mode of deter-
mining expansion, is by means of taking the specific grayity (412)
of the body, at several difierent obsenred temperatures, from these,
and the known expansion of the fluid in which the specific graTity
is taken, the expansion in volume of the body may readily be
determined. One-third of this value will represent the lintar
expansion. This method has the advantage ofbeing independent
of the quanti^ or form of the body examined.
The following table shows the increase in length, or Imear
expannofij of bars of diflercnt substances, from 0** G. to 100": —
Crown-glass 1000792 J Aluminium 1001087$ Copper 1001717
Flint- „ 1*000812 Cast-iron . 1*001127^ Brass . I'OOISM
French „ 1000872 Steel .... 1*001145S Silver . 1001910
Hard „ 1*000897 Iron, drawn 10012355 Tin . . 1*001970
Platinum . . 1000884 S Bismuth . . 1*001392^ Lead. . 1*002818
Palladium . 1001005^ Gold .... 1*001466^ Zinc . . 1002976
Many practical applications have been made of the expansion
and contraction of metals by heat : thus, the tire of a wheel is put
on hot, and by its contraction firmly binds the other parts of the
wheel together ; and if the engineer requires a collar to be veiy
tightly fixed on a rod or bar, he drives it on while hot : and for
similar reasons, boiler plates are riveted together with red-hot
rivets. The successful manufacture of the Armstrong gun, in
which the welded coils are successively shrunk on, is a further
illustration of the same principle. By Molard, the walls of a
building that had bulged, were arawn together by the contraction
of an iron bar passing through them, and secured by nuts tightened
on the outside whilst the bar was hot ; and a similar proceeding
was adopted in the cathedral at Armagh.
The middle of the centre arch of the c^uthwark iron bridge rises
one inch in the heat of summer : and the eflect of a sudden gleam
of hot sunshine on the Britannia bridge, which crosses the Menai
Strait, is immediately perceptible in producing both vertical and
lateral curvature of tne rectangular tubes. The linear expansion
of this wonderful structure is provided for bv the whole being
{)Iaced on friction rollers : and when great lengths of iron pipe are
aid down for the conveyance of steam or hot- water, sliding-joints
are necessar]^ to prevent injury either to the apparatus, or to
the building in which it is placed.
By multiplying the linear expansion of bodies by 8, the total
ZZPANSIOll OF GLABS AND WATER.
761
increment in balk, or the cubical ea^pofinon, is generally obtained
with sufficient accuracy : the algebr€ncal reason of this is obvious.
1351. By the same elevation of tem^rature from 0* C. to 100°,
the increase in bulk of the following ilmds has been ascertained : —
Mercury . . 00019! Ether .... 00070 \ Fixed oil . . 00080
Water . . . 00046! Oil of tup. . 00070 1 Alcohol . . . 00110
JExparmon of Glass, Water^ and Mercury. — An elaborate series
of investigations has oeen made by Dr. A. Matthiessen,* for the
purpose of accurately determining the expansion by heat of water
ana mercury. Having found the expansion of a gla^ rod between
0** G. and 100° to be 0000729 of its length, he employed this value
in calculating the expansion of water and mercury.
Water beug at its greatest density at 4"* C. very nearly. Dr.
Matthiessen found that he could not represent the volumes at
different temperatures (in which calculation and experiment agreed
very nearly) by any single formula involving the powers, up to the
fourth, of t— 4 ; but he found that the volumes from 4** to 32° were
very nearly represented by the formula, —
F/b1-0'0000026S (f-4)+0-0000006389 (<-4)*- 0*00000007178 (<-4}*»
and those between 32° and 100^ by the formula, —
F<>iO-980605+OH)0000647M^-0-000000011fe <*.
It thus appears that the variation of volume is far from constant.
The following table shows the expansion of water for 1 ° at the
temperatures specified : —
«°a
^1+1-^.
Diff.
«°C7.
Vt^i'Vt
Diff.
5°
10
15
20
25
30
35
40
45
50
0*000022
0000098
0000162
0000215
0000259
0000290
0-000345
0 000388
0 000428
0-000466
76
64
53
44
31
55
43
40
38
37
55°
60
65
70
75
80
85
90
95
99
0000503
0-000538
0000572
0-000604
0-000634
0-000662
0-000689
0000714
0000737
0000755
85
34
32
30
28
27
25
23
18
From this teble it appears that expansion oontinuallpr increases
with increase of temperature, but that the rate of increase, as
shown by the columns of differences, is a ^aduall^ decreasing rate.
There is an evident want of smoothness m the diminution of this
rate near the point at which the formula is changed, which was
• FhU. Tmnf. 1830.
762
THEBMICS.
empirically determined by equating the two formnln, and finding
the value of t in that equation.
The coefficient of ex|MinBion of mercury obtained by Dr. Mat-
thiessen from careful weighings in water is
I7«=£?b (1+0-0001812 0;
which agrees very nearly with that obtained from direct measure-
ment by Begnaulti viz. —
In accurate th^rmometric experiments, when the bulb and a porlion
only of the tube is immersed, a small correction is needed for the
expansion of the mercury in the remaining portion of the lube. If
N be the number of centigrade degrees in the column extant, T
the temperature to be obwrved, and t that of the surrounding
atmosphere, then according to Kopp,
correction= ^(T- 0 x 0*0001545.
1352. Gaseous bodies have been supposed to expand equally
for equal increments of temperature, 1 volome of air at 0** C. oeing
increased to 1*375 at 100^ and the same amount of dilatation in
volume was long considered to take place in all other aeriform
bodies.
The expansion of gases has been observed by Rndbeig, and
according to his researches, one volume of gas at 0" C, b^mes
1*865 at lOO"" ; if so, a gas expands 000365 of its volnme for each
centigrade degree, instead of 0*00375, as generally stated. If the
volume of a gas at 0** C. be 1, its volume at any higher tempera-
ture may be readily found by the formula : —
Volume at t* C. = 1 + 000365 1.
According to the more accurate researches of Regnault and
Magnus, the expansions of various gaseous bodies are not precisely
equal ; the following are the volumes of various gases at 100^ C,
the volume at 0* bemg 1 : —
GMes.
•
BegMult.
UAffsn.
Hydrogjen
Carbonic oxide
Carbonic acid
Nitrons oxide
Cyanogen
Solpburous acid
1-36613
1-36688
1-37099
1-37195
1-38767
1*39028
1-36566
1-36909
1-38563
1353. There are very few known exceptions to the general law
of bodies continuously dilating by heat, and contracting in pro-
portion as they are cooled, and the most important of these excep-
▲IB THBBMOICBTKB.
763
tions occnra in the case of water : the others are cast-iron, hismnth,
antimony, and Tulcanized caontchonc. If water at an ordinary tem-
perature be heated to its boiling point, it will expand like other
uqmds, and if then it be allowed to cool, it will be found to contract
in hoik continuously, until it attains a temperature of 4° C. ; at
which point it will be at its maximum of density. . On a further
diminution of temperature, the water will dilate in bulk until it
attains the freezing point, or 0" C. ; and if it be cooled below this
point without freezing, by ayoiding all agitation, it will still con-
tinue to expand. The yolume of equal weights of water at 8^ and
at 0^ is the same. In the act of freezing, a more marked an ount of
dilatation occurs ; the bursting of water-pipes in winter from this
cause is a phenomenon familiar to every one. An iron plug weigh-
ing three pounds was used to close a bomb-shell filled with water,
and on freezing the flaid the plug was projected with violence to
the distance of 415 feet.
The great importance of water being the exception to the gene-
ral law of the condensation of bodies b^ cold, may be illustrated
by a reference to what would occur if this were not the case.
When in winter the surface of our rivers and lakes is covered with
a omst of ice, this would sink to the bottom, and the fresh surface
of water thus exposed would in its turn freeze, and another layer
of ice would sink : and thus congelation might go on, during a
severe winter, until dur lakes and rivers were converted uto
solid masses of ice, and all their inhabitants would perish. But it
has been ordained by Infinite Wisdom that water should expand,
instead of contracting, below the temperature of 4", and thus the
sheet of ice once formed beine lighter than the sub-
jacent water, floats on its surface instead of sinking, -2V* ®79.
and helps to protect the fluid below, with its various
inhabitants, from the further influence of cold.
1354. Before proceeding in his researches on the
properties of heat it is necessary for the inquirer to be
ramished with some means of obtaining a measure of
its intensity. This important object is fulfilled by in-
struments termed ihermometerij or measures of heat ;
all depending for their action upon the expansion of
bodies by heat. The first of these instruments was
oontrived by Sanctorius, an Italian physician in the
16th century, and is now known as the air-thermometer,
because the indications it a£fords depend upon the ex-
pansion of included air. It consists of a glass tube.
Fig. 679| having a bulb blown at one end ; the tube is
then filled as far as the bulb with a coloured fluid, and
inverted in a vessel containing a similar liquid. The
bulb is thus full of air, and on approaching a heated
body towiurds it, the included air expands and depresses the fluid
o
764 THSBMIGB.
in the tabe, a gradaated scale attached to which marin tiie
amonnt of snbBiaenoe of the flnid, and oonseqaently of the ex-
pansion of air in the tube. These instruments are Teiy delicate
in their indications, bat are rarely used, except as therfno§eope»t
in conseqaence of their inability to measure any considerable range
oi temperature,
1355. It may here be remarked that from 0" 0. to 100* the ex-
pansion of air IS proportional to its temperature, as indicated by a
mercurial thermometer, but aboye 100*^ it diminishes slightly.
According to the observations of Magnus, the Yolumes of a given
mass of air under a constant pressure, at different teroperaturafli
as indicated by a mercurial thermometer, are as follows : —
Temperature . 0*C. 100* 200* 300'
Volume of air. 10 1-366508 1-72385 2*0794;
hence the corresponding temperatures, as indicated by a thermo-
meter filled with mercury and one filled with air, are,
Mercury . . 0* C. 100* 200' 300*
Air .... 0 100 197-5 294-5.
1356. Leslie's D'fferential Thermometer. — ^An air thermometer
of a very different kind was devised by Sir J. Leslie, whose in-
strument has been of essential service in
r-^ Fig, 680. ^ elucidating many of the more obscure propep>
J^fiL ^ ties of heat : it consists of a tube bent twice
at right angles, each end tenninatine in a
bulb, Fig. &0. Before hermeticallT closiiuf
both the bulbs, the tube is partly mled with
sulphuric acid, tinted with carmine or indigo,
so that both bulbs are left full of air, and tbe
bent tube partly full of coloured fluid. This
instrument does not indicate any changes of
temperature in the surrounding air, becaose
so lone as the air in both bulbs is equally
heated, the fluid will stand at the same point
in the tube. If, however, a heated booy, as
the hand, approach one bulb, the incloded air
will expand and depress the fluid in the tube
beneath, driving it into the other bulb. The amount of deprea-
sion of the fluid in the tube is measured by means of a grados^ed
scale attached to one arm of the instrument. This convenient
apparatus is termed the differential ikermameter. because it indi-
cates a difference of temperature in the air inclnded in the two
bulbs.
1357. The expansion of liquids has been long used to indicate
diflerences of temperature, and instruments thus constructed have
the advantage or being uniform in their indications, and of being
capable of measuring considerable ranges of temperature. The
8CALB8 OF TEMPBRATUBX.
765
two flnidB now generally used for thermometere are alcohol and
mercury ; of these alcohol is of most service in the «.^ aoi
measarement of very law temperatures, as it has ^' ^^'
never yet been froEen, bat the comparatively low
temperature at whioh it is converted into vapour
renders it unfit for the examination of temperatures
above, or even near to, its boiling point. Whatever
fluid is used in their construction, these instruments
are always similarly formed, consisting merely of a
tube of fine bore, terminating at one extremity in a
bulb, and filled with the fluid raised to a sumcient
temperature to ensure the expulsion of air. The
tube is fixed to a piece of hard wood, ivory, metal, or
glass, on which a scale is engraved. The lower end
of this sometimes moves on a hinge, as in Fig. 681,
BO as to allow the ready immersion of the bulb in
any liquid.
1358. To enable the indications of different ther-
mometers to be comparable with each other, some
fixed points, from which the graduation of the
scale may be made, are absolutely necessary. The
fixed points are the temperatures of melting ice, and of the
steam issuing from boiling distilled water under a mean barome-
tric pressure of 30 inches ^493). The space between these points
has been divided in an aroitrary manner according to the views
of different philosophers. Reaumur divided it into 80 equal parts,
or degrees, of which 0° corresponded to the temp<>rature of melt-
ing ice (or freezing water) ana 80** to that of boiling water : this
gniduation has been extensively employed on the Continent.
Celsius of Sweden divided the same space into 100 degrees, jnving
rise to the centigrade thermometer, now in general use in franco
and Grermany : and it is very much to be regretted that this scale
is not universally adopted, for the sake of uniformity in the nume-
rical estimation, of results. The division of the thermometrio
ecale usually employed in England is that of Fahrenheit, a Ger-
man artist, who assumed for his zero the temperature produced by
a mixture of equal weights of dry snow and salt, ana he divided
the space between this and the temperature of boiling water into
212 degrees, of which the point 32** corresponds to the tempera-
ture of freezing water, and to the 0° or zero of Reaumur's, and of
the centigrade scales ; the space between the temperature of boil-
ing and freezing water has thus been divided by Reaumur into
80, by Celsius into 100, and by Fahrenheit into 180 equal parts.
The scale of Fahrenheit is purely arbitrary, and is not recom-
mended by any one consideration, either theoretical or practical;
and as much confusion has resulted from the use of these different
scales, it is customary to indicate which graduation has been em-
ployed, by placing after the figure the letters R, C, F, respectively.
766 THESMICS.
It 18, however, very easy to convert the indications afforded hj
oae scale into those of another hy remembering the ratio borne by
the degree of one scale to that of the other. Thns, a degree of
Fahrenheit's scale is eqaal to ^ of one of Reaumur's, and to {^ of a
centigrade degree. In practice, the following roles will be found
nsefal for the conversion of the different thennometric degrees into
each other.
To convert the degrees of Fahrenheit into those of Reaumur : —
MvUij^y the numher leas 82 &^ 4, and divide the product bjf 9.
Ex.l86*F.-32 = 153, x4=612,-=-9=68<»R.
To convert the degrees of Reaumur into those of Fahrenheit :-^
Multiply the numher by 2, add iofitj and alio 32 : —
Ex. 16'R. x2i=32+4=36, +32=68'F.
To convert the degrees of Fahrenheit into centigrade degrees: —
Multiply the number less 32 by 5, and divide the product by 9.
Ex.212'F.-32 = 180, x 6=900, -7-9= 100* C.
To convert centigrade degrees into those of Fahrenheit: —
Multiply the numher by 2, nlbtract ^ ofit, and add 32 : —
Ex.20°C.x({=)2-J=40-4=36, +32=68'F.
To convert Reaumur's scale into the centigrade : — add ^.
1359. The curvature arising from the unequal elongation of two
strips of different metals, soldered or riveted together, has been
employed as an indication of the change of temperature by which
the curvature has been effected :^ the metal of greatest expansion
lying, of course, on the convex side of the curve. An instrument
has bcon constructed on this principle by
Fig. 68S. M^ Breguet of Paris : this consists of a
delicate ribbon of platinum soldered to one
of silver, by a very thin layer of gold. This
compound bar is twisted into a spiral coil,
Fig. 682, one end of which is ued to a
support, the other carries a delicate gold
needle as an index. As the two metals,
of which the coil is composed, expand veiy
differently for equal increments of tem-
perature, it follows that the heh'x of ribbon
will uncoil, or become closer twisted, ac*
cording to the temj^rature to which it is
subjected, which ¥nll be indicated by the
motion of the needle over a graduated
circle.
1360. Johnson^ » Deep-tea TJiermometer. — ^The same principle
has been employed by Mr. H. Johnson in the constructioiL of an
inatninieDt for observing the temperatnre at the bottom of the sea.
Two componnd born, b, Fig, 6e3, are riveted at e to a stout plate
of metal, a, thew are oounooted by links at d, d, a. na
with a croBS bar on an aiig eanying an inden, e, ■^'
along a scale, k. Two other unconnected indices
_^ jf, are puahed bv the former to the extremes of
its range, and indicate the mnxunum andmimniutn
temperatarea to vbich the itiBtriunent has been ei
posed. The whole is enclosed in a cylindrical oasi,
1 1, tiimished with rings for Ibe attachment of the
sonDdiog-liDe and the "lead."
1361. Maximum and Minimum Thermottuteri. —
Instruments so coDstmcted, as to indicate the ex-
treme points of temperatnre which the; ha<e at-
tained, have long been in nse nnder the nsme of
dag and night, or nui.ci'niui7) and mtnintum thenno-
meten. In Rntherford's inetnunents the iitenis are
placed horizonlallj in a frame ; and in the farmer,
the point of maxmium elevation hax been msrkrd
by a small piece of a needle introduced into the ti ~
which is pushed forward bv the mercury during
expansion, and left behind at any succeeding de-
presaian, thus indicating the maiimnm temperature
which has been attained. The indei is featured to
contact with the mercury, by placing the tube in an
erect posilion. In Froi; PhillipB' maximum thermometer a short
eolnmn of mercnry is separated from the rest of the column by a
minute bubble of ur; aod if the bore be sufGciently fine for the
index-column to be retained by adhesion when in a vertical podtion.
the ii
chemical researches.
The
0 Dsed; which i
determined by an alcobol thenuometer, (he tnbe oF wliich contains
« small bit of fine iron wire, enclosed in aminule glass tube. Iliis,
by the adhesion of the fluid, c<iDtinues to be entirely immersed
during its contraction, and its extremity con sequontly recedes vith
the surface of tbe column ; but on an elevation of temperature, the
3>irit in expanding flows past the index, the eilremily of wbich
lerefore marks the point of lowest temperature. This, as veil as
the former steel index, must be replocM by a small magnet, after
each observation. The objection lo an alcohol thermometer, espe-
cially in the horizontal position, is that an uncertain quantity may
be retained by adhesion to the sides of the tube, and therefore the
apparent position of the snriiice of the eolnmn may not be correct.
tor very low temperalures, however, alcohol is indispcnsahta, as
it has never yet been reduced to a solid state.
1362. Mereurial MajHmum Thermomeler. — A great improve-
ment lias been made in the maxim nm thermometer by MM.
768 TBKKlflCS.
Negretd and Zambra ; tliis consiits in introducing into the tnbe
near the bulb a minute particle ofiduai which is retained in ita
position b J a bend in the tube. Tne mercory readily pasMa ths
obstacle in expanding, but on subsequentlj oontraclin^, the thread
of metal breaks at this pointy and the mercury retiree into the
bulb, leaving the column itself to indicate its maximum eleYatioo.
The mercury may be readily shaken past the obetmction, in order
to prepare ror another obserration. 'The same ingenious axtitts
have also constructed a mercurial minimum thermometer : in this
instmment a bit of a steel needle, obtusely pointed at the broken
end, rests on the surface of the mercury, and follows ii in deaeent ;
but on the subsequent ascent of the column, the mercoir flows
past the needle, without displacing it, and the lower end of the
needle consequently indicates the minimum temperatore.
1363. (kueSkCs Minimum Thermometer. — ^Thia inatmment
appears to be the most certain in its indications of any that haTe
been oonstracted. A lateral tube, a, Fig. 684, haTing a huge
J^. 684. ^^ comes off from the stem of the
thermometer, near the bolb, and is
bent as shown in the figure. The
wide bore ends abruptly at n, where
a pear-shaped cavity, c, opens into
it by a narrow neck, wider, however,
than the bore of the thermometer.
When adjusted for use, this instn-
ment is placed horizontally, with the mercniy filling tbe tobe ▲
up to B, out leaving the chamber, o, empty. ^ As the temperature
falls, the mercury will be retained by adhesion to the transverse
surface at b, and sink in the stem ; but when the temperature
again rises, the expanding mercury meeting less reaistanee at the
orifice of the chamber, c, than m the stem, will enter it, and
remain stationary in the stem : and thus the point of lowest tesi-
perature will be indicated. The instrument is set for further
observation by gently raising the bulb, until the mercury leaves
the chamber c empty.
1364. The Aetinometer. — ^This instmment for meaauing the
heating effect of direct solar ravs consists of a thermometer
with a large bulb, and an open scale, for indicating small chaogesi
Observations are made with this instrummit by niacing it alter-
nately in shade and in sunshine for equal intervati of a minute or
a minute and-a-half ; then the difference between a reading in
sunshine, and the mean of two adjacent readings in shade, vriU be
a measure of the solar influence. With any given instmment
this measure is arbitrary, and will give only comparative resalts :
in order to render different series of observations comparable, it is
very desirable that some authoritative standard of compaiiaoB
should be established.
A very convenient and portable form of aetinometer has been
INDEX EXROBS. 769
deTised by the BeT. G. C. Hodgkinson.* In order that ob8er?»-
tions may be made throagh a wide range of temperature without
a acale of inconveDient length, a second bulb is made at the top
of the scale to which a portion of the coloured fluid may be trans-
ferred, so as to bring tne observationSi made at any ooe period
within the range of a scale about 10 inches long. A polished
piece of german silver drawn tube 18 inches lon^, and 2^ inches
in diameter, serves as a case to pack a couple of instruments, and
also as a shade, when in use ; for the latter purpose the tube has
at its middle point a lateral socket in which the actinometer may
be fixed by means of a perforated cork, so aa to place the bulb in
the middle of the tube : there is also a means of attachment to a
vertical rod planted in the ^und, and furnished with a ball-and*
socket ioint, so that the axis of the tube may be directed to the
mm ; when a cap corering the end of the tube will shade the bulb.
By means of this instrument Mr. Hodgkinson observed the
ton's heating power at the summit of Mont Blanc to exceed that
at Chamonix by from 20 to 25 per cent.
1365. If, in the graduation of a thermometer, the interval
between the marked points of freezing and boiling water be equally
divided into 100 or 180 parts, then, in order that these degrees
should accurately represent equal increments of temperature, it is
necessary that the bore of the tube, forming the stem of the instru-
ment, should be perfectly uniform. As this is rarely if ever the
case in practice, it becomes requisite in all trustworthy instruments
to determine carefully several intermediate points by comparison
with a standard thermometer of ascertained accuracy. The uni-
formity of the bore of a tube, and its consequent fitness for the
construction of an accurate instrument, may be ascertained by
measuring the length occupied bv a given (quantity of mercury in
diflferent parts of the tube. Where precision is required it is in-
dispensable that the scale should be engraved on the stem of the
instrument ; and this is efifected by a dividing engine, in the hands
of the best makers: the stem being coated with a varnish, a
portion of this is removed by the dividing point, and the figures
being similarly marked by removal of the varnish, both scale and
figures are etched in by nuorhydric acid.
1366. Index Errors. — Fluid thermometers are liable to several
sources of error, both temporary and permanent, in consequence
of a change in the capacity of the bulb ; and in all accurate
thermometric results, these must be allowed for, or corrected.
After a thermometer-bulb has been blown (and especially if it has
not been carefully annealed), it undereoes a gradual contraction,
during a period of several months. In the large and long cvlin-
drical bulbs employed by the writer in the construction of his
thermographs (1367), this error has been observed to amount to
+ 1*6'*F. m tubes tilled shortly after they were blown, but after
• Prooeedingi pf the Boyal Society, Jsa. 1S07.
3 D
770 THEBIIICB.
BIX or eiffht months, no farther permanent change was obserted
to take place : it is therefore desirable that a thermometer designed
to be an instmmeut of exact obseryation should not be graduated
for at least six months after its manufactare.
If, after the freezing-point has been marked on the stem, the
boiling-point is marked, on again immersing the bulb in the melt-
ing ice, the colmnn will be found to fall below the former freenng-
pomt by 0'2* or 0*3°, the dilatation of the bulb not having yet had
time to subside : and several days must elapse before the colomn
again reaches the mark. Hence it follows that, in pointing a ther-
mometer, at least a fortnight ehonid elapse after its expoeore to
any high temperature ; that the freezing-point should be first
marked, then the intermediate points, from "Mow vpward»; and
lastly, the boiling-point.
Also, if a thermometer be pointed in a vertical position, its
readings in a horizontal position will have a small positive error,
and especially if the bore oe large, because the hydrostatic pressmn
of the column will tend to streteh the bulb : a tnermometer shoold
therefore be pointed in the direction, whether horizontal or ver-
tical, in which it is intended to be used.
1367. Thermography orSel/regitterinq Thermometer. — Varioiis
mechanical contrivances for effecting the antomatic registration
of atmospheric temperature have been proposed by Kreil, DoDond,
and others. The photographic method of^ registration applied by
the writer to the magnetometers (624), and to the barometer (495),
has been extended to the thermometer, and peychrometer (1426),
and thus a continuous record of all their respective variations is
maintained. The bulbs of these instruments are freely exposed
underneath a table, which supports a revolving cylinder ooveied
with the sensitive paper, analogous to that represented in FSg.
329 ; their stems pass vertically upwards through the taUe, and
are placed between the opposite sides of the cylinder, and two
lights. A narrow vertical Ime of light, brought to a focus b^ a
cylindrical lens, falls on the stem of the thermometer, and nasBing
through the empty portion of the bore, affects the preparea paper.
The boundary between the darkened and undancvned portioos
indicates the position of the mercury in the stem of Uie thermo-
meter. Fine lines, corresponding to the degrees, are etched across
the stems of the instruments, and broader lines at every lOtk
degree, as well as at certain other fixed points of the scale, namely,
32°, 64°, 76°, and 98* F. The shadows of these lines protect
from the action of light the portions of the photographic paper on
which they fall, the darkened surface of the paper is conseqoentlv
traversed by a series of parallel pale lines, and the relative posi-
tions of the broad and narrow lines readily explain the temperatore
indicated by the register. As this apparatus is necessarily placed
in the open air, when in actual operation, it is provided vnth ss
inner cylindrical zinc case, with sliding doors to protect the sensi-
XfKtLDf ACTOMinO
1368. AppoU'f Aatomatie Temperature-rtguiaUir, — Thi» U
one or the man; ingenious ftpplicationi of phjncsl prindple* to
doniMtic cnmfort made bv Eha Ute Mr. J.Q. Appold, to whom
mbo the world u --'-■^-^ '--^ -'— ■ '"-" • '^—
o. Ths tube, tad
aboDt half of each
bulb are filled with
mercury ; and ball
•□ inch of ether
floata on the menmr;
loaiDtaiDBlhe column
of
higher level in the
bulb, B. Beneath the
wood a wfiffbt, d, ia
■o attached that it
may be shifUd by a
aorew, E, in order to
•Iter the poulion of
the centre orgrsiitv of the whole apparatna, vUch !■ nupended Hko
a acaie-beem on a knife-edge at r. The size of the bdbs is Sncli
that at the ordiuaiy temperatnre of a room a rise of TP. will
laisa an ounce of mercurj 3 inches. A cord, a, is attached to the
cod of a projecting arm, h, which control! the Bupply.ralre of a
stove, or tbe damper of a furnace. It is erident from theconetrDC-
tion that ao elevation of temperature will make B preponderate,
and bj depiouing h, will cloie the valve and damp the Ere ; and
that a fall of temperature will have the contrary eSect. The
position of the weight, d, may be bo adjaated by trial, that the
apparstns ahall maiotain iM mean poaition at any required tem-
perature, and a corresponding ecale may be marked on O. A
thermometer, i, ia sealed into the top of i, and another, l, is
attached to the apparatus, for the purpose of observation and com-
Earieon. The action of the arm, h, is limited in both directiona
V a bent rod passing over it, which ia inserted in holes in a fixed
plate behind it.
Hr. Appold also contrived ■ hjgrometric regulator, by the
movement of which the droppii^ of water "" ■" — ™ — ' — •■--
enpora&in was controlled. This mainly 0
773
lontal tab«, harinK eth«r on the larftM »f th* merciiry i'b bolk
bnlbe, one of which ii covered with wet mBalin, the Tenable er*-
paralion from whicb conl> and coadenMi the etfaar-Tapuiir, aod
thai altera the centre of ^;ravitj of the tuba.
1369. Wedguiood't Pyromtter. — For thajpnipiwe of meaanring
degrees of temperature, mgher than that af^ boiling mcrcnrj, in-
■tnimeats termed pjroinetora bave been emplojed. Of ibeae tliat
contrived bj Mr. Wedgwood va< long held in repute : it coiuiitad
of a «eriea of perfectly similar cjlindera of baked daj, and a na-
dnated scale lo allow of their accurate meaaDremeiit. On«of theae
Gjlindera vae then eipoeed lo the lemperature to be meaanred;
to praportion aa it becione bested it contracted in bnlk, and tkU
cootraotion, meuuted when the cylinder had cooled, became an
indication of the temperature to irhioh it had been anbjecled. In
addition to other eoarcoB of error, the fact that pieces of claj will
nndergo the same amoant of contraclion by a moderate heat leeg
continued, aa bj an exposure for a shorter time to an iatenae beat,
becomes an insuperable objection to a reliance on the iudicstiaDi
of thia inslrutnent.
1370. i>imielri ^/ronieter, — A much more tnalworthy pyro-
meter ia that of Prof. Uaniell: itconaiata es^ntially of a alender
bar of plaliuum or hammered iron, of which the linear expaniioii,
when heated ia a tube of plumbagn, is maaanred by means ot a
little piece of porcelain reating upon the top of the bar, which i*
pushed forwanl during iu expamioD, When the appai«tiu hai
cooled, the displacement of the porcelain iodei becomes a mearare
of the expansion of the bar, and consequently of the temperatwe
to which it has been exposed. The amount of displacement i*
measured by means of a delicately graduated scale, fumiahed with
1371. Bffilrdm'i Hgdro-pynmuitar, — Whenever moderatetr
■mall cbimgea of temperatnre are required to be indicated,
more reliable observaHons may ba ob-
^■^^- tained by means of this inilmiDent,
the principle of which ii that » baU
of metal of known weight eipoaed
to the heat of the funiace oommtmi-
cates its heat to a known weight of
water, the temperature of whidi, bo-
tbre and after tQe accession of best, ii
observed by a mercurial thennonieter.
The contrivance by which the bkll ia
heated and then rapidly transiemd to
the water-chamber, is shown in I^.
686. A perforated ball, *, of steel or
plalinnm, aocordiog to the lemperatnra
to be observed, is carried through the pipe, d r, by a bant wire, e^
lo tbe extremity of a muffle or closed chamber, ■■, which pixgmata
THBBMIO UNIT. ' 773
from the inner suriace of the wall into the fnmace, and the pipe
is then closed by a plug of claj at d. After a snfficient time has
elapsed for the ball to acquire the temperature of the fiimace, the
rod, c, is withdrawn, and the ball beinjr detached from it by a pro-
jection. A, on which the rod rests, rolls down the pipe, a k, and
drops through a Talve, l, into the water-chamber, which consists
of a closed metallic Tossel encased with wood.
With 300 grammes of water, and a steel ball weighing 7 gprammes,
a range of 5" C. of change of temperature in the water*bath cor-
responds with a range of about 1000" in the furnace ; and with
a platinum ball weighing 8 grammes, a change of 4** corresponds
with a range of about 25^" of furnace-heat: it is, therefore,
desirable to use a steel ball up to 1000°, and a platinum ball above
that temperature. From the known data of volumes and soecifio
heats, a table of equiyalent values may readily be calculatea.
1372. Thermic Dnii. — ^AU the requisite means of obtaining
eomparaHve thermometric measurements by differential expansion
having been described, it becomes necessary, in order to obtain
ahtohUe results, to establish some unit quantity of heat, or thermic
unity as a measure of other amounts of heat which it may be
desired to determine. As the dynamic equivalent of heat (1345),
or as it may be termed, the dynamo4hermic unit,* has been de-
fined to be the amount of work used up in raising lib. of water
1** C, so the thermic unit may be conveniently assumed to be the
amount of heat necessary to raise lib. of water 1** C. It will sub-
sequently apjpear that in order to define the unit with complete
accuracy, it is necessanr to specify at what part of the scale the 1"
is situated : it will prooably oe found most convenient to adopt the
interval between 0** and 1°, as shown by a mercurial thermometer.
1373. Conduction of Heat, — It is a familiar fact that beat is
conducted by different bodies with very different degrees of faci-
lity. Thus, a small piece of charcoal may be held by^ one end in
the hand wnilst the other end is red-hot; while a similar piece of
iron would soon cease to be tenable, if one end were plunged in
boiling water only. Among bodies of the same class, as metals,
a great difference of conductibility exists ; thus, a piece of plati-
num can easily be held in the hand whilst one end is red-hot ; but
a piece of copper of the same length similarly circumstanced will
speedily bum the fingers.
Twist together the adjacent ends of a piece of copper and one
of platinum wire, each about six inches long, and place on the
further end of each a minute piece of phospnorus. If the point
of junction be heated by the flame of a spirit-lamp^ Fig. 687,
* It may h«re be oonrenitfiit to adopt tbe Mune mode of phrsMology that
hat been previously (076) adopted in ezpreMiog the relation ot heat and
eUotfieity : a dfpMmo^hervne nnit may be taken to mean the value in work
of an onit of Itiai, and a tkmwihdpia'mio unit, the value in heat of an onii
of work.
•■■«— ^
774
THBEMICB.
in a few seconds the phosphorus attached to the copper wire
- will burst into flam«, whilst
F^g. 887. ^^^ ^^ the platinum will
remain unafiected for a
much longer period.
If a series of cylindrical
rods of equal diameter, and
three or four inches in
length, composed of dif-
ferent material!, soch as
wood, ivory, glass, slate, marble, and various metals, tie inserted
at a little distance from each other in the side of a vessel contain-
ing water kept boiling, and a small bullet be cemented with wax
to the end of each, the relative number of seconds that elapse
between the filling of the vessel and their falling off, will approxi-
mately show the conductivities of the several substances.
The couducting power of several metaUic bodies is shown in the
following table by Wiedemann and Franz, in which that of silTer
is taken at 1000 as the standard: —
Metal.
Heat
Eleo.
MetoL
Heat.
Elee.
1
226
130
• •
107
Metal.
Heat.
64
63
60
18
Dee.
103
• • •
19
1
Silver .
CJopper
Gold .
Brass .
1000
736
632
236
1000
733
585
216 1
Tin .
Iron .
Steel
Lead
146
119
116
86
Platinum .
Palladium
Ger. silver
Bismuth .
The nearly parallel electrical conductivity of various metals
(805), as determined by Lenz, is shown in the second columns of
numbers.
It must hero be remarked that the transmission of htat and of
stngible temperature are not necessarily identical ; this is shown
by an experiment of Prof. Tyndall. If two prisms be made of the
same* shape and equal volume, one of bismuth, and the other of
iron, and one end of each being thinly coated with wax, they be
placed with the coated ends upwards on some heated body, th3
wax will be melted first on the bismuth, although it appears from
the table to be a much worse conductor than iron. This is due
to the greater specific heat of iron, that is, the greater amount ot
heat necessary to produce a given elevation of temperature.
1374. The following general law of the propagation of heat by
conduction has been oetermined : if one end of a bar of metal be
placed in connexion with a source of heat^ it will be found that
for distances measured from this point in arithmetical progressioo,
the excess of temperature above the surrounding memum will be
in ^metrical progression. Tliis may be proved experimentally
by inserting thermometers in holes made at equal distances in the
VARIJLTIOKB OP COHDDCTIVITT.
775
ride of a bar of metal, one end of which is maintained at a constant
high temperature.
1375. tnfiueneeof Organic Structvreon Conductivity, — A series
of experiments has been made by Prof. Tjrndall,* on the power of
yarious kinds of wood to transmit heat in three perpendicnhir
directions, viz., longitudinal, radial, and tangential. For this pur-
pose, cubes 0.3 inch each way were cut so that their edges were
m these three directions ; these were successively placed in each
direction between two cushions of mercury, each bounded by a
plane surface of thin membrane. One of these was warmed by an
inunersed coil of platinum wire heated by a constant voltaic cur-
rent, and the heat transmitted by the wood in one minute was com-
municated by the other portion of mercury to the united ends of a
thermo-electrio element of bismuth and antimony (962), and the
heat was measured by the deflection of a galvanometer needle (856).
A tan^nt galvanometer (861), and a rheostat (804), were inter-
posed m the voltaic circuit, and the deflection of this needle kept
uniform by the rheostat. The relative conductivity of different
kinds tif wood in the several directions was found to be closely in
accordance with the relative elasticities as manifested by the
transmission of sound (546). The heat transmitted in the several
directions tmeasored by the deflections of the needle), by some
varieties of wood is shown in the following table : —
Kind of wood.
Scotch fir . .
Elm . « . .
Oak ... .
Walnut . . .
Satin wood .
Box wood . .
Quebec pine .
American birch
Lour.
Had.
Tang. I
22
10-0
120
24
100
110
27
80
9:4
28
110
130
30
11-9
12-3
31
9-9
12-0
33
10-0
110
35
90
11-0
There appears to be no assignable relation between the hard-
ness of the wood and its conducting power.
1376. Infiuence of Ttmperatwre on ConducUviJty, — Principal
Forbes has carefully observed the conductivity of a square bar of
iron 1*25 inches thick at temperatures varying from 0^ C. to 275**.
His assumed unit of heat is the amount necessary to raise the
temperature of a cubic foot of water at 0° G. one degree ; and if a
body of unit conductivity were formed into a plate one foot thick,
and its opposite rides maintained at temperatures differing by
l^'C, one square foot of this plate would in one minute transmit
an unit of heat. The observed conductivities at different tempe-
ratures were as follows : —
• Phil Trans. 18o3.
■^^•^
776 THEBMICB.
O'C. . 0-01337 MOO'C. . 001012 ) «00'C. . 0*00876
25 „ . 0 01235 ; 125 „ . 000966 225 „ . 0-00651
60 „ . 0*01144 M50 „ . 0-00934 250 „ . 0*00826
75 „ . 001070 i 175 „ . 000904 ! 275 „ . 0-00601
In another bar one inch square smaller differences were ohaerved.
The numerical values here given must be multiplied by 62*321
(425), if required in terras of the deffre£ thermic unit (1372).
1377. Influence of Molecular AggregcUum on Condwetwiiy. —
It has been satisfactorily proved by Dr. Senannont that there is
an accordance between the optical and thermal elasticities of
crystals, su far an his observations extended. He cut thin plates
in different directions from various crystals, and having coated a
surface with a thin film of wax, and passed a wire through a hole
in the middle of the plate, he heated the wire by a constant
voltaic current. The melted surface of wax extended itself gra-
dually from the hole, and ita outline must clearW be an iMotkenitial
lintf or boundary of equal hetU-toave-motion. If the condnctivitr
of the plate be equal in all directions, it is clear that this line wiU
be a circle, but if unequal in two perpendicular directions, it must
be an ellipse.
It was thus ascertained that in some rhombohedral cr^tak the
crystallographic or optic axis (24, V. ; 1203) was the direction of
greatest conductivity, and that in a plate cut perpendicularly to
the axis, the conductivity was equal in all directions ; in this case,
therefore, the thermal, like the optical (1209), wave-front must be
a spheroid.
In crystals belonging to the cubic system (24, I.), whSch are
optically anaxial, the isothermal line ought to be in aU directiona
a circle, and the thermal wave-front a sphere; while in biaxial
crystals, the thermal wave-front would probably in all cases be
found to be an ellipsoid.
A similar temporary variation of conductivity has been obeerv^d
by Dr. Maggi to be produced by magnetic energy in an iron plate
resting on the poles of an excited electro-magnet : the plate being
amilarly coated with wax, and heated bv steam traversing a pipe
passing through its centre. It was founa that the melted sorfaoe
presented an ovial form, the long diameter of which was across
-. _g the lines of direction of magnetic energy ;
^^' ' thus showing that thermic waves meet with
increased resistance in the direction in which
magnetic waves are already travelling.
1378. Li()uids are very bad conductors of
heat ; on this account, if water be froaen at the
bottom of a test tube, more water poured npon
the ice may be made to boil by holding a
spirit-lamp near the upper part of the tube, as
in Fig. 688, and yet the ice will remain unmelted. U, however,
C03IDUCJTIVITT OF BODIES. 777
the heat he applied to the lower part of the tuhe, the ice will
speedily melt, and the whole he made to hoil ; not, indeed, from
heat heing oondacted upwards, hat from the ascent of heated
particles of water from the bottom of the tube, on account of their
being specificallj lighter than the colder portion to which thejr
give place (1382), and thns the whole of the fluid becomes heated
Djr the constant interchange of position of the particles.
1379. Gaseous bodies conduct heat even worse than liquids;
this property is frequently employed to confine or exclude heat, as
in the double door of furnaces, in the double casing of iron sates,
and in the double windows now so frequently used m houses ; the
layers of air confined between the cases being the best possible
barrier to the transmission of heat by conduction. It is on this
account that the contact of very hot air can be endured by the
human body, whilst exposure to a liquid of the same temperature
would produce intense pain.* But if the heated air impinge upon
the surface a» a current^ then its contact will be intolerable, in
consequence of the repeated application of fresh portions to the
surface. The same remark applies to intensely cold air ; thus in
the Arctic regions, men have been exposed to a degree of cold
below that of freezing mercuiy without injury, so long as the air
is calm, but upon the slightest wind occurring, the repeated con-
tact of fresh portions of cold air will carry on so much heat as to
freease the extremities, if they be not most carefully protected.
^ 1380. A practical application of the badly conducting power of
air is found in the various articles of dross, which are generally
wanner in proportion to the quantity of air entangled in the in*
terstices of the material of which they are composed. Count
Bumford suspended a thermometer in a glass tube, and interposed
between it and the bulb of the thermometer the substance, the
conducting power of which he wished to determine. The whole
was first plunged into boiling water, and then removed into meltine
ice. The time required for the thermometer to cool from lOO"* F.
to 54*5" was then noted in seconds, which thus became a com-
parative measure of the conducting power of the body, by which
it was surrounded. In this way he found that when air was alone
interposed it required 676 seconds to cool down to 54*5**: but
when the bulb was surrounded with equal quantities (16 grains
of yarious substances, the times of coolmg were as follows : —
Sewing silk . . . 817*
Finelmt .... 1032
Cotton 1046
Wool 1118
* The wrirer oooe remained for serenl minates in a drying-store heft^ed
to WP F. The sensible effeot of this hi|(h te-Dperiftture wns a momenUrT
priekiDfl; sensation over the exposed sorfiioe of the faoe and hands, followed
Dj proflue perspiration.
Baw silk . .
. . 1284'
Beaver's fur
. . 1296
Eider down
. . 1305
Hare's fur
. . 1315
778 TBBRiiicai.
A beaotiful iUuBtration of this jyropertj ocean in tlie chanffO
of clothing of many animals ; hair being in winter, and in tLe
Arctic repons, n^plaoed by wool ; and feathers, by down.
1381. THa familiar fact that a piece of iron or marble always
feeU colder than wood, flannel, or fnr, although at the same tem-
perature, is explained by their different conducting powers. Thus,
iron being a good conductor, rapidly absorbs heat from the hand,
and hence feels cold : whilst a piece of fnr, or woollen cloth, being
a bad conductor, does not remote heat so rapidly, and thus feels
comparatively warm. That the temperature of these bodies is
really the same, may beproved by means of a thermometer.
1 3iB2. OonveeUon of Meat. — ^As all bodies, the particles of which
are widely separated from each other as in fluids or gases, are had
conductors of heat, it is obvious that when heat is applic»d to one
part of their surface, its rapid communication to other portions of
fluid must depend upon a process distinct from that of conduction.
This has been illustrated oy the case of a fluid heated in a tube
(1378), where the difiiision of heat depends upon the ascent of
heated particles; a process conveniently termed conofcCvm. When
air is heated it ascends, because it becomes specifically lie'hter than
the surrounding colder portion ; this is the rationale of tne balloon
contrived by Montgolfier, which consisted of a lai^ air-tight hag,
having its open mouth downwards ; beneath this a fire was main-
tained in the car, which rarefying the air in the bag rendered the
whole machine specifically lighter than the surrounding medium,
and it oonsequentl v ascended. The heated air at the eouator thns
asceiids, and travels towards the poles, whence an unaer current
of cold air passes towards the tropical regions of the earth (1886).
1388. As heat is diffused through fluids by the process of con-
vection, it follows that whatever diminishes, or interferes with,
the mobility of the particles of fluid, will prevent the rapid trans-
ference of heat from one part of the liquid mass to another.
On this account viscid fluids, as water to which starch has been
added, require a longer time to boil than pure water ; and conse-
quently a longer time to cool.
1884. Hot-waJter Apparatus. — ^The convection of heat by
water is the principle on which various apparatus are constructed
for the purpose of conveying or distributing heat bv the circnla-
tion of not water, in pipe* or other closed channels. For this
purpose one extremity of a system of pipes opens into the upper
part of a closed boiler, and the other extremity into the lower
part; and in order that the cooled water may, by its mater
specific gravity, displace the warmer portion of the fluia, it is
necessary that no part of the circulating system should be below
the level of the boiler. This method of communicating heat may
be familiarly illustrated by the apparatus, Fig. 689, in which a, a,
are two pieces of fflass tube bent at a right angle ; o, d, two re-
tort receivers, of which c has a lateral aperture, and x is a spirit
cientlj indicated. Tbe whole apparatai
being; aupported on t, retort Bt*ad, snd
the Ump K applied to D somewhat late-
rkllj, u in the EKQre, a circnlatinn of
the Snid in tbe direction indicated by
the arrow! will apeedily commence, and
maj be rendered manifeBt hj filling
the tube* and lower BagV wiih water,
and haiins inserted tbem in tbe npper
one, by filling that with water coloured
with a little aolphata of indigo or other
con lenient colouring matter.
The coDYection of heat in caeeona
Bnida may be readily ufaown by the
«une appuatna filled wilh air only, if
the upper globe h» held in the path of
a beam of electric light transmitted
ibroagh a lens, and thrown on a screen :
the flow of heated air will be rendered
1386. Law of'Convectioa by Oatti.
It appean from some ablo researuhes
by HM. Dulong and Petit that the rate of convection by a gna is
entirely independent of the nalnre of the aarrace of the heated
bod;, and is proportional to a conitant power of the diflerence of
s also found that the
if temperature
temperature of tbe body and gas. it was
cooling power of a given gas lor a giren e
depends on its pressure only.
The following formohe for the qnantitiei of heat conreyed, by
certain gases at a presaare of p™*, in one minnte from oDe square
foot of any surface heated t'G. abore tbe gas in contact with
it, expressed in thermic units (1372), have been emprically
detenoiued : —
Hydrogen . 0-0183(^J°"l'«»;
Olefiant gas . 0-0071 (^) t'™:
Air. . . .0.0M3(,^„)2-"i
CarboDicaeid 00368/a^]"" t'-™;
1386. The trade-mindt, and the Gvlf-tlrtam, by which tbe
climate ofdiSenint portlous of the earlh'i surface is so matenaUj
iuflueooed, are large results due to the couTectiou of heat. Tu
780 THEBMIGS.
mass of ftir, beated by contignitj to tbe surface of tbe eartb in the
Tropics, rises continually to tbe upper regions, and its place is
supplied by under currents flowing towards tbe torrid from the
temperate zones. These masses of air proceed from localities of
less to one of greater linear velocity of rotation, and themfore lag
behind the earth's suiface, t.tf., tbe^ have a relative toegterlv
motion, and by the composition of motion (284), the currents north
of the equator will be from N.E., and those south of that line, from
8.E. ; these two currents constitute the trade-winds. SimilaHy
the heated air moving in the higher regions from the equator
towards the poles has both its absolute and relative motions in
opposite directions to the former, and giires rise to S.W. currents
in tbe northeni, and to N.W. currents in the southern hemisphere ;
these constitute the retum-tradei. Tbe direction of those has
occasionally been demonstrated by the course of a cloud of volcanic
ashes thrown up into the upper regions of the atmosphere.
Tbe well known land- ana sea-breezea of summer may be simi-
larly explained: the air being more warmed by radiation from
the earth's surface, than from tbe water, continually ascends, and
is replaced bv cooler air from the surface of the water, forming the
sea-breeze : but in the evening cooling is greatest over the land,
and bonce the land-breeze.
Similarly the equatorial surface-water, being of less density,
flows across the Atlantic from Africa to the West Indies and the
Gulf of Mexico, and is carried direct north-east to tbe shores of
Ireland and Great Britain, and on to Norway.* To this is owing
tbe immense difference between the climate of the north of Canada,
and that of the south of Ireland, which are in nearly the same
parallel of latitude, and therefore similarly circumstanced as to
solar beat.
1387. The relative distribution of heat on the earth^s sarfaoe
has recently been the subject of elaborate investigation ; and by
connecting the points ascertained to possess the same mean tem-
perature, a copious series of isothermal lines has been laid down
by F^of Dove. These have been published under the auspices
of the British Association, and form an important contribution to
physical geography.
1388. Specific Heat, — ^All bodies do not appear to have the same
capacity for heat : thus, water requires nearly twice as much heat
to raise it to a given temperature, as an e^ual weight of ether;
hence water is said to have a greater capacity for heat than ether.
All bodies thus possess a property denominated their spe^c Aeat,
indicating the comparative amount of heat required to raise them
to a given temperature.
If equal Quantities of the same licjuid at different temperatures
be mixed, ttie temperature of the mixture will be the mean of the
* Humboldt, Cosmofl^ vol. 1.
CALORlMETEKa. 781
two ; thos, if a pomid of water at 60" be mixed with the same quan-
tity at 212°) the mixtare will possess a temperature of 136°. But
if equal weights oi different fluids be mixed, the resulting tempe-
rature of the mixture will not be the mean of the two. A pound
of mercury at 40^ mixed with the same quantity of water at 156*,
will produce a temperature of 152*3*. Thus, while the tempera*
tare of the water is only depressed 8'7*, enough heat must have
been evolved to raise the temperature of the fluid metal 112*3*.
Then if the capacity of water for heat be assumed as the standard
unit, that of the mercuiy will be but 0'033, for \
112-3 : 3-7 : : 1 : 0*033. I
A bar of copper weighing a pound, heated to the temperature
of 300**, and immersed in a pound of water at 50*, will give up its
excess of heat to the water, and both will acquire a temperature
of 71*7*. The copper has consequently lost 228*3* and the water
gained 21*7*, and hence the specific heat of copper b 0*095, for ^
228*3 : 21*7 : : 1 : 0095.
Ihe mode of proceeding for the determination of specific heat
pursued in these examples has been termed the tnethoa of mixture,
1389. Calorimeters. — ^This term has been applied to apparatus
employed in determining the specific heat of bodies. In the calo-
rimeter of Lavoisier and Laplace the result is obtained by observ-
ing the quantity of ice liquefied by a body of known weight at a
known temperature. For this purpose a triple cylindrical metallic
vessel is employed, the spaces between the inner and middle, and the
middle and outer vessels are filled with fraCTients of ice, and have
separate outletsfor the escape of the liquefied fluid. Hie body having
been raised to a given temperature ^generally 100* C.) is rapidly
transferred to the empty inner cylinaer, its cover applied, and the
remaining space in the middle cylinder filled in with ice ; this is
then covered, and the outer cylinder similarly filled up with ice
and covered. The only object of the outer jacket of ice is to pre-
vent that surrounding the inner cylinder from liquefaction by
external heat. The amount of ice liquefied by the heated body is
measured by the quantity of water escaping from the middle
space ; and this is an approximate measure of the specific heat of
the body enclosed in the inner cylinder.
This apparatus is liable to several sources of error, the chief of
which is that the whole of the water does not escape, a portion of
nncertain amount remaining adherent to the fragments of ice and
the surface of .the vessel, llie method here employed has been
designated the method of cooling.
1390. More reliable results have been obtained by the calori-
metere of M. Begnault and Prof. Kopp. The former of these
consists of a closed hollow cylindrical vessel, the internal diameter
of which is about one-third the external, supplied with steam from
a small boiler, having also an outlet pipe for the condensation of
782 THBRMIC8.
the steam in a wonn-tub. This cylinder is enveloped by ftn air-
tiffbt jacket to diminish the loss of beat by external radiatioo.
The body to be examined, being reduced to fragments, is placed
aronnd the long cylindrical bnlb of a thermometer in a arenlar
basket of wire gauze, which is suspended by silk threads in the
central cavity : the stem of the thermometer passes throngh a
cork plug by which the top of the inner cylinaer is cloeed, and
which supports it independently of the basket
When the thermometer has attained a constant temperatnre
(which requires from 2 to 3 hours) a very thin brass vessel, con-
taining a Known weight of water, and suspended by silken cords
to three uprights on a stand mnidng on castors, is placed beneath
the former vessel, and a valve being withdrawn from the bottom
of it, the wire basket and its contents are rapidly lowered into the
water-vessel ; this is then removed, and the water stirred to
equalize the heat acquired from the heated body, and its tern-
Sirature accurately noted, and from the heat gained by the water
e specific heat of the body may be readily calculated.
1391. Prof. Kopp*s process differs somewhat from the pi^
ceding : the body to be examined is placed in a test-tube ; and if
solid, together with some fluid in which it is^ not soluble, to aid
the conduction of heat to every part of the solid fragments. This
supported by a wire inserted in the cork, is immersed in a cylinder
containing mercury, which is surrounded by a jacket of oil, which is
heated by a spirit-lamp, until a thermometer placed beaido the
test-tube* shows a constant temperature.
When the constant temperature (which should be about 50* C.)
is attained, 10 minutes were found to suffice for the tube waA
its contents to assume the same temperature : it is then rapidly
transferred to a water-vessel similar to that of Regnanlt, and tho
water is stirred by a pair of horizontal plates moved np and down
by a wire to which they are attached. It was found desirable to
have the temperature of the water-bath a little below that of the
sir, so that its maximum of derived heat should about bring it np
to that of the surrounding atmosphere.
If, in regard to gain or loss of heat, the glass in contact with
the water be equivalent to x parts of water, and
/, the weight of the liquid m the tube,
pf the specific heat of that liquid,
ffi, the weight of the solid,
M, the weight of the water, together with the equivalent in
water of the immersed thermometer, and the vessel, and
stirrer;
* In Prof. Kopp*s drawing (Phil. Trsai. 1866, pt i.) the thennomefcr bss
a tpherieal bulb : in all cmm in which the tempentiire of a fluid b^Ui it
required to be aooarately determined, the writer prefim a long €fU»dr»eml
bnlD, by which the Tarying temperaturea of diflbrant parts are iniegrated,
or redaeed to an aTarage.
■PBGIFIC HEAT.
783
- T, the teroperatare of the mercar j bath ; and
t/ that to which the contents of the tnbe are estimated to sink
in the water;
tf that of the water-bath before, and
I', the maximnm after the immention of the tnbe ; then
sp. heat=.-^(^-^)-l^t/^)iJ^O
The flnid employed in the tabes was coal-tar naphtha carefnllj rec-
tified for this parpose.
This methoa has an advantage over that of Regnault in enabling
the experiments to be made much more rapidly : but as very much
smaller quantities were operated on, the errors of observation
have a greater relative value ; and the results obtained by Regnault
must b« looked upon as the most trustworthy.
1392. It was established by Dulong and retit, that the specific
capacity of bodies for heat increases as their temperature rises, so
that it requires less heat to raise a body from 100* to 105", than
from 200"* to 205", although in either ease equal increments of
temperature are indicated by the thermometer. Thus, the specific
heat of water at 0" C. being taken as 1, that of water at 100" will
be 1*01. The variation of specific heat is generally small at a
considerable distance from the point of fusion ; but it increases
rapidly where sensible softening commences.
The following table gives the specific heat of several bodies,
chiefly from the accurate experiments of Begnault, that of water
being taken as unity : —
Alcohol . .
0-6603
Graphite .
0-2018 j
' Copper
00951
Ether . .
0-6207
Glass . .
0*1977
ArHenic .
0-0814
Nitric acid
0-4425
Phosphorus
0-1895
Silver . .
00570
Oil of turp.
0-4259
Diamond .
01468 ;
[Tin . .
0-0562
Sulphuric ac
8o<uum. .
. 0-3330
Manganese
0-1217
Antimony
00507
0-2934
Soft steel .
01175 ;
; Thallium .
00336
Magnesium
0-2499
Iron . .
01137 :
Mercury .
00333
Charcoal .
0*2411
Nickel. .
0-1086
; Platinum .
00329
Aluminum .
0-2143
Cobalt .
01069 1
Gold . .
00324
Sulphur
0-2026
Zinc . .
00955
; Bismuth .
00308
In this table may be noticed the remarkable difierence in the spe-
cific heats of carbon under its allotropic forms of charcoal, graphite,
and diamond. As the specific heat of this crystallized body is
considerably less than that of the other forms, and as it may be
reduced to somethine between these forms by the energetic action
of heat (althongh tne converse transformation has never been
effected), it is not improbable that the latent conversion-heat in
this case ma^ be analogous to the so^alled '* latent'* heat effusion
in other bodies : and it is further probable that in all cases the
augmentation of specific heat is really due to softening-heat ; for
784
THERMICS.
it is well known that the loss of elasticitj (in a steel spring, for
example) is gradnal from tbe lowest temperatures. Possiblj some
other changes of the apparent specific heat may be similarly
accounted for.
An extensive table of the specific heata of various substances
has been given by Prof. H. Kopp.*
1393. influence of TempercUure.— It has been determined by
the investigations of MM. Dulong and Petit that the spec^ heat
ofeoUdi incretuee with the temperature.
llie following table shows the mean specific heats of varioos
substances between 0* C. and lOO"*, and between 0** ( \ and 300*.
Sobstanoe.
O^C.to
100°.
0»C.to
300P.
Bubitaiioe.
O^Cto
lOOP.
CPCio
Glass . .
Iron . .
Copper .
Zinc . .
0-1770
0-1098
0.0949
00927
01990
0-1218
0-1013
0-1016
Silver . .
Antimony
Platinum
Mercury .
00567
00607
00355
00330
00611
0-0549
0*0355
0-0350
The specific heat of water at various temperatures has been
investigated by Regnault, who has given toe following mean
values of the specific heats, that at 0" C. being 1 : —
0•C.to40^..1•0013j0''C.tol20^..1•0067i0•C.to200^..1•0160
„ 80...10035J „ 160 ...10109J „ 230 ...1*0204
1394. The specific heat of gases and vapours has been investi-
fated by difierent physicists. The process generally pursued
as been to heat the g&B to a given point, and to observe how
much in cooling to a given temperature it raised the temperature
of water through which a current was^ conducted by means of a
spirid tube. Another mode was contrived by Dr. Apjohn, which
consists in vaporizing water by a current of the heated gases,
when the latter will be cooled with a rapidity inversely propor-
tionate to their specific heats.
But the most accurate observations on this subject have been
made by Regnault, from which he has deduced the following
laws : —
1. The epecificheat of a given weight ofgae doee not vary with
the temperature.
2. The tame does not vary with the preeeure, and Aenoe the
spfcifie heat of a given volume varies with the density,
8. The specific heats of equal volumes of incondensibie gases
are equal: but neither of these laws holds good for the rmiiiy
oondensihle gases.
Some of Kegnau1t*s results are given in the following table, in
which the specific heats are those of equal volumes,
• PhiL Trans. 1805, pi. i.
LATEKT BEAT. 785
Hydrogen . 0'2359 ' Oxygen . 0*2405 J Marah gas . 0-3277
Nitrogen . 0*2368 Nitnc oxide 0*2406 Carbonic acid 0*3307
Carbonic oxide 0*2370 Chlorine . 0*2964 Nitrous oxide 0*3447
Air . . . 0*2375 Ammonia 0*2996 j defiant gas 0*4160
1395. Atomic Heat. — It was considered by MM. Dulong and
Petit* in 1819 to be an established law that the moleculea of aU
BimpU bodies have exactly the same capacity for heatj or in other
words, that the product of the atomic weight and the specific heat
is a constant quantity ; this they showed to be true in a great
many instances. It was further shown by Neumann in 1833 that
in analogous chemical compounds the products of the atomic
toewhts and the specific heats are approximatdy equal.
FtoL Kopp, from an extensiye series of observations on nearly
200 different chemical compounds, has shown conclusively that
(taking the double ec^uivalents for those " simple" bodies which
purely chemical considerations have indicated} the law of Dulong
and Petit holds good for 41 out of 49 recognised elements ; and
that if for these 6*4 be taken as the atomic heat (the product of
the specific heat and the atomic weight), and 5*4 as that of 2 S
and P, 5 for Fl, 4 for 20, 3*8 for 2 Si, 2*7 for B, 2*3 for H, and
1*8 for 2 C, the law of Neumann then agrees very closely with
the observed results.
1396. The capacity of gaseous bodies for heat bears an inverse
ratio to their density; thus, according to MM. Clement and
Desormes, a given weight of steam contains the same amount of
heat at all pressures. The following are experimental illustra-
tions of 'this law : a sufficient amount of heat is disengaged from
air, violently compressed by a piston in a closed cylinder, to ignite
anj readily inflammable matter; an apparatus for this purpose
18 well known. The hand will be speedily scalded by steam
emerging from the spout of the tea-kettle, but a jet of high-pressure
steam will not produce the same effect, the temperature of the
whole being too much reduced bv expansion.
If four or five atmospheres of moist air be condensed into a
strong vessel, and a sufficient time elapse for the whole to cool
down to 50** or 60** F., on allowing a small escaping current to
impinge on a bad conductor, as a thin glass flask, a small particle
of ice will be deposited : this arises from the increased capacity
for heat in the expanding air, whereby it is able to deprive the
accompanying vapour of its heat, and thus reduce it, not only to
the fluid, but even to the solid form.
1397. Latent Heat. — As the mixture of equal quantities of
water at different temperatures possesses the temperature of the
mean (1388), it follows that when a pound of water at 32** is
mixed with a pound at 172*, the mixture ought to be of the tem-
perature of 102**; and experiment proves that such is the case.
• Ann. Ch. Fhyi. [2] vols. vii. tad x.
3 E
786
THSBiace.
But if a poond of ponnded ice or diy mow at 32^ be added to the
same weight of water at 172**, the mixture will be found to poaeen
a temperature of only 32° : it is therefore obvious that some cause
must exist to account for this apparent loss of 140 degrees of heat,
differing entirely from the law of specific capacity already ex*
plained. The heat that has disappeared must have been absorbed by
the ice, in passing from the solid to the liquid state, yet without m-
creasing its thermometric heat ; hence the 140 degrees of heat
must have become concealed, or latent in the water.
The phrase " latent heat" is rather an unfortunate one, aa it
has conduced to, and corroborated, much misapprehension as to
the real nature of the phenomenon. ** Latent'* neat has ever been
held up as the great stumbling-block of the dynamic theory,
because it is impossible to conceive motion to be reduced to a
state of quiescence, but remaining still ready to start again into
action, xhis, however, is merely a confusion of ideas : the fact
being that when any substance passes from the solid to the liquid,
or from that to the gaseous form, a certain portion of the imprcwsed
heat-force is continuously occupied in overcoming molecular attrac-
tion, and thereby effecting change of form ; and cannot be im-
parted to other bodies, so loug as the change of form is maintained.
1398. If a vessel of water be exposed to a freezing temperature,
a thermometer immersed in it will gradually fall to 3i2* ; when
the water begins to solidify, the thermometer will indicate no
further depression of temperature until the whole quantity is con-
verted into ice, yet it must, duriog the entire process, be evolving
that heat which, when in the form of water, preserved it in the
liquid state. If, then, the ice be placed in warm water, it will
absorb heat, thereby assuming the form of water ; whilst, as before
shown (1397), the temuerature of the resulting mixture will not
exceed 32**, the original temperature of the ice. Theae discoveries
are due to the researches of Dr. Black.
1899. The comparative quantity of heat rendered "latent**
during liquefaction has been in many cases verv accurately
determined. The following table shows the results of some recent
experiments by M. Person ; the first column of figures shows the
amounts expressed in thermic units (1372) of the latent heat of
1 lb. of eacn substance, and the second, the ratios of the latent
heat to that of water taken as unity : —
Bubstanoe.
Th. units.
Bstios.
Th.ii]uti.
Ratios.
Water . .
79-25
1-000
Cadmium .
13-66
0172
Nitr. soda .
62-976
0-794
Bismuth .
12-640
0-159
Nitr. potash
47-371
0-598
Sulphur . .
9*368
0-118
Zinc . . .
2813
0-356
Lead. . .
5-369
0-067
Silver . .
21-07
0266
Phosphorus
5034
0-068
Tin . . .
14-252
0179
Mercury .
2-83
0-035
DIBTILLATIOH. 787
1400. The remarkable absorption of beat prodaoed bj the
liqoefaction of solids, eoables us to produce extreme degrees of
cold at pleasure. Thus, if a quaotity of nitrate of potass be stirred
into a quantity of cold water, an intense degree of cold is produced
In consequence of a large amount of heat being absorbea, which
becomes latent in the solution. A mixture of snow and common
Bait rapidly liquefies, and absorbs as much heat during the process
aa to furnish us with a very available mode of producing low
temperatures: the zero of Fahrenheit was determined by this
freezing mixture, as being the lowest temperature he was able to
produce. If chloride of calcium be substituted for the salt, so
great a depression of temperature is produced, that mercury may
uius be readily reduced to the solid state.
1401. The evolution of heat in a sensible form occurs whenever
a fluid becomes solidified. This may be shown by pounng a
boiling saturated solution of sulphate of soda into a flask, and
securing the mouth bj tying over it a piece of moistened bladder.
When cold, the solution will retain its liquid state without pre-
senting any appearance of crystallization, until a hole is made in
the bladder, when in an instant crystals will begin to shoot, the
fluid will become nearly solid, and so much of the latent heat will
be evolved, that the vessel will feel sensibly warm to the hand.
1402. Whenever fluids assume the gaseous state, an analogous
conversion of sensible into latent heat occurs. Thus, if water be
expoeed to heat in an open vessel, it will on attaining 212° boil,
and evolve considerable volumes of a gaseous vapour or steam,
but during the whole time the ebullition continues, although re-
ceiving fresh heat every instant, neither the temperature of the
water, nor of the steam, will much exceed 212**. The enormous
quantity of heat thus absorbed bv the steam and becoming latent
in it, may be rendered sensible by causing it to traverse a tube
surrounded by cold water : the steam in condensing will give tip its
latent heat to the water as sensible heat, the increase in tempera-
ture of which will be an index of the quantity of heat latent in steam.
1403. DUtiUation. — ^The practical process by which a volatile
fluid is separated from less volatile matter is termed distillation : it
consists in boiling the mixture in a closed vessel, when the most vo-
latile substance present, t.s., that which at the^ existing atmospheric
pressure is wholly converted into vapour at the lowest temperature,
will first be evaporated. The vapour passes into a tube coiled spi-
rally in a vessel of cold water, commonly called the " worm-tub,*'
and is there condensed, and flows into some convenient receptacle.
For distillation on a small scale in the chemical laboratory,
Liebig^s condenser is generally employed ; this consists of a
straignt tube from one to two feet long, surrounded by a "jacket,"
through which a continuous stream of cold water is poured.
When several difierent fluids, as for example the hydro-carbons,
whiph boil at temperatures differing but a few degrees from each
other, are thus separated, the process is termedyractumol diatiUatioTi,
3e2
788
THBBinCS.
1404. Drv /^eam.'^The term " steam*' w ambigaonslj ased :
if this tenn oe meant to designate the trae vapour of water under
any pressure not less than that of the atmosphere, then what we
Bee issuing from a tea-kettle or a locomotive engine, which in
common parlance is called steam, is not really such, but merely
minute particles of water suspended in the air, just as the Tisihfe
so-callea vapour of a 8ummer*s evening has ceased to he vapour
when it became visible. The invisible vapour of water at or above
the boiling-point is technically termed dry steam ; and if it be
raised to a nigher temperature than that necessary to maintain
the state of vapour, it is called superheated steam.
A jet of visible steam from a closed boiler may be at any point
reduced to vapour and rendered invisible by burning a gas-jet
under it ; and the vapour of water under the receiver of an air-
pump may be instantly rendered visible by a few strokes of the
pump, which cools the contained air and vapour by expansion,
oelow the temperature at which the whole can remain as vapour.
1405. If steam be conducted into eight ounces of water until its
temperature is raised from 60° F. to 188^ and the whole when mea-
sured be found to be nine ounces, it is obvious that the latent heat
of the vapour of one ounce of water has been able to raise the
temperature of eight ounces from 60* to 188**, or 128^ But as
there were eight ounces, the whole heat when contained in the
vapour of one ounce was equal to 128 x 8 = 1024*. This must not
be regarded as all latent heat ; for the steam while condensing
shoull have formed water of 212*, whilst the temperature of the
whole was only 188* ; hence, as 212—188=24. we must, to get
the true proportion, deduct this from 1024, whicn leaves 1000*, as
the measure of the latent heat of steam. It is the enormoas
quantity of heat thus taken up in steam that renders it so import
tant as a heating agent. One gallon of water converted into steam
will c6ntain sufficient heat to raise 6i gallons from 32* to 212*.
Any other vapour, even presuming that it could be as readily
procured as steam, would not be so efficient as a heating affent,
in consequence of its containing a smaller quantitr of combined,
or "latent '* heat. The following table contains the numbers re-
presenting the latent heat of vapours ; derived from the researches
of Favre, Silbermann, and Andrews : —
Yspour of
Th.aiiita.
RatiOB.
Taponr of
Th.iuiiti.
Balioa.
Water . .
535d
1000
Sulph. ether
90-46
0169
Wood spirit
263-7
0-492
Bisul. carbon
86-67
0162
Alcohol . .
202-4
0-378
Ozal. ether
72-72
0136
Acet. methyl
110-2
0-206
lodid. ethyl
46-87
0087
Formic ether
106-3
0196
„ methyl
46-07
0-086
Acet. ether
92-68
0-173
Bromine .
46-60
0086
BBOELATIOir. 789
1 406. The expansion of fluids in yolame on assnming the state of
vapour, generally decreases with the amount of heat latent in the
▼aponr. Thus, a cubic inch of water is converted into 1689 cubic
inches, or nearly into a cubic foot of steam ; while the same
quantities of alcohol and ether become respectively 493*5 and
212*18 cubic inches of vapour.
1407. llie temperature at which a fluid assumes the form of a
solid, or vice verad^ differs materially in different substances ; this
temperature is known as the congealing or melting point of the
bod^, and for the following substances this point is shown in the
subjoined table : —
Nitrons acid— Se*" F. ^ Acetic acid . . 60' P. ; Tin . . 45rF.
Ether. . .—47 | Phosphorus . . 1 U '5 < Bismuth . 512
Mercury. . — 38 | Potassium . .136 ! fjead • . 620
Sulph. acid.-30 Wax. . . . 149 Zinc . .680
Bromine. .+ 9'5 | Sodium . . . 2077 | Antimony 810
Milk. . . + 28 3Sn+4Pb+8Bi210 Silver . 1832
Water . . + 32 j Sulphur . . . 239 } Gold . 2282
1408. Influence of Pressure on the Melting Point. — ^In a body
that, like water, expands in becoming solid, it might readily be
anticipated that under considerable pressure the necessary expan-
sion would be retarded, and that hence a lotcer temperature
would be required for solidification ; and conversely, that in a
body which contracts in becoming solid, the effect of pressure
would be to aid the conti'action, and therefore to aceelerctte the
solidification : and both these results have been verified in prac-
tice. It was found by Sir W. Thomson that under a pressure
of about 17'**, the freezing point of water was lowered by J of a
degree. On the contrary, Prof. Bnnsen found that under a pres-
sure of 156*^, the point of congelation of spermaceti, which con-
tracts in solidifying, was raised 6".
1409. Effect of Mixture of Elements. — ^As a j^eneral rule the
alloys of different metals are found to become fused at a lower
temperature than the average of their melting points : of this a
conspicuous example, the " fusible metal,*' is given in the pre-
ceding table (1407). But this is not confined to metallic mix-
tures, for silex accelerates the surface-fusion of iron, as in the
process of " welding ;" and borate of soda equally aids the fusion
of " spelter" in the process of " hard-soldering." Saline or other
substances having this power are termed fluases.
1410. Begelatton, — The remarkable tendency to cohesion in
two masses of meltine ice was first noticed by Faraday ; he
showed that if a slab of ice were simply superposed on another,
they would cohere, even if immersed in warm water. This action
is probablv analogous to the welding of two pieces of iron, depend-
iug on a plastic or viscous condition of the immediate surface, in-
790 . THEKMICB.
termediate between tbe solid and flnid states. The slow Init
ever-progresBing glacier-motion has been no doubt rightly attri-
buted to this plastic property of ice : and Prof. Tyndait has shown
that a flat case of ice may be converted into a hemispherical cap
by simple pressure in a mould, a very short time being required
for the regelation of the displaced and compressed fragments.
It may be remarked that generally fluids may be cooled several
degrees below their point of congelation, without assuming the
solid form, when their particles are perfeclly quiescent ; some dis-
turbaoce being requisite to initiate the change of state. The
same remark is applicable, but in a less degree, to the point of
ebullition, under a given pressure.
1411, .Ebullition. — ^The temperature of the ebullition of flnids
varies accordiDg to the pressure to which they are subjected, and
is not a fixed point like that of congelation. Fluids boil at a
lower temperature when the pressure to which they are sulgected
is diminished. In an ordinary air-pump vacuum, ether wiU boil
at 38"* F., alcohol at 49^ and water at 88J. If a tumbler of hot
water be placed under the receiver of an air-pump, and the air be
partially exhausted, the water will speedily begin to boil ; and the
same result may be shown in an apparently more paradoxical
manner by filling a flask half full of water, boiling it b^ a lamp
until the air is mostly expelled, and then closely corking it : if the
flask be then completely immersed in a vessel of cold water, it will
immediately begin to boil, by the condensation of the vapour, and
consequent diminution of the pressure on its surface. Hence it
appears that on ascending a lofty mountain, the boiling-point of
water will fall continually : at the summit of Mont Bumc, that
gdnt was found by Saussure not to exceed 187" F. In fact, tiie
11 of this boiling-point has been made available as a means of
measuring the height of mountains. This is effected by means of
an apparatus devised by Archdeacon Wollaston, a thermometer
with a lar^ bulb and a very fine bore, so as to give a veiy open
scale, ran^ng from about 182** F. to 212° ; or still better by three
separate instruments, each comprising about one-third of this
range. These, with a small boiler (best made to draw out like a
telescope), and a spirit-lamp, are all the apparatus necessai^ for
these hypsometric observations. It has been roughl;^ estimated
that the Doiling-point of water falls 1** F. for each height of 5S0
feet ascended ; wnich is equivalent to a diminution of 0*589 inch
in barometric pressure.
The effect of diminished atmospheric pressure, in facilitating the
evaporation of fluids, has been advantageously applied to phar-
maceutical operations, especially in the preparation of vegetable
extracts, by enclosure in an exhausted vessel ; many of the oi^^anio
chemical compounds being partially, if not entirely, decomposed
by a temperature nearly approaching that of boiling water.
1412. The converse fact is equalfy true that the boiling-point
BOILINO-FOINT.
791
is raised by increasing tbe presRore on ' the flnid. A table of the
temperatures of steam at various pressures of from 1 to 45 atmo-
?>heTes is ffiven in p. 288. This is the principle of the well-known
Spin's digester, a strong iron vessel capable of being closed
air-tight, in which various nutritious animal subeCances may, by
preventing the escape of steam, be submitted to as high a tem-
perature as mav be necessary for the solution of animal matter
not soluble at the ordinary temperature of boiling water.
In order that the " boiling-point" of water may be accurately
defined, the British Commisidoi) on standard weights and measures
have assumed 212** F. to be the temperature of steam under a
Sressure of 29*905 inches of mercury at 32^ in the latitude of Lon-
on ; the pressura corresponding to each tenth, from V below to
1° above that point is given in the following table : —
T«mp.
PreMore.
Tamp.
PrcMore.
t
Temp.
Presrare.
2110
29-816
211-7
29-727
212-4
30-143
2111
29-374
211-8
29*786
212-6
30-203
211-2
29-432
211-9
29846
212*6
30*263
211-3
29-491
212-0
29-906 •:
212-7
30-323
211-4
29-660
212-1
29-964
212*8
30-384
211-6.
29-609
212-2
30024
212-9
30-444
211-6
29*668
212-3
30083 <
213-0
30-506
The following table contains the boiling-points, in Fahrenheit's
scale, of a few liquids at a mean barometric pressure of 29*9 inches.
Chlorid. ethyl. 61*9 j Wood-spirit
Aldehyde . . 69-4 | Acetic ether
Ether . . . 948 1 Alcohol .
Bisulp. carbon 118*6 | Benzole .
Bromine . . 146'4 f Formic acid
149-9 \ Acetic acid . 243*1
164-9 i Oil of turn. . 560*0
173-1 I Linseed oil . 6020
176*8 ! Sulphuric ac. 6400
221-5 } Mercury . . 662*0
The material of which the evaporating vessel is composed,
makes a marked difference in the boiling-point of many fluids,
especially if they be capable of forcibly adhering to its surface ;
thus, water will boil at 212° in a metallic, but at 214° in a porce-
lain vessel : and in a glass vessel the temperature may be nearly
2° higher still.
1413. Prof. H. Kopp has clearly established some remarkable
relations between the ooiling-points of certain organic fluids, and
their chemical composition. He has found, on comparison of the
boiling-points assigned by a large number of observera with his
own calculations, that in many membere of the alcoholic series
represented by the formula C||H«+,0„ and of acids represented
by C«H«04, as well as the isomeric compound ethera, an elemen-
tary difference of p x C,H, is attended oy a corresponding diffe-
rence of 2» X 19** C. in the boiling points : also, that a compound
792 THESlilCf.
ether Cnl^n^* ^^<i <^^ ^ tempemtnre 82* lower than its isomeric
acid. It was also observed that in many cases one substance coo-
taining CV more or less than another, boils at r x l4-y higher or
lower temperature ; which corresponds with r x 5*" lower or higher
temperature accordingly as it contains Hr more or less : and in
many instances that the 'boiling-point of an acid is 63* hi^er than
that of the corresponding metnyl-ether, 44** higher than thai of
the ethyl-ether, and 13° lower than that of the amyl-ether: and
in a few sabstances other than acids, the substitution of C^ H^ for
H is attended by a decrease of 44° in the boiling-point ; and that
of C,H, for H, by a decrease of 63^
It has also been observed that isomeric compounds belonging to
the same type, but not having the same chemical character, have
different boiling-points : these various facts are certainly remark-
able links in the chain of evidence of the correlation of physicml
forces.
1414. In order that ebullition may be freely carried on, it ap-
pears to be necessary that some kind of gaseous matter sfaoold
be present, sgainst the molecules of which the vapour may expand.
When, for example, water is boiled in a nearly closed vessel, as a
flask, the bubbles of vapour at first rise very freely; but as the
air held in solution in the water is exhausted by ebullition, the
escape of vapour becomes fitful and explosive, a higher tempera*
ture being now necessary to overcome the adhesion between the
surface oi the glass and the water.
1415. Spheroidal State. — A remarkable fact connected with
the evaporation of fluids, which has attracted much attention was
first investigated by Leidenfrost. If a few drops of water be
allowed to fall into a metallic cup, as a platinum crucible, heated
considerably above the boiling-point of water, the rapidity of
evaporation will decrease with the increase of temperature of the
vessel above 212°. If the crucible be red-hot, and the drops of
water be watched, they will be observed to assume the form of
spherules rolling about the vessel, and on the temperature of the
latter falling, they will be suddenly dissipated witli a sort of ex-
plosion. The cause of this curions phenomenon seems to be that,
at an elevated temperature the vessel and the water are separated
hy a stratum of vapour, hence the drops of water assume a sphe-
roidal form, as mercury ordinarily does, from the mutual attraction of
their molecules being uncounteracted by adhesion. As the tempera-
ture is lowered, this separation lessens, and at a certain point, Uie
water loses its spheroidal state, comes into contact with tne vessel,
and is instantly dissipated. It is remarkable that water in this
spheroidal condition has a temperature of about 7° below the
boiling-point, although actually rolling over a red-hot surface.
1416. Ether is capable of assuming a similar spheroidal state,
and is thus actually repelled by a red-hot metallic surface.
Iodine, when thrown on an ignited platinum crucible, melts, and
FOBMATIOR OF YAPOUBS. 793
forms a spberoidal mass like a black fluid, rolling over the
enrface of the vessel, and giving off hut a very small quantitv of
vapour. In this state the liquid iodine does not come into
actual contact with the platinum. On allowing the crucible to cool,
contact occurs, and a sudden evolution of iodine vapour takes place.
14] 7. M. Boutign^, to whom we are indebted for many obser-
vations on these curious facts, succeeded in freezing water in a
red-hot crucible, by availing himself of this spheroidal state ; he
made a platinum crucible nearly red-hot, and poured into it an-
hydrous sulphurous acid, and afterwards an equal bulk of water.
The rapid evaporation of the acid caused the conversion of the
water mto a mass of ice, which could then be removed from the
still ignited crucible. Faraday placed in an ignited crucible solid
carbonic acid and ether, afterwards pouring in mercury, when the
latter was frozen in the red-hot vessel. In both these experiments
a thin layer of badly-conducting vapour kept the freezing body
from contact with the red-hot crucible.
1418. TrcmafomuUion of Fluids into Vapaurt. — It has been
observed by M. Caignard de la Tour that if a portion of a volatile
liquid be enclosed in a space not much exceeding its own bulk,
in a strong glass vessel capable of resisting very hirge pressures,
at some certain temperature the whole mass passes suddenly
into the state of vapour and becomes invisible. The existing
pressure, when this chan^ of form takes place, was observed by
means of a communicating tube with a nne bore containing a
globule of mercurv, which, as the pressure increases, is driven up
towanls the closed end of the tube, and the remaining length of
empty bore inversely measures the pressure. The following obser-
vations on the corresponding pressures (expressed in atmospheres),
and temperatures, were made : —
Ether 37-6^* 369-6*'C. j Bisul. carbon 66-6^ 604-6°C.
Alcohol 1190,, 497-7 (Water ?„ 7730
A difficulty in estimating correctly the pressure in the case of
water arose from the solubility of glass in water at that high tem-
perature, by which the capillary bore was altered.
It thus appears that at a certain temperature a liquid becomes
wholly gaseous ; and it has been conjectured by Faraday that
no amount of pressure that we could exercise would restore the
liquid form : he also expressed an opinion that the temperature
of —160'* F. is <ibove the limit of fluidity in oxygen, hydrogen,
and nitrogen.
Dr. Andrews observed that above the temperature of SS^ F. a
pressure of 400*^' was insufficient to liquefy carbonic acid. He
further observed that as the temperature of liquid carbonic acid
rose, the capillary curve bounding the surface of the liquid gra-
dually lost its curvature, became fainter, and finally di^ppeared.
794 THERinos.
1419. The tendency of volatile flaids to eraporate is so ^retA,
that the vapour in riaing will abstract from the flaid a portion of
its heat ; and the rapidity of evaporation increases as the preasare
at the atmosphere is diminished. On this property, the mode of '
freezing water by its own evaporation, contrived by Sir J. Leslie,
depends. Let a shallow porous earthen vessel be filled with
water, and placed over a saucer filled with sulphuric acid, under
the receiver of an air-pump. On exhausting the air, a portion of
the water robs the remainder of its heat to Secome converted into
vapour, which is instantly absorbed by the acid : fresh evaporatiim
then goes on, and at last so much of the heat contained in tne water
is abstracted, that the fluid remaining in the porous vesael is
converted into ice. As the only use of the acid is to absorb the
vapour as soon as it is evolved, and thus to reduce the pressure of
the vapour, anv porous body capable of freely absorbing aqueous
vapour, as freshly dried oatmeal, may be substituted for the add.
1420. Water may be readily frozen in the air-pump vacuum, bv
the evaporation of ether. Let a test-tube be partly filled with
ether and immersed in a much wider one, the mterapace being
filled u|} with water. On exhausting the air, the ether will very
soon boil, and rob the water of its heat so rapidly, that in a few
minutes the tubes will be found to be tightly lirozen together.
^ Ice-making MachtTies. — ^llie evaporation of ether has been prae-
ticalljr applied to the artificial production of ice in considerable
quantities. For this purpose the vapour rising from ether con-
tained in a closed vessel is continually exhausted by an air-pump,
and then condensed and returned into the vessel, and so rapid
is the abstraction of heat by the vapour, that a temperature of
—20" F. may be obtained. Thin rectangular chamben, not much
more than an inch in thickness, containing water, are aorroiuided
by the cooled ether, and their contenta are eoon converted into
square cakes of ice. Machines have been constructed to produce
as much as 10 tons of ice daily : in these the air-pump is worked
by a steam-engine : they have been found an invaluable boon in
tropical climates, where the transport of ice by ships is rarely remu-
nerative, owing to the great waste in transitu.
The machine just mentioned is too cumbrous and expensive for
domestic use; but a very compact and ingeniously-oontrived
machine has been produced by M. Carrd, of Paris. A cylindrical
vessel B, Fig. 690, containing a very strong a(^ueous solution of
ammonia, communicates by a pipe, o, with a jacket, d, capable
of holding the cylinder, e, which contains the fluid to be mnen.
To prepare the machine for use, it is first laid on its aide, to
allow any fluid in d to run back into b ; the vessel, b, is then placed
over a small portable iiirnace (or even over an open fire), wnile d
is kept cool by immersion in the tub, a ; and b is gradually
heated up to about 270* F., at wbich temperature nearly all the
ammonia will have been expelled from the water, and condenasd
wou.i.*TOn*H aKTOPHOcm.
D, nhile tlio water »lone renu
J, -D aid conducti... ,
is snrniuDded with felt, orother good
non-coDdnctor nf heat. Ai the water
Ed b becomei cooled, the ammoDia in
D Tuea io TapOQr, abetracting heat
from the ramainder, and panin^ back
tbrouKh the tube, c, into b, u ab-
MTbed bj the water ; and thie ci
tinnes, until the content* of l I
froien. As it _i> vary delirable
Temore all the air from the interior
the boiler, the firat portion of miied Taponr that rises is allowed
to escape bj a small tabe beneath o into a small vessel codImo-
ing wiier, which absorbs (he Ammonia, and allows the air ta
escape throagh it in bubbles ; and Che escape is permitted to
continne, nntn the air-bubbles cease to appear.
1421. Wotiatkm'i CryophoruM. — The prodnclion of ioe bj the
OTBporationof water (1419) is well shown in an elegant contri-
Tance of Dr. WoUastoD, which he termed the eryophortu or &ost-
bearer; it conBisti of a tube bent twice at nght angles, and
ftimishsd with a bnlb at each
•nd. Fie. 691. Enough water to nf.m..
ueariv half fill one of the bulbs is , ■■ -.
introduced, and after bein(! made JC ~jL
to boil violently for a few minutes, ^^ _J b^d
the apparatus is hermetically ^"^
sealed : thus it containe a quantity of water conSned in vacun, or
nither in an atmosphere of aqueous vapour. If the empty bulb
be placed ia a Ireeiing mixture, the vapour will be condensed, and
a vacnom being tbns restored, further portions of the water in the
other bulb will be evaporoited, and carrying off beat, will past
over into the cooled bulb, leaving at length the reit of the water
(Hmverted into ice. In the constmction oF this inatniment, it is
necanary Chat the bulb containing the wal«r sbould be a little
less than half filled, as the freenng water is likely to burst the
bnlb by its expansion, if more than a hemispherical space be
occupied by the ice.
1422. Although at ordinary pressnres water binis at 312* yet
■low evaporatioD will go on from its aor&ce at any temperature,
even below the fiMdiw point. The vapour thas evolved mixes
vith, and is dissolved by, the ur, which is never absolutely dry,
bnt always contains a certain portion of aqueona vapour. The
wanner the air, the greater the proportion of watery vapoDr it ii
796 THBXMICB.
capable of holding in solution. The pressure of aqneooa Yaponr
at various temperatures has already been considered C519).
1423. The Dew-point. — ^If a solidi surrounded by a mixture of
air and vapour, be cooled down below the temperature corre-
spottding to the density of the vapour in the mixture, the stratnm
iu immediate contact with the solid will be cooled, and the ex-
cess of vapour contained in it will be deposited on the mrface of
the solid, in the form of dew ; which may be made to disappear
by raising the temperature of the solid above that which cone-
Bponds to the density of the vapour.
The lotcett temperature at which the whole of the vapcnr con*
tained in any mixture of air and vapour is capable of remaining
in the elastic state, is called the dew-point. It may be determined
practically by cooling a bulb of glass, or polished metal, and ob-
serving its temperature when dew begins to be deposited on it :
then suffer the temperature of the bulb to be gradually raised, and
observe the temperature at which the dew disappears. These two
observed temperatures are one less, and the other greater, than the
dew-point, but will be found to differ very little from each other,
if the experiment be carefully conducted ; their mean may there-
fore be considered as the dew-point.
1424. DaniePs Hygrometer, — Observations on the dew-pmnt
may be conveniently made by this instrument, which consists of
a pflass tube with a bulb at each end of it, which is bent near the
middle twice at right angles, so that both extremities may be
vertical, and at a re w inches' distance from each other. The bulb
of a small and delicate thermometer is enclosed in one of the
bulbs, which also contains a little distilled water, its stem occupy-
ing the tube ; the other bulb is covered with muslin. When an
observation is to be made, the covered bulb is wetted with ether,
the evaporation of which cools its contents, and thus causes ooo-
densation of the vapour, whioh How begins to rise from the water
in the uncovered bulb, the temperature of which is thus lowered,
as is shown by the enclosed thermometer, which is observed at
the instant that the deposition of dew commences. Afler the
evaporation of the ether, the bulb containing the thermometer
gradually regains the temperature of the surrounding atmosphere ;
and the point of disappearance of the dew may be observed.
Raemtz* considered this to be the best instrument, but it is not
convenient for repeated observations.
1425. RegnauWe Hydrometer. — ^This, which is probably the
most convenient form ot instrument, consists of a very sensitive
thermometer, a, divided in half degrees, the stem of which passior
through an air-tight plug, the bulb is enclosed in a highly polishea
silver bottle, b, which at the time of observation is sufficiently
filled with ether, that the bulb may be entirely immersed. A
* Httdbaeh der Meteorologie.
MA801I 8 HrOKOlffETEB.
797
jPV- 6es.
flexible tube, d, is attached to a mudl silver pipe, that enters the
neck of the bottle, and passes down nearly to tne
bottom of it ; and the cavity of the hollow stem, c,
opens into the bottle at the point of support. To
make an observation, the bottle bein^ carefnlly
wiped quite dry^ and su£BcientIy filled with ether, a
current of air is blown through that fluid by means of
the tube, d ; the temperature of the bottle and its
contents is thus lowered, until a deposition of dew
is observed to commence, when the temperature in-
dicated by the thermometer, a, is recorded, as well
as tbat of the air, by the thermometer attached to
the stem, c. Allowing the instrument to remain at
rest, the temperature marked by ▲ will gradually
rise to that ofthe surrounding atmosphere, and the
point at which the dew dUappears^ is also noted.
7!lie mean between this, ana the point previously
noted, will be the exact dew-point; and the two
observations, if carefully made, will be found to
differ very little from each other.
1426. MoMorCs Hygrometer. — ^The dew-point is
frequently obtained by this instrument, which
consists of two nearly e(]|ual thermometers placed
side by side at a small distance from each other,
on a stand, the bulb of one being covered with muslin, and kept
wet with distilled water. The evaporation of the water from
the muslin lowers the temperature of the covered bulb, and the
amount of depression depends on the rapidity of evaporation,
which itself depends on the dryness of the atmosphere. If the
the temperature of the dry- and H that of the wet-bulb thermo-
meter, p the pressure of the vapour in the atmosphere, j/ the pres-
sure corresponding to the tempejatore f, and II the pressure of
the atmosphere, then, according to the researches of Aug^,
|/=|)-002239(t-f) —5—.
Pr. Apjohn^s formula, which has been frequently made use of in
this country, differs slightljr from that of August.*
It has been stated by rouillet, on the'authority of August, that a
current of air does not affect the result, although it increases the
rapidity of evaporation.
The Pnfekrometer, — ^The thermometer, the bulb of which is
covered with muslin, and kept constantly wet, is sometimes called
a peyekrometer, on account of its being employed to measure the
quantity of moisture suspended in the atmosphere.
1427. The abstraction of heat by evaporation is of great im-
* Uneh UMfbl informatioo on this labject mav be obtained fVom Mr.
Glslshec^i elaborate little work on the Dry- and Wet-bulb ThemometerB.
798 TBBKM1C8.
portance in the animal economy, aa being the principal means by
which vital heat is regulated. In the healthy condition of the
frame, superfluoaB heat is carried off, and the temperature of the
surface lowered, by a copious evaporation of fluid, under the well-
known form of perspiration, secreted by the wonderful mechanism
of the skin, which it would be out of place here to describe. The
oppressive feeling of a damp warm day, and the burning heat of
fever, are alike due to the suppression of this natural process ; in
the former case, by the already saturated condition ol the atmo-
sphere, and in the latter, by the absence of secretion from the lUn.
The absence of injury from the almost fabulously high tempera-
tures, which some individuals are eaid to have sustained, is en-
tirely due to the same cause, dryness of the air being in these cases
an essential condition of immunity; the excessive secretion of
fluid to which the skin is stimulated, carries off by evaporation the
intense heat, as rapidl v as it is imbibed by the surface of the frame.
1428. Transiti4m from the SoUd to the Gaeeous State.— TYaa
transition appears to take place in some substances without the
visible intervention of the liquid state ; this is probably owing to
the extreme volatility of the flnid, which is no sooner formed than
it is dissipated in vapour. Arsenic, benzoic, and carbonic acids,
iodine and camphor, present examples of this change of form. It
is also well known that a gradual evaporation takes place from
the surface of ice and snow : the writer has repeatedly observed
the crystalline structure of a sheet of ice developed on its suriaoe
by evaporation, just as that of a plate of zinc or tin may be, by
the solution of the immediate suiiace by an acid.
] 429. Condensation of Gcuea. — Some gaseous bodies assume the
liquid form from their strong affinity for some liquid : thus the
eager absorption of ammonia by water has alreadv (1420) been
spoken of as a source of cold. Hydrochloric and hydronaorio
acids, and in a much more limited degree, carbonic aad, are ex-
amples of condensation from this cause. It is also highly probable
Uiat the phenomena of transoiration of gases, which have recently
been elaborately investigatea by Mr. Graham, are further exam-
ples of temporary liquefaction by affinity.
All gases are probably capable of liquefaction at sofne tempera-
ture, and by a sufficient amount of pressure, but the necessarj
temperature and pressure vary very considerably. Faraday was
one of the first woo pursued these investigations ; the result of
his experiments has already been given (10).
Caroonic acid, which, it appears, is uquefied at the freenng
point by a pressure of 36**', is frequently employed as the best
known means of producing intense cold, by its own spontaneous
evaporation. The apparatus for this purpose, which was first con-
trived by Thilorier, consists of a strong-closed iron vessel, in
which are placed some substances capable of disengaging carbonic
acid, as, for example, hydrochloric acid and broken wnite marble ;
this communicates with an equally strong cylindrical iron receiver.
COLD PRODUCED BT ETAPOXXTIOIT. 799
in which the gas is collected, and reduced to the flnid state under
its own pressure. When the receiyer is sufficiently filled with
the liquid acid it is allowed to escape by a stop-cock through a
tube entering tangentially the side of a cylinorical box, which
has two tubular apertures in the direction of its axis. A portion
of the issuing liquid is instantly converted into vapour, and carries
off so much heat, that the remainder is reduced to the state of
snow, which is whirled round the circumference of the box by
the force of the jet, while little more than the vapour escapes by
the central tubes.
This frozen carbonic acid, at a probable temperature of — 1 06** F.,
evaporates very slowly in a moderately cool atmosphere ; but
evaporation constantly goes on from its surface, in consequence of
which a portion may be placed in the hand, or even on the tongue
with impunity, being prevented from actual contact with the living
tissue by a film of badlv-conducting vapour, I'ust as in the case of
liquids in the spheroidal state (1415). For the same reason, buys
at an iron-foundry will sometimes step over the molten metal with
naked feet, provided only it be hot enough ! — that is, — to raise
from the surface of the skin a protecting layer of vapour.
In order to bring the frozen acid into contact with other matter,
it must be wetted with ether or alcohol ; the former is generally
employed on account of the additional cold resulting from its evapo-
ration : mercuiT placed on this mixture is rapidly frozen.
B^ placing this mixture of ether and frozen acid under the
receiver of ah air-pump, Faraday attained the probable temperature
of — 166° F. But a much lower temperature nas been reached by
Matterer, who by means of a mixture of liquid nitrous oxide and
bisulphide of carbon, placed under an exhausted receiver, attained
a supposed temperature of ~220''F.
1430. The chemical agency of heat is of the highest importance,
as without its aid, a very large proportion of the results of modem
chemical analysis must have been for ever concealed from us : the
student will find this matter treated of in all works on chemistry.
l*he heat evolved during chemical combination has been carefully
studied by MM. Favre and Silbermann, and by Mr. Andrews ; the
numbers of thermic units due to several combinations of lib. by
weight of various substances, are given in the following table : —
With Oxygen. \ Zinc . . . 1301 s Arsenic. . 994
Hydrogen . 34135 Tin ... 1233
Marsh-gas. 13085 < Copper . . 602
Olefiant gas 11900 j^ruj, Cfhlorine.
Carbon . . 7990
Copper . . 961
Antimony . 707
With Bromine.
Iron. . . 1277
Phosphorus 5747 > Potassium . 26o5 [ t«. , ,
Iron . . . 1745 Wuh lodwe.
Zinc . . . 1529) Zinc. . . 819
Carb. oxide 2417
Sulphur . 2263
Iron . . . 1576 \ Tin. . . . 1079 Iron ... 463
800 THBBMICS.
1431. There is a peculiar action of beat, wbich has Doi yet
been sufficiently investigatedi evinced in its power of actoating a
chance of chemical composition; one result of this action has
already been considered (1321). Mr. Grove has shown, in a
communication to the Royal Society, that at a considerable ele-
yation of temperature, the compound gases or vapctars are resolved
into their constituents, as if the
^' <^99. disruptive power of heat had
been sufficient, not only to sepa-
JU,^^ ^ rate molecule from molecule,
/^•■•^^^^"v^ %J ^* ^^^^ ^ ^^^ their consti-
cr^jC ^^^l^^w^^ tuents from each other. He
\>y§ ^*^"\0 found that an intensely ignited
\ J y^ V}^ of platinum, indium, or
^ silica, plunged into water, de*
V. composed the evolved steam
intooxygen and hydrogen. The
best mode of showing this im>
portant fact is by bending a tu]be into the shape abb. Fig. 693,
having platinum wire, z c, passing across its bulb. The whole
is filled with water, and allowed to rest in a vessel of water. On
connecting z c with a battery consisting of two of Grovels ele-
ments (776), the water in the bulb will soon boil, and the bulb
will be filled with steam ; the wire traversing it, becoming red-
hot, will decompose the steam into oxygen and hydrogen, minute
bubbles of which will rise through the water.
1432. Terrestrial Heat. — The solid mass of our earth owes its
warmth to what is termed terrestrial heeU, for which it is not in-
debted to the sun's rays, but to some internal cause. So far ss
researches have extended, it appears probable that the tempera-
ture of the earth increases one degree (Fahrenheit) for every 60
or 70 feet we descend beneath its surface, so that at a depth of
a few milea the mass of the earth must be actually red-hot. A
large proportion of the terrestrial heat is radiated into space, and
is subsequently absorbed by the aqueous vapour suspen^d in the
atmosphere ; the vapour itself then becomes in its turn a good
radiator, and hence the temperature of the air decreases on
ascending to any considerable elevation : this diminution of heat
has been assumed to be one degree for every 290 feet above the
level of the sea. From repeated observations it appears, that the
mean, or averase, temperature of any place corresponds to the
heat of the earth at a oistanoe of about 30 feet below its sur£ue.
801
CHAPTER XXIV.
SAOULHT HEAT.
Iv the preceding chapter the influence of heat on the state and
physical propertiei or matter, and the laws of its transmission
from molecnie to molecule of matter in its several solid, liquid,
and gaseous states have been discussed : it now remains to con-
sider the influences of matter on its transmission by radiation,
preciselv in the same manner as light is transmitted through trans-
parent bodies.
1433. Every one, when standine near a fire, must be aware
that he feels a sensation of warmu, and consequently, if actual
particles of matter do not pass from the fire to him, that some
Lind of energy must emanate from it, which, on reaching his sur-
face, excites tne sensation of heat, in the following account of
the properties of radiant hecU, it must be recollected that this
expression is applied to the effects of those undulating movements
which are assumed to excite the sensation of heat, and not as
referring to any form of matter. In this view also, a ray of
heat must have a definition analogous to that already given of a
ray of light (1019, 1020). When a body is heated in the air or in
a vacuum, it continues to evolve rays of heat ; and manifestly so,
until it attains the temperature of tne surrounding medium, lliese
rays pass off in straight lines, and obey the laws of reflection
and refraction, precisely like those of light.
1434. If a heated body, as a red-hot iron ball, a, Fig. 694, be
placed in the principal focus (1040^ of a concave metallic mirror, b,
radiant heat will pass from it to tne mirror, from which the rays
will be reflected in pandlel lines. These, if ooUected bv a second
mirror, o, placed ten or twelve feet from the first, will be equally
brought to a focus, and the bulb of a differential thermometer, d,
S laced near that pointy will be immediately acted upon by the re-
ected heat, the fluid falling in the tube. In this manner phos-
phorus or gunpowder may be easily inflamed at a considerable
distance from the source of neat, by concentrating the thermic rays
by means of a concave metallic mirror.
1435. If a mass of ice be substituted for the hot ball, the ther-
mometer in the focus of the second mirror will indicate a de<
pression of temperature. This was once erroneously assumed as
ui illustration of the reflection of cold ray$, which in fact have no
dF
802
RADIAHT HEAT.
existence ; cold being merely the negation, or abBence, of beat. Id
this arrangement of the experiment, the ball of the thermometer
being warmer than the ice, plays the same part as the red-hot ixtm
Fig. 694.
ball (1434) ; it radiates bent, which is reflected by the mirror
in the focus of which it is placed, and reaching the ice, becomes
latent (1 156) in converting a portion of the ice into water.
1436. Theory of Exchange*. — ^The facts herestated are embodied
in this theoiT of Prevoat, which assumes that every molecale emits
continually heat rays, the energy of which is proportional to its
temperature ; consequently, a particle that emits less enei^ than
it imbibes, gains heat ; on the contrary, one that emits more energy
than it appropriates, loses heat.
It follows as an immediate consequence of this theoiy that the
radiant and absorbent powers of any substance are necf«Mri2y
equal : for suppose two bodies to be placed in a space surrounded
by a complete envelope maintained at a constant temperature,
they will soon arrive at, and subsequentlv maintain, tne same
temperature. Suppose now that one of tnese bodies is able to
radiaJte n times as much heat as the other, it follows of necessity
that it must at the same time absorb n times as much heat, in
order to maintain the constant temperature. It will subsequently
appear that this law has been abundantly verified in the invaluaUe
researches of Prof Tyndall.
Suppose now one of the two enclosed bodies capable of reflect-
ing 90 per cent, of the rays absorbed by the other, it can therefore
absorb only 10 per cent, of those rays, and as its radiation must
necessarily equal its absorption, it must radiate onlv 10 per cent,
of the heat it receives ; therefore, aood r^Uctort of h^U are had
radiators^ and vice verad; this law nas been proved ezperimentsdly
by Leslie and others*
Suppose the hypothetical envelope to be spherical, for the mke
THXOBT OF EZCHAXGB8. 803
of simplicity ; it is self-evident tbat the temperature of the body
wiQ not be affected by its position in the cavity : this mav m
proved theoretically by the same geometrical oonsiderations woich
were employed by Newton, to prove that the attraction of a spherical
shell for a particle within it is independent of the position of the
particle. This law has been proved experimentally by Lambert
and Leslie, and may be thus enunciated : — If a poiiU receive from
a given eurfaee of indefinite extent rays bounded by a conioal
ewrface of any form, the same amount of heat vnU arrive at the
potntf whatever may be the distanee, form, or poeUion of the «tir-,
faeef provided only that the eeetionm area of the cone, at a given
distancefrom the point, remain eonetant. This is merely a gene-
ralization of the experimental proof in 1440.
Suppose again one of the two included bodies to be a plate of
some substance capable (as probably all substances are more or
less) of absorbing some heat-rays and transmitting others — acting,
in fact, as colouness glass does towards the supra-spectral rays
(1114\ or coloured glass to some portion of the visible spectrum
(1093), — rock>salt, ror example: it is clear that this plate; will
ei^er transmit or intercept the radiations towards the other body
from a portion of the sunace of the envelope ; but as the same
amount of heat must reach the second body, as when the rock-salt
plate is not interposed, that body must be capable of radiating
the same quality, as well as quantity, of rays that it absorbs ; or
in other words, a body abeoros moet freely those rays which it is
capable of emitting: this has been proved experimentally by Mr.
Balfour Stewart, with regard to rocK-salt.
1437. It is a noteworthy fact, and strongly corroborative of the
identi^ of the nature of lignt and heat, that precisely the same laws
that have here been enunciated with regard to heat-rays, hold good
with respect to light : thus the converse relation of radiation to re-
flection 18 shown by the fact that if the dross be skimmed off a pot of
molten metal (as may be seen any day in a foundry), it glows much
more brightly than the clean, and, therefore, good-reflecting surface
of the metal, which may be observed to brighten up as the surface
again becomes oxidated. Aeain, the coincidence of absorption
and radiation may be shown thus : —
I. It has been shown bpr Mr. B. Stewart, that if an encaustic
tOe with a black and white pattern on it be made red-hot, and
viewed in the dark, the black part of the pattern will come out
bright, and the white part dull.
II. A piece of coloured ^lass will radiate much more light than
a colourless piece, if both be raised to the same temperature.
m. When the piece of coloured glass has become heated in a
olear fire, the colour of the glowing coals behind it will be very
little affected, because it has acquired the power of replacing the
absorbed rays by its own radiations. There is an obvious analogy
between this result, and the reciprocation of sound (540).
8f2
804
RAVILKT HEAT.
IV. If a tonrmaliDe be rendered incMidescent bj being pboed
in the centre of a red-hot iron sheil, having a hole Uirongn whidi
the crystal may radiate transrereely to its axis, the radiated Hgfat
was found by Mr. B. Stewart to be polarised in a plane petpendicn-
lar to that of the polarized rsrs wnich it wonid have tran»mitf$d.
1438. Bate of Cooling bv Madiation.-—\t is foond by experiment
that a body radiates a fizea proportion of its heat, or in otber words^
that the amount of radiation at any moment is pn^rtionaltotbe
temperature only : for if as** be radiated to a given point from a body
at a temperature of y**, then if the body be raised to 2 y*, 2 x* wiH
be radiated to the same point. Hence the rate of cooling hj
radiation most be some function of the temperature only ; and if
a body at a temperature T+ <, be surrounded by an envelope at
a lower temperature, t^ then its rate of cooling will be a rmming
balance of its debtor and creditor account with heat. If the rate «
cooling at the temperature x be represented by i^x), tben the rata
of coofing of the supposed surrounaed body wilibe
/[T+o-/:*).
If this expression represent the loss of heat by the body in
question, in order that it may also represent the loss of tempera-
dire, it will be necessary to assume that the specific heat (1888)
of the body remains constant during the interval of temperatore
under observation.
Accurate observations on the numerical values of this function
were first made by Dulonp: and Petit, by means of a thin sphere
of copper, blackened inside, and immersed in a water-bath of
known temperature, into the middle of which the bulb of a ther-
mometer at a higher temperature than the bath was inserted,
and the decrement of temperature during one minute was ob-
served. In order to divest tne radiation of the interference of the
air, as much as possible, the hollow sphere was exhausted by an
air-pump. The observed rates of cooling for several different
values of T and t are given in the following table : —
VslBMOf
2*0.
Valnetofl
•
0°C.
20*
40=»
aop
80"
240
10-69
12-40
14-35
• ••
• ••
220
8-81
10-41
1198
■ *•
• ••
200
7-40
8-68
1001
11-64
13-45
180
610
704
8-20
955
1105
160
4-89
6-67
6-61
7-68
895
140
8-88
4-57
5-32
6-14
719
120
302
3-56
415
4-84
564
100
2-30
2-74
3-16
3-68
4-29
80
1-74
1-99
230
273
8*18
60
• • •
1-40
162
1-88
2-17
KATE or OOOLIRG.
805
It will here be found that while tM socoeeeiye Talnee of i have
a eongtant common difference of 20% the corresponding rates of
cooUng, for each and every valae of T have a eonetant common
raHoot ri65 very nearly: hence result the general laws that.
The rates of cooling of a heated body are in a geometrical cro-
greesion, when the exceeeet of temperature aiwve that of the
eurrounding medium are in an arithmetical progression: and
The common ratio is the same for the same common difference^
whatever be the excess of temperature of the heated body.
From these laws MM. Dnlong and Petit have inferred the form
of the fonction/. The common ratio 1'165 found for 20*" must
evidently be the 20th power of the common ratio x for 1% or
«»=1165,
in which the value of x is 1*0077 ; and for <° the common ratio
will be 1*0077^: and the first term of each geometric series, con-
tained in the second column, depends on T alone, and may be
pnt under the form f{T): hence is derived the equation
/(r+o-/(o=^(r;ioo77<=a<^(r).
expanding /(?*+<) in ascending powers of Thy Taylor*s theorem,
and expanding ^{T) also, there results
f{t)-\-def{t). T+ &o.-/(0-a<(i4 T+B T« + &c.)
then by equating the coefficients of like powers of T,
dif{t)^A.a^=blogt a.a\ suppose,
then by integrating this equation
f[t) =b.a^ + const.
similarly / (^ + Q = 6 . aT+ * + const.
subtracting, /( r+ 0 —/(O =&(«''+*- «0.
=6.o«(ar-.l).
In the value thus found for the rate of cooling, the determina-
tion of the value of the constant a has been purely empirical ; but
its value has been confirmed by the investigations of MM. Pre-
▼ostaye and Desains: the value of the constant b will depend
on the nature of the radiating body.
For determining the absolute radiation from any particular
substance and surface, the value of b in thermic units (1372)
must be ascertained ; this has been done by the late Mr. W.
Hopkins in a few instances, viz. : —
Glass 6=1-367
Pry chalk 1*230
New red sandstone . . 1*197
Buildiuff sandstone . . 1*269
Polished limestone . . 1*301
The same unpolished . 1*829
The object of this quantitative determination bv Mr. Hopkins,
was the determination of the probable thickness of the solid crust
of the earth ; by ascertaining from the amount of radiation the
806 RADIAHT HEAT.
dej^h at which the temperatifre wonld be above the point of fiitton
of its ordinary materials : this problem is, however, still veiled in
much uncertainty, and very dinerent estimates hare been fonned,
according to the basis on which they have been made.
1439. BeUe of Cooling by Convection in Octaes. — This has been
determined by a laborious process, bv MM. Dulong and Petit^ in
relation to certain gases : tney have deduced the following laws : —
1. The rtUe of cooling hy gaseoua convection it independad of
the nature of the turface. This is not the case with the rate of
cooling by radiation.
2. Thie rate oc {excess of temperature) ^**".
8. This rcUSt for a g^ven excess of temperature, is a function of
the pressure only.
Combining the two last laws, it appears that —
Rate of cooling =:m .p» . <***■•.
It has been determined irom the researches of Mr. Hopkins,
combined with the above, that the amount of heat lost by con-
vection by one square foot of surface heated f* above the adjacent
gas, the pressure of which is p centimetres of mercury, will be
expressea in thermic units (1372) by the following formulse: —
Pydrogen, 00184 (^) •'" t '« ;
(•I \ 0.S17
JL) ti-w.
720/
1440. Reflection of heat takes place from the surfaces of bodiea,
and generally, the more highly polished these are, the more com-
pletely do they reflect heat. If 100 rays of heat be incident at an
angle of 60° on reflecting surfaces of the following substances,
the proportion per cent, of heat reflected will be as f^lows : —
Polished gold . 76 ; Unpolished brass 52 5 Glass blackened
at the back . 12
Metal blackened 6
„ silver . 62 5 Lacquered brass 41
„ brass . 62 \ Looking-glass . 20
When heat is reflected in parallel rays from the surface of
a good reflector, it scarcely seems to be afiected by the snace it
may happen to traverse, except in being slightly diminianed in
quantity oy the absorbing power of the medium through which it
passes. If, however, heat be radiated from a small sorface, its
intensity, if examined at different distances from the radiant
FORM or THBRMOPILI XMPLOTSD.
807
bodyi will be found to decrease as the square of the distance
increases, as is the case with light (1026) : this is, however, abso-
lately true for radiation from a point only. The truth of this
proposition may be readily demonstrated by directing the conical
guard of a thermopile (968), b. Fig. 695, towards any extended
and uniformly heated surface, and noting the deflection of the
needle. If the thermopile be now removed to any neater or less
distance, the deflection will remain unaltered, provided only that
the area representing at the heated surfaoe the base of the cone
produced, he wholly within that surface. Now it is clear that
this area, and consequently the number of points radiating
heat to the nile varies as (distance)' of the pile from the surface ;
but the total amount of radiation is found to be constant, there-
fore the heat received by the pile from each point of the heated sar-
fiace must be inver$ely as its (distance)' from that surface : q.e,d.
1441. Heat, like light, admits of single and double refraction,
and polarisation, — properties for the discovery of which science is
almost exclusively indebted to the labours of M. Meiloni, and of
Principal Forbes of Edinbureh : the latter physicist, indeed, is
the diMoverer of the plane and circular polarization of heat. The
application of a delicate thermopile (Fig. 548), in which an electric
current, capable of affecting the needles of a sensitive galvanometer
(856), is excited by very minute changes of temperature, has been
the main source of these important discoveries, by enabling the
physicist to detect changes of temperature otherwise utterly un-
appreciable.
The most complete apparatus of ^ this kind is that devised by
Forbes, Fig. 695. The thermopile is enclosed in the case, a, and
JFV. 095.
supported by a stand, so as to be moveable in any direction. The
bent wires, c, are connected with the terminal elements of the
808 BADIAXT BBAT.
Httle batterji and oonnexion is thas readilj made with a delicate
galvanometer, d, the indications of which are examined through a
telescope, b, so that very small doTiations of the needle are easilj
observed. The battery is so minute, that the section of a pre-
sents an area of o^y 0*4 inch. The rays of heat are frequently
concentrated on the* thermopile by means of a conical metalHc re-
flector, B, and so delicate is this apparatus, that the mere approach
of the hand towards the mouth of b will excite a current capable
of deflecting the needle through seyeral d^rees. When observa-
tions are made with this instrument, it is usual to inteipoee a
screen consisting of two parallel plates of wood or pasteboard
separated by an interval ot about an inch, between the sonroe of
heat and the mouth of b, or the extremity of the pile, when the
reflector is not used, and to remove it at the instant that all ia
arranged for observation.
^ 1442. B^raeUcm of JJeot.— Heat-rays are capable of refrac-
tion through prisms and lenses, in the same manner as lominoos
rays. In experiments on heat, however, the refracting medium
must be a substance capable of readily transmitting heat of aU
kinds ; and for this purpose rock-salt is almost the only sohstaoce
that can be employed, the extraordinary &cility with which it
transmits more wmh 90 per cent, of incident heat, rendering it to
heat-rays what glass is to those of light, or quarts (1114) to the
invisible rays of hiffh refrangibility.
If a convex lens no made of transparent rock-salt, it will with
facility bring radiant heat and eopecially the rays of dark heat,
or as they may be termed, hypo$peetral ray*^ to a focua. The
rays from a vessel of boiling water can thus be brought to a ibcuB
by a salt lens, with as much facility as luminous rays are by a
lens of glass. The higher refrangibility of the heat-rays in the
solar beam enables them to pass through ordinary convex lenses of
glass, and the common burning-glass affords an instance of solar
heat being thus brought to a focus toMther with the light.
1443. if heat be incident on a rocx-salt prism, it is, like light,
resolved into a series of rays of unequal refrangibility (1088), and
a heat-spectrum is the result. Tne refraction of heat may be
readily shown with the following arrangement, in which the rays
emanating from a vessel of boiling water, a. Fig. 696, after passing
through an opening in a screen, d, are incident on a rock-salt prism,
B, where they are refracted in the direction B c, and may he de-
tected by the thermopile, o.
1444. When heat-rays emanating from different aouroes are in-
cident on the prism in this apparatus, it u found that the angle
of their incidence must be changed by moring the source of heat,
in order to obtain the maximum action on the galvanometer ; or,
what comes to the same thing, the position of the thermopile most
be slightly altered. ^ The explanation of this is readil v found in
the unequal refrangibility of the rays of heat : thus M Jlooi fonnd
BSFRACnOV OF BBAT.
809
that the heat from incandescent platinum was refracted more than
that from a hot plate of blackened copper.
1445. The existence of rajs of heat of yarions refrangibilitj
piesents a key to the phjrsicd alteration produced in heat after
trayersinff plates of different bodies. Thus, rock-salt allows
rays of all refraogibilities to pass in nearly eqaal propNortions, a
plate of alum intercepts all saye the least refrangible rays.
Melloni coyered a plate of rock-salt with soot, and found that only
rays of the highest refrangibility could pass, the plate becoming
to heat what yiolet-colonred glass is to light; ana by combining
with this a plate of alum, which refuses to transmit any but the
less refrangible rays, dU heat was absolutely stopped, the combi-
nation becoming absolutely atUathermanouSf or opaque to heat.
When a plate of alum is combined with one of ereen glass, the
brilliant light of a lamp, or eyen of the sun, is reaaily transmitted,
hat their heat is absolutely stopped : these experiments show yery
satisfactorily the analogous relations of light and heat
It has preyiously been remarked (1107) that the most intense
rays of the heat-spectrum are situate beyond the red end of the
light-spectrum, and the curye she, Fig. 594, is intended to re-
present the relative intensities as observed by Sir J. Herschel.
With better means of observation, MiUler determined the point of
maximum intensity of solar heat to be at a distance equal to
about half its length beyond the extremity of the visible spectrum.
The relative intensities of the heat-rays are represented b;^ the
ordinates of the dotted curve ▲ b o, Fig. 697, of which the ordinate
i> B represents the termination of the visible spectrum.
1446. But as it has been demonstrated by Prof. T^ndall that
aqueous vapourisapowerfulabsorberof the heat-rays, it is more than
probable that the sunbeam is robbed of most of those rays before
It reaches us. And this supposition is fully borne out by the obser-
tations of Prof. Tyndall on the heat-spectrum ofthe electric lamp,
obtained by transmission through a lens and prism of rock-salt.
In tbi« cue he found the btinntj of the ■pcctram to he tvfn-
■ented br the cnrre c E r D, the ordinate Di of MulWi corre
being tftken u the onit of compariBoa. The point of BUxiiniUB
mteniilf, r, coneipond* with e, that «f Unller'i cnire, bat th«
amount of heat ii then mora than doubled. Semihle rsn of heal
vere obaerred to extend u much ai half the leDgth cf the fiaiUe
epectmni further into the itA ipace bejond A.
1447. Oalortteeiiee. — ABtheinlentilroflheobaciinra^efbeit
appears to adfance^Niri HOW with the total heat emitted. It became
an iotereding subject irf inquiry to diacover BomemeaDaofwpa-
rating the Inminooi from the obscure heat-ran, and a higfaly im-
portant aerioa of inreatigationB was made b; F>«f. Tj>BdaD.* He
diacorered that iodine and bisolphide of caj'bon are both fnclj
permeable hj obscure beat-rajs, and as iodine is tbtj aoloUe in
that fluid, a stroog solutioD, eDclosed in a glass cell terminated bj
pUles of rock-salt, was foutid to intercept entirely the lomiiMKts
rays, but to tnnimit about 90 per ttaa. at
I the obacnie raj ■ of heat. Tbess beiag bno^t
I to a focus, Knnpowdsr, pnper, and otha* com-
I boBtible bodies ma; readily be ignited.
' But a still more temarkable [JieDOBeiwa
wasobserred when sooie incombustible bodies,
I capaUe of freelj taking up the raTt, were pre-
I sented to the focns. & [ueoe <^ platinum-fbal
I platinised, i^., coated with flnel^-dirided pla-
tinun b; electrolytic precipitation, being so
placed BB to receive a focal image of the carboii
electrode*, that image in a few seconds h-^sTnt
plates ot rocK-saJt,
B
SURPACB-BADIATIOV. 811
words, their refrangibilitj is increased. This phenomenon, the
converse of fluorescence (1116), haa been designated "calorescence''
by Prof. Tjndall.
The same arrangement that heated the platinnm-foil to white-
ness with the electric, or ozy-hjdrogen light, produced only dull
redness with direct solar rays, thus demonstrating the absorption
of the obscure rays by the atmosphere.
1448. The state of the surface of the radiating body materially
a£fects its radiant power. It is not improbable that the impedi-
ment that a polished surface offers to the radiation of heat, may
bear some analog to the internal reflection of light (1060). Ab
a general rule, lor the same substance, the radiating power is
diminished by even slightly compressing its surface, as in bur-
nishing ; and in the case of metals, it is increased by tarnishing
or oxidation.
The radiant power of bodies may be roughly illustrated by re-
placing the hot ball in the focus of one of tne concave mirrors
(1434) by a cubic canister of tin filled with hot water, the angles
oeing provided with grooves, so that plates of substances may be
slippea in. Or two sides may be coated with different substances,
as lamp-black and white-leaa, if the radiant powers of such bodies
are to be determined, and one of the remaining sides polished,
while the other is made roueh by scratching it. In every case,
that side of the canister which is to be examined, must be turned
towards the surface of the mirror, in the focus of which it is
placed. The indications of the thermometer, placed in the focus
of the second mirror, become a measure of the heat radiated by
the substance under examination. The radiating power of lamp-
black is the most considerable of all bodies, and is assumed as tne
standard of comparison with others in the following table : —
Lamp-black . . 100 ^ Ice 85 ^ Clean lead 19
Writing-paper . 98 e Plumbago . . . 75 > Polished iron ... 15
Crown-glass . . 90 < Tarnished lead 46 \ Other bright metals 12
1449.' The rapidity of the cooling of different bodies depends
jointly upon the radiating power of the substances of which they
are composed, and upon the nature of their surfaces. Leslie
filled a polished tin globular vessel with hot water ; it cooled
down to a certain temperature, as indicated by a thermometer, in
156 minutes. On repeating the experiment, after covering the
vessel with a thin layer of lamp-black, it cooled down to the same
Soint in 81 minutes : thus the rapidity of cooling was nearly
onbled by increasing the radiating power of the surface of the
vessel. Count Rumford allowed hot water to cool in two polished
brass cylinders, leaving one naked, and covering the other with a
fold of linen: in the former the water cooled 10 degrees in 55
minutes, whilst in the latter, it lost the same amount of heat in
361 minutes. The good radiating surface of the linen thus acoe-
812 EADIAHT HBAT.
lerated tbe loss of heat. For a similar reason, regetable ioliuioiiUL
as tea, are best prepared in brislit metallio Tessels; nnglaaea
earthenware, and especialljr a black tea-pot, radiates a laigo
amount of heat. In heating rooms with tubes of hot air or
steam, their surfaces should be roughened or blackened, to focili-
tate the radiation of heat into the apartment ; whilst that portion
of the pipe employed to convey^ the source of heat into the room
should be kept bright or polished; or, still better, "jacketed,"
that is, enclosed in a second tube with a space of air interroning,
in order to prevent unnecessary loss of heat by convection.
1450. During the nieht the temperature of the air ia alwaya
many degrees colder £an in the day. The earth, therefore,
radiates into space a portion of the heat it had absorbed in the
day-time. Thus becoming cooled, a depoeition of the aqneoos
vapour of the air takes place upon its surface, which is familiarly
known b^ the name of dew. This, in oold weather, freezes in tlie
act of being deposited, and constitutes hoar-frott, which is the ice
of dew. Ihe greatest quantity of dew is always found deposited
on that portion of any surface which radiates best; hence a
meadow will often be found covered, whilst the smooth road by its
side is nearly free, in consequence of grass radiating freely. If a
polished plate of metal be exposed at night by the side of a piece
of wooUen cloth, the latter will, in the morning, be found covered
with dew, whilst the badly radiating metal wiU be free. Deposit
tions of dew may be readily prevented by opposing any obstade
to free radiation ; eveiy gardener is aware that he can prevent the
deposition of dew over a portion of ground by merely supporting
over it, by means of slips of wood, a thin cloth or handkerehi^
which prevents the free radiation of heat from the suifaoe thus
protected. The demonstration of the real source of dew and
noar*frost is due to the researches of Dr. Wells.
1451. In order that observationson terrestrial radiation may he in
any degree comparable with each other, it is absolutely neceasaiy
that not only the construction of the instruments used, but alM
the mode of using them, should be strictlv defined, since otiierwise
widely differing results may be obtained under preciselv similar
circumstances. It appears from the observations of Mr. F. W.
8tow,* that an extreme difference of 2*5*' F. was observed be-
tween thermometers of different constructions, placed in juxta-
position on the same spot. It is probable that the most sensitive
instruments will be found to be those having a bulb made of black
flass, the surface of which has been eroded by fluorhydric add :
ut it is very desirable that both the form and the sixe of the in-
strument most suitable for such observations, and the precise
nature of the surface on which it is to be placed, should be
authoritatirely determined.
* Proceedings of the Ifeteorolof ieal Society, roL iii. p. 188.
MOSm's FIOUBES. 818
1452. The relation of the state of Buriace with the depoeition
of Tapoar is well shown in the carious phenomena discovered bj
Prof. Moeer, and known as MUnr't fgure$. To oheeire these,
place a coin upon the surface of a piece of looking-glass, or of
common glass, having the back covered with tin-foil, and allow a
few sparks to fall upon the ooin from the prime conductor of an
electnc machine. Bemoye the coin,^ and gentlj breathe over
the surface of the glass^ when the outline of the impression on the
coin will become partially defined upon the glaiis in minute glo-
bules of water. It a series of plates be superposed, and the coin
placed upon the upper one, and the sparks allowed to fall upon it,
the upper surface of each plate will present similar phenomena
when breathed upon. These figures may be renderea visible by
exposure to the vapour of iodine or mercury, quite as well as by
breathing upon them. Similar efifects have been shown, by Mr.
Hunt, to result when a coin, gently heated, is allowed to rest on
a plate of polished silver : on removing it and breathing on the
plate, or exposing it to the vapour of meroury, the figure of the
coin will be rendered distinctly visible. If a clean coin be allowed
to rest on a looking-glass for some time in the sun, and be then
removed, a tolerably distinct outline of the coin will appear, on
gently breathing on the glass.
1453. The altered condition of the surface of the body, on
which the vapour is deposited, must be owing to radiation from
the coin or otner body placed on it. Founded on these curious
iSscts is the art of Thermography, to which attention has been
especially directed by Mr. Hunt. He found that to obtain a good
image, the superposed body must be of a different material from
the plate on which it is placed. Thus, when a sovereign, a shil-
ling, and a penny are placed on a polished copper-plate, the latter
gently warmed by passing the name of a spirit-lamp under it,
then allowed to cool and the coins removed, pictures or the sove-
reign and shilling will appear on exposing the plate to the vapour
of meroury, whilst a scarcely visible image of the penny win be
obtained. Pieces of blue, red, and orange-coloured glass, of white
crown, and flint glass, mica, and paper, were placed on a plate of
polished copper, and allowed to remain in close contact for half
an hour. On removing them and exposing the plate to the
Tapour of meroury, distinct images of the red, orange, flint, crown-
slass, and paper were obtainea, whilst the blue glass and mica
had scarcely produced any impression.
A plate of copper being amalgamated, so as to present a bril-
liant reflecting surface, by rubbing it with nitrate of mercuiy, a
sheet of printed paper was placed on it with the letters downwards
and pressed in close contact by several folds of paper on which a
weignt was placed. The whole should be allowed to rest on a
warm surface. In half an hour some emanation, probably of the
yapour of oil, from the black letters will have produced a marked
814 KADU.HT HEAT.
althoQgli as jet invisible, effect on the snrfaoe. To render this
obvioQs the plate should he exposed to the raponr of mercnrj,
which will adhere to those parts which corresponded to the white
portion of the printed paper. It should next be exposed to the
yapour of iodine, whicn will blacken the parts to which the mei^
curia! vapour has not adhered, and an accurate copj of the printed
page will result.
1454. It has already (1486) been remarked that bodies which
possess a high radiating power, are also universally endowed with
the property of readily ah$orbing heat; and whilst the beftt
radiatons are the best absorbers, they are the wont reflectore.
In the experiment of the two mirrors (1350), it is found that the
metallic plates of which they are composed do not become sensibly
heated by the rays from the red-hot ball impinging upon them.
But if their concave surfaces were covered with lamp-black, the
mirror nearest to the ball would become hot from the abscxrptioa
of heat, and scarcely any would reach the second mirror. The
blackened surface would, however, continue to radiate the heat
acquired from the ball until its temperature is reduced to that of
the surroundine atmosphere.
The observed reciprocity of radiation and absorption was as-
sumed by Prof. Stokes to be a probable means of accounting for the
existence of dark lines in the solar spectrum : the correctness
of this hypothesis was fuUy demonstrated by the researchei of
Bunsen and Kirchhoflr(1108, 1111).
1455. A highly important and exhaustive series of observations
has been made bv Prof. Tyndall on the relations of the colours of
bodies, and of pulverization to radiant heat.* From the time of
Franklin*s experiments on the absorption of heat to the present
time it has been accepted as an axiom that dark-coloured bodies
are the best absorbers, and colourless or light-coloured bodies the
worst, but Prof. Tyndall's researches have entirely nullified this
conclusion. For example, if any two similar surfaces, as the bnlbs
of a differential thermometer (1356) be coated, one with powdered
alum, and the other with powdered iodine, which is to the eye
quite black, and equally exposed to a source of heat, the move-
ment of the column will show a great excess of absorbent power
in the white powder. This result depends simply on the fact
that the molecules of iodine are not in unison with — are incapable
of appropriating to themselves — the wave-motion of the greater
part ofihe heat rays ; while those of the alum do not labour under
this disability. It would hence be naturally inferred that iodine
would be found to be diathermanous to those rays which it it
incapable of arresting, and it is proved by experiment that snch is
the fact : while the notoriously adiathermanous quality of alum
led to the trial of its relative absorbing power.
1456. One of the most important results of these invostigatioiit is
• Fhtl. Trans. 1808, part I.
RAOIATIOH FKOM POWDERB. 815
the establiBhmentof the paramovnt influence of chemical constitu-
tion on the reciprocal phenomena of radiation and absorption. In
the ezperimentB of Melloni, and also in those of Masson and
Coort^p^,* the powdered substancCf on which they experimented
was spread on a surface mixed with a solution of gimi or gelatins ;
and it is somewhat surprising that it appears never to have occurred
to these physicists to separate in a mat measure the influence of the
substance examined from that of the adhesive material with which
it was coated, by first lading the solution on the required surface,
and then sifting over it as much of the powder as coold be re-
tained by adhesion. ^
As gum and gelatine are both good radiators and absorbers, Prof.
Tyndall p»referred to employ sulphur, which is yerv transcalent, as
the adhesive material, and a solution of this in bisulphide of carbon
was employed to attach the powders to the surfaces of observa-
tion. When thus compared, tne radiatinfl^ powers of red oxide of
lead, and biniodide of mercury Tboth bright red powders) were
found to be as 74*1 : 39*7, or nearly as 2 : 1 ; but when mixed with
gum, they were both alike 80, thus the influence of the particles
was entirely masked by that of the varnish which coated them.
In regard to chemical constit0tion, the results invariably showed
that amongst powdered substances consisting of the same elements
variously comoined^ increased complexity of atomic constitution
is accon^anied bv increased powers of radiation and absorption.
On comparing black and wnite powders with each other, their
radiating powers were found to vary as extensively as in the case
of red powders ; thus —
Black : — ^Electrolytic platinum 59*0 ; protoxide of iron 81.3.
White: — Chloride of silver . . 32*5; carbonate of sdno 77*7.
As in the comparison of alum and iodine, so in other cases, it
appears that the radiating or absorbing power of a white powder
far exceeds that of a black one ; thus m white hydrated oxide of
zinc it ia 80*4, while in platinum-black it is only 59*0.
1457. The radiating power appears to be very'slightly influenced
by mere mechanical suodivision; for when surfaces of glass and
rock salt were coated by simple adhesion with their own powders,
the radiation was very little afiSscted by the removal of the powder
with a camelVhair pencil : but this does not hold equally for the
extreme state of comminution obtained by precipitation ; thus
with bright platinum foil the radiation was only 6*0, while with
the same platinized it was 45*2 : moreover this experiment shows
the influence of surface-condensation (1448).
Prof. Tyndall subsequently found that the powders might be
made to adhere sufficientlv by electrical attraction only ; but the
observed results did not oiffer materially from those obtained by
means of the sulphur-cement.
* Comptei R«ndos» vol. zzr. p. 038.
816
B^DIAST HSAT.
In all these experiments the heat employed waatbat of boiling
Uer, in a Leslie's cobe ; the following table shows the reUtiTe
water
amoimts of radiation : —
Babttuoe.
Bad.
BvbiUiiM.
Bad.-
Kock salt . . .
Biniod. mercury .
Iodide of ,,
Chloride of lead .
Flnor
Carbonate of lime .
35-8
39-7
46-6
55*4
68-4
70-2
Red oxide of lead .
Sulphate of lime .
Rea oxide of iron .
Hjd. oxide of sine
Black oxide of iron
Lamp-black . . .
74-2
77-7
78-4
80-4
81-3
840
The relative amounts of absorption were found by experiment to
be very nearly the same.
1458. The quantity of heat radiated having been detennined, the
question next arose as to its quality ; and this was tested by die
permeability of rock-salt to the radiations of various substances at
the same temperature of 100** C. It was found that the amount of
transmission was generally proportionate to the amount of radia-
tion, but to this there were some exceptions.
It was concluded by Melloni and Knoblauch that rock-salt is
equally permeable to all kinds of beat-rays ; but the experiments
of Mr. B. Stuart and Prof. Tyndall show that this is not ^e case ;
the following are the ratios of absorption by a plate of rock-salt of
heat radiated from the several sources.
Sooree.
Abt.
Source.
AIM.
Blnck platinum
Black oxide of iron
Red ,,
3-7
130
160
Suffar .....
Chloride of silver .
Rock-salt. . . .
17-3
22-6
29^9
Thus it appears that while rock-salt absorbs only 4 per cent,
of the rays emitted by black platinum, it absorbs 30 per cent, of
its own rays. It is quite natural to suppose that the molecnlea of
a body would be found most in accordance with, and theieibre
most absorptive of, its own emitted wave motion.
It was also demonstrated by direct experiment that the relative
absorption of various bodies closely corresponded with their relative
radiation.
1459. A simple and convenient mode of comparing the abeorptive
powers of two surfaces has been employed by Prof, l^dall: two
tin plates are placed vertically on two stands and connected by a
copper wire; to the middle of the back of each is soldered a amall
bar of bismuth, and these bars are connected by wires with the ter-
minals of the galvanometer coil. A source of heat being placed
somewhere in a line joining the centres of the plates coated
TSAMSCALBHOT OF 80LIDB.
817
witli two substances to be compared, if uneoaal atnonnts of heat
are absorbedf the needle will be deflected 07 the difference of
the two contrary currents, but if these be equal, it will remain
unaffected. The squares of the relative distances of the source
of heat from the two plates, will give the ratio of the absorptions
bj their surfaces.
1460. Radiant heat is partially absorbed in trayersing the most
transparent media ; it is supposed that the heat of the sun loses
one fifth of its intensity in traversing a column of air 6000 feet in
]en^h. It must not, however, be supposed, that those media
which are the most transparent with regard to light, possess the
same property with regard to heat. Indeed, it has oeen satis-
factorily proved that a piece of smoky quartz, so thick as to in-
tercept the piissage of a considerable portion of light through it.
yet allows the paHsage of rays of heat which are entirely absorbed
by even thin plates of absolutely transparent alam, or citric acid.
Media which allow of the free passage of heat, are termed diather-
manouaf or transcalent^ those terms bearing the same relation to
heat, that " diaphanous " and " transparent " do to light.
1461. Transcalent bodies differ greatly not only in the quantity
of heat which they are able to transmit, but also in its quality,
distin&;ui8hed only by its refrangibility. In general, beat accom>
panied by light, or in other words that produced by undulations
SabatancM.
Looatelli
Red-hot
Copper at
Copoer at
lamp.
400»O.
lOOPC.
Bock-salt . . .
92-3
92-3
92-3
92-3
Sulphur . . .
74
77
60
54
Fluor ....
78
69
42
33
Calcite . . .
39
28
6
0
Plate-glass . .
39
24
6
0
Rock crystal
38
28
6
3
Smoky quartz .
37
28
6
3
Chrom. potash .
34
28
16
3
White topaz
83
24
4
0
Felspar . . .
23
19
6
0
Amethyst . .
21
9
2
0
Artificial amber
21
5
0
0
fiorax ....
18
12
8
0
Common gum .
18
3
0
0
Selenite . . .
14
5
0
0
Citric acid . .
11
2
0
0
Natural amber .
11
6
0
0
Alum ....
9
2
0
0
Sugarcandy . .
8
0
0
0
Ice
6
0
0
0
3 a
818 Badiaut rsat.
of sufficient rapidity to affect the yisa&l organs, is ca|>able of
penetrating most diatbermanous media, whilst the rays of dark
neatj or those of less velocity, as those emanating from a metal
heated below redness, or from boiling water, are absorbed by manv
▼ery transparent bodies. Solar heat, again, readily passes through
glass, whilst the luminonn heat of a bright fire is almost com-
pletely intercepted by a plate-glass screen. The preceding table
presents the results of MeUoni's experiments on the percentage
of rays of heat from several different sources transmitted by plates
of various bodies, each 0*1 inch in thickness.
1462. Of the 20 substances in the above table, 12 only arepenne-
able both by dark and luminous heat, but in very different propor-
tions; the other eight are diatbermanous to luminous heat alone.
The difference in quality of transmitting power is striking^v
shown, in the relative transcalency of Fluor and Calcite. Of all
bodies hitherto discovered, rock-salt transmits most of the incident
rays of heat, and of all kinds of heat in very nearly equal propor-
tions ; hence it must be regarded as the true glass of radiant heat.
The oil-lamp used in these experiments is a very steadily-burning
one, that of Locatelli. The incandescent platinum consisted of a
coil of wire, ignited in the flame of a spint-lamp, and the copper
used as the other sources of heat was blackened.
1463. After the thermic rays have traversed a transcalent body,
thev appear to have undergone some physical change, for if allowed
to &1I upon a second plate of the same substance, a much larger
proportion of those transmitted by the first plate will traverse the
second. It thus appears, that in the act of passing through a
medium, the rays are separated into two portions, one of which
is changed (1114) or absorbed, and the other transmitted; and
thus, sifted from the non-transmissible rays, those transmitted are
better able to traverse a second portion of the same medinm.
From the last table it appears that but 9 per cent, of the rays
emanating from an oil-lamp are transmitted by a plate of alum,
but if these transmitted rays be allowed to fall upon a second plate
of this substance, 90 per cent, will permeate it, and connequentlv
but 1 0, instead of 91 per cent, will be absorbed. On the other hano,
Melloni found that a slice of green tourmaline transmitted 18 per
cent, of the calorific rays incident upon it, whilst it allowed but 1
per cent, of heat which has passed through alum, to traverse it,
thus intercepting 99 per cent. The same slice of tourmaline, al-
though thus nearly impervious to heat transmitted through alum,
vet freely permitted the passage of 30 per cent, of the mys which
had previously passed through black glass. There is also a striking
contrast between the transcalency of chromate of potash, and of
white topaz ; in the former, the ratio of red heat to dark heat
transmitted is rather less than 2:1; in the latter, the same xatio
is6:l.
1464. The probable reason why the thermic rays, that exist at
TEAHSCALCHOT OP FLUIDS. 819
And bejond the lower end of the visible spectrum (1107), are not
luminous, may be found in the fact that they are aosorbed hy the
humours of the eye, and do not reach the retina. If a spectrum
be formed, in a pencil of rays from an electric light, by a prism of
rock-salt, and the thermopile be interposed in the path of the rays
Dear the lower end of the visible spectrum, the galvanometer
needle will be considerably deflected. If a elass ceU, containing
the transparent vitreous humour (1 179) of the eye of an ox, be
now so placed as to intercept the rays falling on the thermopile,
the needle will shortly fall back to zero, although the light of the
portion of the spectrnm passing through the cell will not be sen-
sibly diminished ; thus showing that the hypospectral rays have
been wholly absorbed. This inference has been impugned by
Ftof. Tyndall, and undoubtedly under ordinary circumstances the
heat that reaches the eye is cleared of its hypospectral rays bj
soueous vapour: but the point requires some further consideration.
Tnese phenomena bear a striking analogy to the absorption of
certain portions of the spectrum by coloured media (1093), and
possibly to the conversion of the more refrangible rays to others
of lower refrangibility (1114).
1465. Rock-salt is represented in the above table as equally
transcalent to heat from all the sources employed ; but it appears
Irom the accarate experiments of MM. Prevostaye and Desains,
that this is by no means the case with heat from all sources :
and it has been shown by Mr. B. Stewart that rock-salt is peeu>
liarly opaque to the radiations from a heated piece of the same
substance. This fiict strikingl v confirms the universally observed
reciprocity of both radiation ana absorption with transmission. The
radiations from rock-salt, glass, and alum heated to the same
temperature increase progressively just as their respective powers
pf transmission are diminished.
1466. TranacaUncy of Fluida. — A series of experiments was
made by Melloni on the proportion of heat radiated by an Argand
lamp with a ^lass chimnev, which was transmitted through
various fluids, m a glass cell containing a thickness of 0*36 inch
of the fluid. The pronortion per cent, of the total radiation trans-
mitted by several nuios is here given : —
Bisulphide of carbon 63 ( Hydrate of ammonia 15
Oil of turpentine
Olive oil . .
Naphtha . .
Oil of lavender
Ether . . .
Sulphuric acid
31 < Niiric acid .... 15
30
28
26
21
17
Alcohol 15
Hydrate of potash . 13
Acetic add ... 12
Solution of rock-salt . 12
Distilled water . . 11
Here it appears that the variation in the transcalency of fluids
is not nearly so lu|;e, as in the case of solids ; it, however, amounts
8a2
d20
&i.DIAJrr RSAT.
to nearlj the proportion of 6 : 1. It ap]>ears alao that water
maiotains itN opacity alike in the solid ana in the flaid form.
Bat in these results the truth is considerably masked by the
known opacity of glass to a lai^e proportion of the rays of neat«
Much more reliable results have been obtained by Prof. TyndaU,
by confining the liquids under examination between two plates of
rock-salt, and by separating these to different distances fmm each
other, the relatiye aosorptions of the rays from a red-hot platiniim
spiral by different thicknesses of the fluids were obtained. The
aosorptions per cent, of yarious fluid plates from 0*02 to 0*27 inch
in thickness are giyen in the following table : —
Nune of liquid.
Bisulphide of carbon
Chloroform . . .
Iodide of methyl . .
„ ethyl . .
Benzol
Amylene . . . .
Sulphuric ether . .
Acetic
Forroic
Alcohol
Water
II
II
O'OS.
0-04.
007.
0-14.
6-5
8-4
12-6
15-2
16-6
250
350
460
36-1
4C-5
53-2
65-2
382
50-7
690
69-0
43-4
66-7
62-5
71-5
58-3
65-2
73-6
77*7
63-3
73-6
761
78-6
• ■ «
740
780
820
65-2
76-3
790
840
67-3
786
83-6
85-3
80-7
86-1
88-8
910
0^.
17-S
44*8
68-6
71-5
73-6
82-3
85-2
86-1
87*0
89-1
91-0
1467. InUrrud Absorption of Heat. — It is clearly shown by the
experiments of Prof. Tyndall that the absorption of heat is an in-
ternal molecular, and not a superficial, action ; and the amount de>
pends on the thickness of the medium. This is equally true in
respect of light ; for the light of an electric Ump transmitted
through 15 feet of pure water, which in a tumbler is absolutely
colonrless, assumes a decidedly bluish-green tint. Melloni has
shown the same to be equally true with regard to radiant heat :
the following are the results of Bome of liis experiments with platea
of glass and of seleuite of different thicknesses, the same four
Boui ces of heat having been employed : —
ThiolnieM
in mm.'
Loontelli
lamp.
Red-hoi
platinum.
Copper ni
40U«C.
Copper at
100»C.
« ( 007
1 0-5
o (2-6
77
54
39
67
37
24
34
12
6
12
1
0
• I 001
1 ] 0-4
64
38
14
51
18
5
32
7
0
21
0
0
TRAV8CALEHCT OW OASBS. 821
In this table there is a manifest difference, in these two snb*
stances, in the piwresidve rate of absorption, both of red-heat,
and of the higher dark heat, indicating different values of mole*
cular absorptive power.
1468. QitalUy of Badiant Heat. — Many of the preceding re*
snlts clearlj indicate the fact that the difference in quality of the
thermic rays of different refrangibilities must be precisely analo-
gous to that of the luminous rays. Thus of the visual rays trans-
mitted by any coloured glass a larger proportion will be transmitted
throngh another piece of the same, than through a piece of any
other colour : — so in heat ; if plates each 2'6™"' in thickness, of
rock-salt, selenite, alum, and oichromate of potash, successively
receive the heat-rays from a lamp, and the transmitted rays be
again passed through a second plate of the same substance and
thickness, the following percentage of the rajs surviving the first
transmitted will also survive the second :—
Rock-salt . . . 92*3
Selenite .... 91
Alum 90
Bichromate of potash 71
A comparison of this with a preceding table (1461) manifests the
great difference that exists in each substance between its ab-
sorptive power of the total rays, and of its own transmission : thus
alum, which absorbs 91 per cent, of the total heat of the lamp,
absorbs only 10 per cent, of its own transmitted rays.
Siiuilarlv the beam of an electric lamp concentrated by glass
lenses, and falling on the bulb of an air thermometer at 15 feet
distance, produces no sensible effect on the instrument : because
in its transit through the glass lenses, and the intervening air,
the beam has been filtered of all the rays that either glass or air
is capable of absorbing.
1 469. Transcaleney of Oases. — An extensive series of researches
has, within the last few years, been made by Prof. Tyndall on the
transmission, absorption, and radiation of heat by gaseous bodies,
not less able and exhaustive, than those of his great predecessor
at the Ro^al Institution on the properties of electricity.^ The
means of investigation employed was the differential action of
two sources of heat on the thermopile, the gas or vapour under
examination being interposed on one side of the pile in a tube
closed at both ends by plates of rock-salt, and connected with an
air-pump : and in some cases, in order to divest the results of any
conceivable error arising from the action of vapours on the surface
of the transcalent plates, the effects were examined as the vapours
flowed throngh the open tube, and also without even the inter-
vention of any tube at all. By bringing the radiations from the
two sources to an equality, the needle of the galvanometer stood
at zero, and was, therefore, in the position most sensitive of small
changes of energy. For all the precautionary details of Prof.
TyncUtll's ingenious apparatus the reader must be referred to hii
RADULRT BEAT.
origin&I memoirs, or to his lucid treatise on heat : it need onlj
be mentioned tbat, in all observations on the transcalent pro-
perties of air, it was found to be absolutely necessary to free it
entirely from aqueous vapour and carbonic acid : the first was best
removed by passing the air through a tube filled with fragments
of glass moistened with strong sulpburic acid, and the second br
a tube containing fragments of pure white marble, moistened with
caustic potash.
1470. When a Leslie's cube containing boiling water was em>
ployed as the source of heat, the pure gases, oxygen, hydrogen, and
nitrogen, and consequently air divested of all but oxygen and ni-
trogen, were found to be almost completely transcalent to non-lmni-
nous or hypospectral heat Olefiant gas, on the contraiy, was
found to be very opaque ; and it was ascertained by experiment that
for very small densities of the gas, the absorption of neat was pro-
portional to the pressure : thus the unit of absorption being taat
due to filtered air at mean pressure, and the unit volume oif the
gas being 0*02 of a cubic inch, the observed absorptions, and
those calculated by multiplying the absorption of one volome by
the number of volumes, are given in the following table : —
Units.
OU.
Calc.
UniCt.
Obs.
Calo.
13-2
Unite.
Obs.
Cafe.
1
2-2
22
6
120
11
24 0
24-2
2
4-5
4-4
7
14-8
15-4
12
26-4
26*4
3
6-6
6-6
8
16-8
17-6
13
290
28-6
4
8-8
8-8
9
19 8
19 8
14
30-2
29-8
6
11-0
110
10
220 220
16
33-6 330
It may here be remarked that the capacity of the experimental
tube was 220 cnbic inches: consequently, the density of the atmo-
sphere being 1, the density of one measure of gas in the cube was
only 0*00009, and the pressure, 00027 inch of mercury.
1471. The vapour of ether was found to be much more absorbent
than olefiant gas ; up to a pressure of four inches of mercury the
mean absorption was found to be 2*26 timc9 greater than that of the
gas, but the ratio decreased gradually wiih increasing pressure,
llie law of the constant ratio of absorption to tension was much
earlier deviated from in the vapour than in the gas.
The absorption of carbonic oxide was found to be about ten
times less than of olefiant gas ; but the above law of the ratio of
absorption to tension was observed to hold much fifirther than in
the latter gas.
1472. The fact of the parallelism of radiation and absoiption wae
demonstrated by allowing a stream of heated gases to pass across
the mouth of the conical reflector of the thermopile, xhe radi*-
ABBOEPTIOV BT OASES.
828
tioni from oxygen, nitrogen, air, and hydrogen were found to be
insensible ; while those from four compound ^ases are compared
with the absorptions of the same at a tension of 5 inches of
mercury : —
BmL Aba.
Carbonic oxide . . . . 12 . . .18
Carbonic acid 18 ... 25
Nitrous oxide 29 ... 44
defiant gas 53 ... 61
The nearly corresponding pro^ssion of the powers of radiation
and absorption is perfectly obvious. It hence appears that those
puieous molecules which possess the greater power of arresting,
nave also the greater power of imparting, heat-motion.
1473. The absorbent powers of various gases and vapours were
•compared at pressures of 30 inches and 1 inch of mercury, the source
of heat in these experiments being a plate of copper heated by an
uniform jet of gas : the absorptions ot 0, N, and II being in each
case taken as unity, the relative absorptions were found to be as
foUows : —
Ou.
30 in.
lln.
Om.
30. in.
lin.
Chlorine . .
Bromine . .
Carbonic oxide
Nitrous „
39
a • >
90
355
60
160
750
1860
Sulp. hydrogen
Sulphurous ac.
Olenant gas .
Ammonia . .
390
710
970
1195
2100
8800
7950
7260
It appears from this table that the excess of absorbent power in
compound above that in simple gases advances greatly with dimi-
nished tension : it is also noteworthy that the only coloured gaseous
bodies, chlorine and bromine, are the least absorbent. Likewise a
striking difference may be observed in the relative absorbent
powers of sulphurous acid, and of ammonia, at the two different
pressures mentioned.
1474. The entire independence of the relative transparency and
transcalency of different substances has just been exemplified in
chlorine gas and bromine vapour : but precisely analogous facts
may be observed in regard to both liquids and solids. Thus a
layer of bromine contained between two parallel plates of glass,
sufiBciently thick to extinguish entirely the flame of a lamp, will
freely transmit the heat from a copper ball heated to dull redness ;
also the absorption of heat by bisulphide of carbon was found not to
be sensibly increased by a quantity of iodine in solution, sufficient
to render it completely opaque. Similarly lamp-black, though per-
fectly opaque to light, is by no means equally so to heat ; on the
contrary, it is found to be considerably transcalent to the dark
rays. A plate of rock-salt, so smoked as to be perfectly opaque,
824
KADIAST BBAT.
will neyerthelem transmit 38 per cent, of the radiatiooB from llie
black cube at the temperatm^ of boiling valer.
1475. Absorption by Vapours. — The amount of obserred absorp-
tion bj Be viral different vaponrs, at pressures of 0*1, 0*5, and 1 inch,
of the heat from Leslie's cube, is as follows: —
Yaponr of
Bisnlphide of carbon
Iodide of methyl .
Benzol
Chloroform . . .
Amylene ....
Ether
Formic ether . . .
.Acetic „ . . .
Boracic „ ...
0-lin.
0*6 in.
Itn.
No.ofaL
16
47
62
3
35
147
242
5
66
182
267
12
85
182
236
5
182
535
823
15
300
710
870
15
480
870
1075
11
590
980
1195
14
620
■••
•••
25
From the fourth column of this table, containing the number of
atoms in each compound molectile, it appears that the absorbent
power proCTesses irregularly with the complexity of atomic con-
stitution ; just as friction is greater against rough, nneren surfaces,
than it is against smooth ones.
1476. With vapours at a tension of 0*5 in. the absorption per cent
of the total heat radiated by the red-hot platinum spiral (which was
enclosed in a glass globe with a lateral aperture for free radiation)
was as follows : —
Bisulphide of carbon . 4*7
Chloroform . . . . 6'5
Iodide of methyl . . 96
Iodide of ethyl . . .17-7
Benzol 20*6
Amylene 27*5
Alcohol 28-1
Formic ether . • .31*4
Sulphuric „ ... 31 '9
Acetic .. ... 34*6
II
With the first six of these the order of succe.«sion is the same
as that of the fluids from which they were derived, but in the
four last the order is varied thus : —
• Liquids.
Sulphuric elher.
Acetic ether.
Formic ether.
Alcohol.
Vspoors.
Alcohol.
Formic ether.
Sulphnric ether.
Acetic ether.
This apparent deviation from the general law that the pR>>
perties of radiation and ab8or|)tion are molecular, aud not de-
pendent on the liquid or gaseous form, may be readily explained.
The vapours at a constant pressure do not contnin equivalent
quantities of matter: if, however, the specific gravities of tha
fluids be divided by their vapour-dettsities, equivalent vaponr-
47
Roeemary •
. 74
60
Chamomile .
. 87
65
Cas8ia . .
. 109
68
Aniseed . .
. 372
▲B80BFT10I BT AKOMATIC YAFOUBS. 826
Tolames will be obtained ; and if equal portions of tbeae be
taken, the order of succession is foona to be the same for both
fluids and their vapours.
1477. Ahaorptionhy Aromatic Vapours, — ^The vapours of variourt
aromatic essential oils were introduced into the testing cylinder,
and the absorption in anits of filtered air was in—
Sandalwood . 32 | Neroli . .
Clove . . . 33'5 1 Lavender .
Bose . . . 36*5 | Lemon . .
Beigamot . . 44 | Thyme . .
As the density of these vapours must be exceedingly small, it is
evident that they roust possess, in common with other hydrocarbons,
a very high absorbent power: but how much of the above diffe-
rences depends on specific absorption, and how much on vapour-
density, has not been determinea.
It was fui-ther observed by Prof. Tyndall that although oxygen
in its ordinary condition is very slightly absorbent, it is consider-
ably so in its allotnipic form of ozone, rrobably the dense volume
of ozone obtained from the ozone-generator already described (824)
would have presented an almost opaque medium.
1478. The heat generated by friction when air was admitted into
the nearly-exhausted testing cylinder, and the cold resulting from
its expaniiion, when withdrawn by an air-pump, were alixe de-
monstrated, wherever a feeble atmosphere of some good radiator
was present to tell the tale ; and the relative radiations from the
simple and compound gases themselves, when similarly admitted
into an approximate vacnum, and subseouently withdrawn, were
closely in accoi-dance with their observeu absorptions (1473).
It has likewise been shown that if a heated body, whether solid
or even gaseous, be coated or varnished with a layer of a good ga*
aeons radiati)r, the radiation will be augmented ; just as the rama*
tion of a metallic surface is increased hy ordinary varnishing.
1479. Internal Radiation. — ^The fact that radiation of heat is
not a superficial, but an individual molecular, action has been fully
established by the observations of Mr. B. Stewart on rock-salt ;
from which it appears that the amount of heat radiated from
plates of rock-salt of difierent thicknesses, and heated to the
same temperature, increases with the thicknesa of the plates.
The same observer has also shown in a round-about way that if
fi be the index of the mean refraction of heat by a given transpa-
rent medium, the amount internally radiated to a fixed unit of
surface external to the medium will oc — |. The truth of this is
evident from an inspection of Fig. 573, from which it appears that,
K E and B p being the corresponding directions of incidence on the
sur&ce of the medium, and refraction out of it, the heat radiated
from a circukr area of which the radius Vi dk towards th«
826 SADIAHT BBAT.
point, B,^ will be diBtributed bj refraction over the area of wbich l^e
radius is cf\ and as the ratio of these areas is as e/* : dJ^^w
AS fi* : 1, it follows that the amount of those rays of beat that reach
an unit of the surface ef will oc — | : q.e.d.
The same dependence of radiation on thickness has been shown
hj Prof. Tjndall to be true for gases. From columns 3 and SS
inches in length of four different gaseous bodies similarly heated
bj frictioni the following radiations were obtained : —
defiant gas Sin. 39. . . 33 in. 63
Sulphuric ether vapour . . . „ 1 1 . . . „ 64
Formic „ „ . . . „ 12 . . . „ 09
Acetic „ „ . . . „ 15 . . . „ 70
Here it appears that from the denaitj of the defiant gas, a
considerable portion of the heat-rajs from molecules in the hinder
part of the long tube is absorbed by the molecules in front of
them, therefore the total radiation is not at all diminished in pro-
portion to the diminished length of the radiating mass : but the
much more attenuated vapours offer far less obstraction to the
waves from behind, although their influence is still considerably
felt : thus while in defiant gas, the volumes of the radiating matter
being as 11:1, the radiations are less than 2 : 1, in sulphuric
ether the radiations are nearly 6:1.
1480. Ahnorption by Aqueous Vapour. — ^It most be remarked
respecting this, the most important to organic existence of the re-
lations of heat and matter, that the term " vapour" has been as
much popularly misunderstood, as that of " steam.'' That which
we see hovering over the meadows in a summer evening, called in
common parlance " vapour " is not vapour at all ; — it ceased to
be vapour when it became visible : just as steam (the vapour ot
boiling water) ceases to be steam when it becomes visible. Both
alike are merely minute particles of water (eroundlessly assumed
to be vesicles) floating (sometimes electrically suspended) in an
atmosphere, of which the temperature is so reduced as to render
it incapable of holding them any longer in solution.
Takine as before the absorption of filtered air as the unit, it
appearedthat with the atmosphere in its average state of moisture,
the absorption was about 70 ; it was also found that when filtered
air was saturated by passing through a U tube containing frag-
ments of glass moistened with distilled water, the absorption
amounted to 98. It was also found that in saturated air the ab-
sorption progressed uniformly with the density : the observed are
here compared with the calculated results : —
PreMora. AhBorptioa.
20 in Obs. 64, Calc. 64
25 II • • • II 82, „ 80
Frenvre. Absorption.
5in. . . . Obs. 16, Calc. 16
10 ,1 ... „ 82, „ 32
1 ,1 • • • y, 49, ff 48
Ov ti • • • tt 9o> II 96
QUALITATITB ABSORPTION.
827
For Tarioas climatic applications of these physical facts the
reader is referred to Prof. I'yndall's excellent treatise.
1481. lnfiv£fnce of Temperature on Absorption. — Hitherto the
quantity of heat absorhedl)/ vapours has been alone considered;
bat different resalts will be obtained when the qiMlity of heat is
▼aried by change of temperature. Il is evident that in order to
enable the moiecnles of a substance to *' absorb'* or take np any
particular kind of wave-motion, there must be a congruence be-
tween the period of vibration of the wave and the molecule, and
the oft exemplified analogies of light and heat irresistibly lead to
the inference that the same kind of selective absorption, which is
seen to take place in the various phenomena or colour, takes
place equally with regard tc the rays of heat. As the temperature
of an incandescent hody, the platinum spiral for example, is raised,
it visibly emits in succession rays belonging to the higher portion
o( the spectrum, and if the light- and heat-rays belonging to the
same point of the spectrum, i.e., of the same degree of refran^-
bility, be not identical, they must at all events be isoperiodic.
Hence it is easy to conceive that this kind of selective absorption
may take place in gases, and therefore that the relative absorp*
tions may not be the same for heat of different wave-periods ;
and such is proved to be the fact. Although the introduction into
the heat-beam of rays of higher refrangibility necessitates the
increase of the intensity of toe total beam, it must be borne in
mind that the variations of absorption depend not on the increased
intensity of the heam^ but on the varied quality of the rays.
The absoi-ption by various vapours of the heat emitted by the
platinum spiral, at fiur different degrees of incandescence, was
observed, and the following values per cent, of the total radiations
were obtained :-^
Name of Yapoor.
Barely
▼isible.
Briehfc
red.
White
heat.
Near
fuBion.
Bisulphide of carbon .
Chloroform ....
Iodide of methyl . .
Todide of ethyl . .
Benzol
Amylene ....
Sulphuric ether . .
Formic „ . .
Acetic „ . .
6-6
9-1
12-5
21-3
26-4
35-8
43*4
46-2
49-6
4-7
63
9-6
17-7
20-6
27-5
31-4
31-9
34-6
2-9
6-6
7-8
12-8
16-6
22-7
26-9
251
27-2
2-6
39
■ • •
• • •
• • .
• ••
23-7
21-3
The diminution of absorption by increase of temperature may
here be remarked throughout, but the relative amount of absorp-
tion by different bodies at different temperatures varies con-
siderably : thos in the bisulphide of carbon tne ratio of absorption
828 ' R1.DI1.HT HB1.T.
of barely ^inible to that of white heat is as 2*24 : 1, wfafle in
chloroform the same ratio is only 1'62 : 1 ; from which it apoean
that chloroform absorbs both absolutely and relatively more neat-
rays belonging to the visible spectrum, than bisulphide of carbon.
On referring to the absorption by vapours of heat emitted by
Leslie's cube (1475), it will be seen that the ab^rption of chloro-
form considerably exceeds that of iodide of methyl, thus again de-
monstrating the selective power of absorption. Again, while sul*
phuric ether absorbs less rays from the lower end of and beneath
the visible spectrum than formic ether, it absorbs absolutely more
ravs i'rom the higher sources, and especially from the highest of
all ; hence its relative absorption of the highest rays mast be con-
siderable, and it may be said that formic ether is much morv ther-
mally " blue " or " violet '* than sulphuric ether.
1^1. Radiation from Flamet, — ^The absorption percent of the
rays emitted from the brightest portion of a jet of g^ was com-
pared with that of the heat accompanying the feebly luminous
rays of a Bunsen's burner, in which the gas is mixed with air
hefare ignition^ and the results were : —
Jet
Ban.
Jat
Bub.
Bisiilph. carbon
9-8
Ill \
Amylene . .
30*2
24-2
Chloroform . .
12 0
6 2 \
; Formic ether .
34-6
33-3
Iodide of ethyl
196
140 1
; Sulph. ether .
35-7
31-9
Benzol . . .
22-0
179
■ Acetic ether .
38-7
36 3
It^ here appears that the relative absorptions of amylene and
formic ether vary considerably, while in bisulphide of carbon and
chloroform the amount of abaorption is reversed.
The capacity of (gaseous bodies to absorb their own radiations
was conspicuously demonstrated in the absorption by aqueous
vai>our of the heatrajs emitted by a flame of hyarogen gas, which
mainly consists of incandescent aqueous vapour: tne atmosphere
in its ordinary state of humiditv was founa to absorb more than
20 per cent. ; and even a thin layer of water proved equally ob>
structive to the rays from the hydrogen flame.
The power of aqneoos vapour to transmit the heat-rays of high
refrangibility, but to absorb the lower rays is the chief and potent
means of iire venting the undue dissipation of terrestrial heat by
radiation from the earth^s surface. These, being hypospoctral rays,
are frequently almost entirely anvsted by the aqueous vapour
combined with the atmosphere. The S'imeprinciple was not less
manifest in the absorption by carbonic acid of the rays fiom a
flame of carbonic oxide: so much so that the physical test of ab>
sorption was found to be nearly as accurate as the chemical test
of combination, for the quantity of carbonic acid emitted from the
lungs in respiration.
The relative absorptions, by dried carbonic acid and olefiant
gases at various pressures, of the heat emitted by a carbonic oxide
flame are as follows : —
In.
O.G.
O.A. ]
; !»•
O.G.
C.A.
1 . .
. . 23-2 . .
. . 480 ;
; 4 . ,
. . 50 6 . .
. . 65 1
2 . .
. . 34-7 . .
> . 55.5
; 5 . .
, . 55*1 . .
. . 68-6
3 . .
. . 440 . .
. . 60-3 \
! 10 . .
• . 65.5 .
. . 74.3
829
1483. PdarizatUm of Heat, — Bays of heat are capable of being
)K>1arized bj processes analogous to those by which light is pola-
rized (Chap. XXI.) : these phenomena were first investigated by
Principal Forbes. If a ray of heat be refracted through a very thin
plate of brown tourmaline, it emerges partly polarized in one
plane, the polarization being never so complete as when light is
similarly treated. If the emergent ray be incident upon a second
similar plate, it will be partly abflorbed, and the rest transmitted
or dispersed, according to the positions of the axis in the sections
of tourmaline. When the axes of the two plates are parallel, a
considerable portion of the heat transmitted by the first plate
iMisses the second also, and may be measured by the thermopile ;
out if the axis of the second plate be at right angles to that of tho
first, the greater portion of this heat will be absorbed. By com-
paring the quantity of heat which reaches the thermopile, when
the axes of the plates are parallel, with that which n*ache8 it when
they are crossed, the proportion of heat polarized by the first
tourmaline may be ascertained.
1484. The most satisfactory evidence of polarization U obtained
when plates of mica are used instead of tourmaline. These should
be split extremely thin, and a film should be ignited for some time
in a clear fire. By the action of heat, the film becomes split into
innumerable laminfe. and then is capable of exerting an exceed-
ingly powerful polanzing power on heat and light. These i(;nited
films need never be more than 0*001 inch in thickness, and should
be placed in wooden or pasteboard tubes, to allow of readily mani-
pulating with them.
The films of mica thus prepared should be so placed either in
the wooden tubes, or on the frames of thin wood, that the rays of
heat may be incident upon them at an angle of about 56**. When
beat is incident in the direction c, upon one film placed as shown
at A, Fig. 699, a large proportion of it emerges polarized, and this
will be either trans- j« ^^^
mitted or absorbed by '
a second inclined plate, ^S. /N^
according to its posi- ^s^ > ^^7 \b]
tion. It will be trans- ^-^^ ^—^ U--^
mitted if the plates be 1* 2.
parallel, as at 1, and absorbed if they are crossed as at 2, just as
would happen to light, under similar circumstances. In this
manner the following proportions per cent, of radiant heat from
difierent sources may be polarized : —
Argand lamp 82
Incandescent platinum 79
I
I
I
t
880 RADIAVT HSAT.
Brass heated to 440" C. 68
The heat from ditto transniitted through glass . 73
Boiling-water • 49
1485. Principal Forbes sncceeded in polarizing heat by refrac-
tion through thin inclined plates of rock-salt, just as light is pola-
rized by a bundle of glass plates (1217) ; when heat was incident
at 35", ne found that with three plates, one seventh, and with six,
one half, of the incident ra.ys were polarized.
When plates of split mica were arranged so as to reflect inci-
dent heat at an angle of 56", heac from three different sources
was polarized in the following proportions per cent. : —
Red-hot platinum, 65; brass at 440" C, 61 ; Argand lamp, 65.
These last results are explained by the angle of incidence pro-
bably approaching nearer to the polarizing angle for beat ra-
diating from red-hot platinum, than to that of heat from the two
other sources ; as the same observer has proved that heat of diffe-
rent refrangibilities is unequally polarizable.
1486. It has been observed by M. H. Knoblauch* that when
rays of solar heat polarized by transmission through a Nicol's
prism are incident on a bundle of parallel plates of glass, the
amount of transmitted rays is augmented, as the angle of incidence
increases from 0" up to about 55", the polarizing angle, when the
plane of refraction is perpendicular to the plane of polarization ;
and it is a maximum at tne polarizing angle. With three plates,
the amount transmitted is increased by about one-fourth, and
with twelve plates, it is more than doubled by inclination. When
the plane of polarization coincides with the plane of refraction,
the amount of transmitted heat is always diminished, and mors
rapidly as the number of plates is increased.
1487. It has been shown that polarized light is prevented
reaching the eye by crossing the tourmalines fl215): and when
reflecting plates are employed, by placing the planes of reflection
and polarization at right angles to each other (1218). If a thin
plate of mica, or selenite, bo placed between the polarizing and
analysing plates, it causes the polarized ray to undergo a physical
change termed depolarization (1330), by which it is enabled to
undergo reflection and transmission, producing a brilliant display
of complementary colours. Precisely analogous phenomena oocnr
in the case of polarized heat, and are readily detected by the ther-
mopile ; but, of course, no vinble effects occur, as is the case with
light.
1488. The depolarizing effects of a thin film of mica are best
observed, on account of the great diathermancy of this substance.
For this purpose, let the heat radiating from any source, as a
coil of platinum wire ignited b^ the flame of a spirit-lamp, a.
Fig. 700, be polarized by refraction through the inclined film ef
* Poggendorff, Amisleo, May, 1886.
OIBOULAB POLABI«ATIOar OF BXiff BT BEFBACIIOH. 831
mica, A, in the manner before explained (1484). These rajs will
be partly intercepted by the second mica plate, b, bo that bnt 21
per cent. wUl reach the thermopile, d. Having observed the e£fect
Fiff. 700,
on the galvanometer, produced by these transmitted rays, place
between a and b, a film of mica, c, and if the optic axis (1202) of
the film be inclined to the plane of polarization of the rays of heat,
an increased deflection of the needle will be observed. This arises
from the plane of polarization of the heat refracted through a,
being altered by the doubly refracting film of mica, c, and thus a
portion of the heat previously absorbed by b, now becomes enabled
to traverse it. In four experiments, Principal Forbes found that
the proportion of heat thus depolarized, when the optic axis of the
film was inclined 45^ to the plane of polarization, compared with
that which reached the thermopile wnen it corresponded to that
plane, was as
126 : 100, 120 : 100, 120 : 100, 113 : 100;
the mean of which is the ratio of 120 : 100, or of 6 : 6, very nearly.
When the principal section of the mica plate, c, corresponded with
the plane of polarization, no depolarizing eSeci was observed, pre-
cisely as in the case of light.
14^9. Circular Polarization of Heat. — This has been efiected
in two modes, by refraction through an exceedingly thin film of
mica, and by FresnePs method of internal reflection. A test of
the circular polarization of heat is found in the fact that if whilst
the polarizing plates a and b. Fig. 700, are crossed, so that a mi-
nimum of heat reaches the thermopile, a bod^r be interposed between
them, capable of con verting the rectilinearlyintocircularly polarized
heat, it will produce such a physical change in the thermic rays
passmg through b, that no great difierence of efiect shall be shown
by the thermopile, in whateverposition the analysingj^late is placed.
1490. A film of mica, which in ordinary polarized light ap-
peared of the pale reddish-white colour of the first order of
X^ewton's rings (1135), and which so far interfered with the plane
polarized light as to partiallv convert it into circularly polarized
tight, was first employed. When introduced between the two sets
of mica plates, a and b, this film produced such a physical change
in the heat, as to cause it to reach the thermopile in nearly equal
proportions, whether the polarizing and analysing plates were
parallel or crossed. When mcandescent platinum was employed
aa the source of heat^ the quantity of circularly polarisea heat
832 KADIAHT BBAT.
irUch thns passed the analysing plate under the iDfloence of tho
mica fihn, amounted to 40 per cent, of the whole quantity whidi
would have passed if ▲ and b were parallel, and the film absent.
1491. A very ingenious mode or causing plane polarized heat
to ac<^nire the same properties was contrived oy Principal Forbes
in imitation of Fresnel s mode of obtaining circularly polarised
light by internal reflection (1259). For this purpose he procured
a rhomb of i*ock-8alt haying two angles of 45**, and of sufficient
length to allnw of the emergence of a ray after two internal re-
flections. The most convenient arrangement for the experiment
is shown in Fig. 701, where s is the source of heat, i, the pola-
rizing, and K, the analysing mica plates placed in tabes of wood,
JV* 701.
and B is the rhomb of rock-salt. It was found that when the
plane of reflection in b corresponded with, or was perpendicular
to, the plane of polarization, the rays underwent no change, and
either emerged from, or were absorbed by k, according to its posi-
tion : but when the plane of internal reflection in b was inclined
at an angle of 45" to that of polarization, the rays emei^ged from
B, circularly polarized ; and neariy an equal proportion of them
passed through k, whatever angle its plates formed with the pri-
mitive plane of polarization.
Repebbkces.
The treatise on heat in the " Encjclopndia MetropoUtana" is an
able digent of all that was known of heat at the date of its appear^
ance; and the "Elementary Treatise on Heat,'* by Mr. Baifour
Stewart, comprises most of the important investigations of more
recent continental observers. But by no one has so great an ad-
vance been made in the amount of existing knowledge of the
nature and properties of heat as by Prof, Tyndall ; and to his ad-
mirable treatise on " Heat Considered as a Mode of Motion,'* the
reader must be referred for many important details, the insertioD
of which would have inconveniently extended the dimensions of this
work. The original memoirs of Melloni, Fourier, Pre vest, Duloog
and Petit, Clement and Desorraes, Forbes, Despretz, Regnanlt,
Wiedemann and Franz, Favre, Siibermann, Andrews, and others ,
may with great advantage be consulted.
LIST OF TABLES.
J^J3.—Tk« numb$ri refer to paragraph §, not to pag§9.
Abforption of heat bj fluids
It ft |j"BCB ••• ••• a.. ,,,
n f» perfiimes
n M .• at different tempentares
If II BOllQB ••• ••> >•• ••>
„ ,1 Taponra ...
Adhesion of varioas floida to glasa
,1 metals to mercory
Amounts of reflected heat
Boiling jpolnts of fluids
CapiUiuncy of fluids
Compasa-dsTiatlons in iron ships
Compresiiibility of liquids
ConuuctiTity (electric) of metals
Crystals exhibiting dichroism
Density of water at different temperatures
Diamagnetic bodies
DifiVisiye powers of ffases
DisperslTe powers of various bodies
Elasticity of various bodies
Electrieal relations of various bodies
H rosistance of metals and alloys
Electro-positive and negative elements
Elementary combinations of pore mechanism
Elliptic optical constants
Expansion, by heat| of solids
•A y I U UlUo •■• ■•• ••• ■••
* A M laH^S** ••* *•■ ••• •••
„ „ water at various temperatures ...
Heat-conducting powers of metals
n n materials of dothing
PI pf W^OOuS ■•• ••« ••■ ••«
Heat reflected flrom various bodies
Heating of metallic conductors by electricitv
Height of the barometer in different latitudes
n „ n at different altitudes
Indices ofrefraction of various bodies
„ „ Fraimhdfer's lines
Intensity of light in differentparts of the spectrum . . .
Light lost by reflection at dlflerent angles
Magnetic and diamagnetic bodies
HagnUyiug powers of the compound microscope
Memng pmnts of various bodies
3 H
• ••
• ••
1406
■ ••
1470. 1473
• ••
■ ••
1477
• •«
• • •
1481
• ••
• ••
1461
«■■
• ••
1476
■ ••
• ••
35
• •■
• ■•
35
• ■•
• •»
1440
••s
• ■•
1413
• ••
• ••
87
• ••
• ••
fl07
• ••
• ••
382
•••
• • ■
805
■ ••
• ■«
12S0
■ ••
• ••
422
• ••
•••
648
■ •■
• • •
47
• ■•
■ ••
1097
• ••
• • •
280
■ ••
• ■•
667,668
« ••
■ « ■
9^,943
• •■
• ••
764
• ••
• ••
160
• •a
• ••
1279
• •■
• ••
1350
• ••
• ••
1361
««•
• ■ ■
1362
■ • •
• ■•
1361
• ••
• ••
1373
• ••
• ■•
1380
• *•
■ •■
1375
■ ••
■ • *
1440
• •■
• • •
733
m sm
»••
493
• ••
• ••
419
• ••
• •■
1067
• •■
• ••
1101
• •■
■ ■ •
1104
• ••
• ■ ■
1031
■ •t
• ■«
648
• • •
• ••
1164
• ••
• •a
1407
834 LIST OF TABLES.
Miudcal intoiTolB ... ... ... ... ... ... ... ... 670
Komben of reflections required to polurize light 12S3
„ teeth and Btaves in wheels 189
„ Tibrations in coloured mjs 1087
Optic axes in uniaxial crystals U08
„ „ biaxial ciystals ISO
PrassureofaqueousTapour atTBrioostemperatares 519
„ steam „ m ... ... ... 680
ProjMftioDB of heat transmitted through rock-salt 1468
RJMiating powers of various bodies 1467
Bates of cooling by radiation 1438
Beflection of heat from TarioussurfiMses 1410
Befraction of coloured rays lOM
Belative temperature and pressure from 211° F. to 213° 1412
Besistances of TariouB bodies to oomprossion 18
Botation of the planes of polarization 1288
Specific gravities of solids, fluids, and gases 425
„ heat of solids and fluids 1902
M yi ICfUSvB ••• •■• ••• «•« ••■ ••• ••• ■•• A^HFS
Temperature variation of dectric condQctivity 9^949
Tenacity of various metals 15
Thermoelectric series of metals 982
Tranacalen<7 of solids 1461
M UulQB ... ... ... ... ... ... ... A^K^v
ff gases ... ... ... ... ... ... 147vy 1473
„ aromatic vapours ... ... ... 1477
•y T ttpOUlD ••■ ••• ••• •«• ••• ■•• ■■• A»V W
Variahons of concert-pitch in music 671
„ magnetic declinatioa 609
„ „ inclination €14
Velocity of sound in some bodies 646
M wXDClo sst ••■ ••• ••■ ••■ ••• ••• «•• 09v
Width of coloured bands in the solar speotnim 1083
ALPHABETICAL INDEX.
Jf.B. — Tht number9 refer to paragraphe, not to pagee.
ABB
AIBI AIB
Abemtion, ohromatio— , 1119;— of
Hght, 1023 ; leaat mrole of—, 1047 ;
—of a reflected ny, lOM; spheri-
<ml— of lenaee, 1081.
Abflolute cold, point of—, 1348 ;— mo-
tion, 270; — refractire poirert, 1057.
Abeorption of heat bj gaaes, 1470,
1473;— liquids, 1466 :— solids, 1461 ;
Tapours, 1476 ;— perfdmed— , 1477 j
— and radiation parallel properties,
1464;— of light, 1082.
Abutments of an arch, 108.
Accelerated motion, 270.
Accelerating force, 273 ; relation of
— , to time, space, and Tdooity, 300,
806.
AceiimQlation,lateral— , of waTes,468.
Aooorate graduation of thermome-
ters, 1366.
Achromatic lenses, 1120;— for the
camera (photography), 1294, 1296;
—object-glass, 1167; — adjustment
of—, 1169.
Achromatism, 1119.
Adinlo lines, 612.
Aooustio figures of vibrating plates,
583; optical— ,6^;— lens ox Sond-
hans, 664;— shsdow, 666.
Actinism, 1286.
Aotinometer, 1384.
Action, line of—, 168.
Adhesion, 34; intensity of—, 36;—
of gases, 48.
A4justment of object-glass, 1169.
Agate, action of—, on light, 1214.
Agonic lines, 609.
Air-balloon, 1882.
Air-gun, 610.
Air-pump, simple—, 608 ; Seuokebee'e
— , 604; Smeaton'e—, 606; Cuth-
h0re»^, 606; Orw^e—, 606;—
gauge, 507 ;— syphon-gauge, 608.
3 I
ABC
Air-thermometer, 1864 ; — and mercu-
rial—compared, 1368; differential
— , 1356.
Air-Tcssel of foreing pump, 437.
Airy' 9 method of producing oirenlsr
polarisation, 1269; obserfations on
astigmatisui, 1188.
Ajutages, 434.
Alarum, electric—, 918.
AlbumJuixed photographic paper,
1314.
Aldiw^e researches, 966.
Amalgam, electric, 696 ; — of smmo-
nium, 840, 841 ;— of potassium, 838.
^sMci't prism, 1168.
Ampkre e electro-dynamic formula,
862 ; — ^theory of magnetism, 883.
Ampmtype, 1318.
Anelectncs, 668.
Anemometer, lAnJPe—, 524; ISoMn-
son'*—, 624.
Aneroid barometer, JBoiirdoa's— .
497; r«d:y'«^, 496.
Angle of incidence, and reflection,
1083; polarixing— rel^bed to index
of refraction, 824; risual— , 1079.
Angular Teloeities in Unk-work, 162;
— ^in rolling contact, 163.
Animal mechanism, 148—162; elec-
tricity, 976—1017.
Annual rariation of the barometer,
492 : — of the magnetic needle ; 618.
Annulus, formation of an — , 326.
Anorthic system of crystals, 24^ tx.
Ansell's flre-damp indicator, 48.
Anthotype, 1820.
Antimony, thermo-electric proper-
ties of, 962, 968, 976.
AppoUTe centiifhgal pump, 448;—
temperature-regulator, 1368.
Arch, equilibrated—, 108—104; uni-
form-, 106.
2
836
INDEX.
ABO
BAB
ilreAiM«i«», hydroBtatio prinoiple of
—,410; aorew of— , 463.
Argentotype, 1305.
Arm of ft oonple, 81.
ArmUrcn^* oonoiusion fiuo, 280; —
hydro-electrio maohinft, 699 ; — seg-
ment shell, 280.
Artilioiftl oiTstals, 20 ; — marnets,
691 ; — ^respiration, ftpparfttos for — ,
613.
Ascent of balloons, 61.
Assaj balance, 116.
Astatic needle, 836; eqailibrinm of
— 867.
Astigmatism, 1188.
Atmosphere, its constitntion, 477;
dansitf . 478 ; elasticity, 611 ; pres-
sure, 491; reeistanoe, 612;^topro*
jeotiles,8l4; weight, 480.
<* Atmospheres,** pressure measured
bj— . 620.
Atmospheric electrioitj, 747—766.
Atomic heat, 1396.
Atoms, their form and sixe, 6, 6.
Attraction of atoms, 9.
Attraction, adhesire— , 84; capillar^
— , 86—40 ; oohesiTe— , 82 ; electric
— , 656 ; mftgnetic— , 691;— of elec-
tric corrents, 864 ; — of gravitation,
68—62.
Attractiye forces, 80 ; law of—, 31.
JMwood't machine, 3(33.
Ju0t8e$ formula, 1426.
Aurora Borealis, 760; magnetic influ-
eoce of, 925.
Automatic registration of the baro-
meter, 496 ; — magnetometers, 624;
—thermometer, 1367.
Axis, denolariiing— , 1230; maene-
crjstslno— , 662 ;— of rotation, 869;
instantaneous—, 367 ; optic—, 1202.
Axle, wheel and—, 131.
BaeoMio*9 yegetable battery, 1013.
Bain'$ electric clock, 962;— electro-
chemical telegraph, 932.
Bands, and puUeys, 208— 210 ; mode
of shifiing— , 211.
BakttMU'B electric copying telegraph,
9i6.
Balmoe, 114 ; chemical, or assay-,
116; false—, 116; BobenaFa—t
128; sensibility of the— , 114 ; tor-
sion-, 290.
Bar, magnetic, 693.
Barker* § mill, 480 ;— acting by steam,
621.
Barometer, principle of the— ^ 483 ;
Bourdon' $ aneroid — , 497 ; Vedjf't
aneroid — , 406; annual variation
of—, 492;— applied to measuring
BAB
CAL
heights, 404: conical—, 489; oor>
reotion of— lor temperfttnre, 487;
diurnal variation ot— , 400 : mean
diurnal height of — , 491 ; mean
height of—, 403 ; mercurial — , 484 ;
sel^registering— , 405 ; stuidard— ,
486; syphon— ,485.
Bart9n'9 buttons, 1139.
Battery, electrio— , 724^ 726: tbet^
mo-dectric— a 968, 909; voltaie— ,
766—796; Bun9en*»—, 777; Ontik-
tkan^t^, 793; DamieiTa—^ 794,
795; frog—, 990; Oro9^»—t 796;
SS-, 819, 820; L4e$im*»—, 781;
arie Davy't—, 792 ; IfMOoO— ,
779; Pulverwuuker'M — , 790; JKe-
hertiTi—, 780; SeAoii6«wi'e— , 773;
Sme^i—, 771, 772 ; fitrinafdlot^9^,
791; froUa«to»'»— , 703.
Beam and scales, 114.
Beats in music, 649.
Beequtrelf on electrolysis, 632; his
battery, 835 ; thermopile, 971.
Bell-erank work, 221-223.
Bells, electric—, 711, c.
Bent lever, 112, 118.
BemotUUPa formula for gMeoos em^
rents, 621;— theory of the tides,
473.
Bevilled wheels, 177.
Biaxial crystals, 1206, 1207.
BiiHar magnet, 622
Binocular vision, 1195; — mioroaoope^
Wenham'*, 1163.
Biot, on circular polarisadon, U87 ;
— ^polariscope, 1217, 1218; <m the
length of sound-wavea, 660.
Bismnth,diamagnetio propertiea of—,
646 ; thermo-electric — , 062, Ott.
Boiling point related to oompoastioD,
1413.
J9oiwr« astronomical chronograp]i,068.
BourdmCa aneroid barometer, 407.
BomtignviOn the spheroidal state of
fluids. 1417.
BojfWa law of gaseous praaanre, 108
—600
Bramak'M press, 804.
Bregutfa thermometer, I860.
BrewtUr*9 kaleidoscope. 1088; »c-
searohes on polariaed lights UI1-—
1224;— stereoscope, 1100.
Bridge, TFAmMm^s— , 800, 801.
BritUeness, 17.
Brush, electric—, 708.
Hafwrn't battery, 777.
B]f9ir6m*$ hydro-pyrometer, 1371.
CeBsram disoovered by
analysis, 1106.
Caloqatliig machlBWi, 28^—234.
INDEX.
837
CAL CUB
Caleidophone, W\eaUton»*9—f 375.
California pump, 439.
CalUn amd BipU/9 multiple motion,
206,207.
Caloite, coloured rinn in — , 1236.
1237 ; double refraction in—, 1200
—1205.
Caloreacenoe, 1447.
Calorimeters, 1389; Regnaulta—,
1390 ; Kopp'^-, 1391.
Calotype, 1306.
0am, the, 147; yarioua forma of—,
254.
Camera lucida, 1162 ;— obeoura) 1147 ;
photographic—, lj93.
Canary glau, fluoreeoence in — , 1114.
CapaoitT of bodies for heat, 1392;
speoiflc mduotiTe— , 680.
Capillarity, 36;- In tubes, 37, 89;
•—between plates, 38.
Capillary depression, 41.
Capstan, the, 131 ; Chinese—, 138.
Carbon-printing, 1336.
Cauejprain'a telMCope, 1144.
Cassia, oispersiTe power of oil of—,
1098.
OatenaiT ourre, 84^ 106.
OavalUft table of electrics, 667.
Cftustics by reflection, 1043; by re-
fraction, 1067.
Celestial photography, 1336.
Centre of grari^ 85 ;— of a system of
points, 86 ;— of a line, 87 ;— of a pa-
rallelogram, 88 ;— of a triangle, 88 ;
—of a plane polygon, 89 ;— of any
plane ftnire, 90 ;— of a ring, 91 ;—
of a body not homogeneous, 92.
Centre of oscillation, 847; — and of
suspension reciprocal, 349.
Centre of percussion. 351.
Centre of pressure of a fluid, 401.
Centrifugal force, 820— 324 ;— orac-
tioally demonstrated, 321; figure
of the earth due to—, 324; an-
nulus formed by-—, 326.
Centrifrigal drill, 82;— pump, 412;
AppoUr*^, 413.
CkanniHft process (photography),
1811.
Charcoal, Toltaio ignition of—, 806,
809.
Charge, electric—, 718; residual- ,726.
Chemical action, electricity from — ,
762;— of light, 1106, 1107; 1281-
1288.
Chemical balance, 116.
Chords, Tibrationa of, 576, 677 ; Tocal
Choroid membrane of the eye, 1179.
Chromatic phenomens, 1082 — 1140;—
aberration, 1119;— cUspersion, 1096.
CUB COM
Chromotype, 1319.
Chronographs. 958, 959.
Chronoecope, WkaaUttnu'*", 967.
Chrfsotype, 1315.
Circle, least— of aberration, 1047.
Circular arc, oscillation in a — ,
332.
Circular polarisation of light, 1256 —
1274 ;-of heat, 1489—1401; infln-
ence of solution on — , 1274;— of
temperature on—, 1273.
CleanUness in photography necessary,
1341.
CUment and De$orme§ on capacity of
gases for heat, 1396.
aick, ratchet and—, 866—267 ; the
silent—, 268.
Clock, electric— V 08 1—936;— 'trains,
228—231.
Clothing, rationale of—, 1880.
Clouds, dectricity of—, 751, 756.
Coddin^on*$ lens, 1164.
Coefficient of friction, 53.
Coercing magnetic force, 638.
Cog-wheels, 174
Cohesion, 32 ;—flguree of oils, 83.
Coil machine, 898 ; primary—, 896 ;
secondary — , 896.
Cold produced by CTaporatiou, 1419,
1420;— by expansion, 1396;— by
radiation, 1486 ; — by a Toitaio cur-
rent, 817.
Collision, effects of, 288 ;— of elastic
bodies. 283; — ^practically demon-
strated, 284, 296:— of ineUtetio
bodies, 291, 292.
Collodion process, 1383 — ^1326 ; — ^ne-
gatives. 1826;— positives, 1327.
Ccnour-bUndness, 1194.
Coloprs, incidental — , 1190—1193;
complementary — , 1091 ; — of thin
plates, 1132 p-oi a soap-bubble,
1131; Ntwlon'i table o^-, 1134;
prismatic — , 1082;— of small par-
ticles, 1138; — of i^o&err« lines,
1139; primary—, 1090; tpectral- ,
1190-1199.
Colour-top, Qorham'^—, 1098.
Combinations, voltaic, 771—796 ;— of
pure mechanism, 160.
Communioator (electric telegraph),
915.
Compass needle, 602} deyiat^ons of
—.608—607.
Complementary colours, 1091 ;— in
polarized light. 1231, 1236.
Composition, physical and chemical,
7 ;— of motion, 283^286 ;— of rota-
tions, 366 ; — of statical pressures,
69, 70 ;— of TelociUes, 284.
Compound leTcrs, 125 ;— microteope.
838
ISDEX.
coif
CRA
1166; SoM*!— 1163;~bftr thermo-
meter, 1369.
Comprenibilitj of flaids, S82.
Comprenrion and rettitntion, 19, 289.
Concvre mirrors, reflection of light
from—, 1098—1041 ;— of he«t from
— , 1434, 1435.
Coaoert pitch in mario, 671 ; tables
of Tsriations of—, 671.
Concord in music, 674.
Concossion ftue, Anuirong^$—, 280.
Condensation of gases, 10; 1429.
Condensing syringe, 609.
Condenser, electric, 738—740; Gil-
Uee'i—, 1167 ; optical, 1166—1168.
Coudnoting mediom neoessaiy for
sonnd, 633;— plate, 768; — wires,
magnetic propertiee of—, 862, 863.
Conduction of heat, 1373.
ConduotiTity of metals, table of, 805;
-T4nflaenced by stmctore, 1376 ; —
by temperatare, 1876 ;— of liquids,
1378 ;— of gases, 1379.
Conductive discharge, 716, a..
Conductors of electricity, 667, 668;
rcTolation of—, 867. 868, 874, 879,
882 : Tibrauon of—, 868 ;— of heat,
1378.
Cone, equilibrium of a—, 101.
Conical barometer, 489;— pendolnm,
338.
Conjugate foci of a mirror, 1039 ;— of
alens, lora.
Connectors, wrappings— , 208 ; motion
by-, 244^-246. ^
Conservation' of vis viva, 340, 341.
Contact, rolling— , 164; sliding, 164;
—motions, velocity-ratio in—, 163;
sliding in—, 164.
Contraction of fluid currents, 488.
Convection of heat, 1384, 1386;— by
gases, 1386.
Convective discharge, 716, o.
Convex mirrors, reflection of light
from—, 1042.
Cooling, rate of—, by eonvectlon,
1439;— by radiation, 1438;— de-
pends on surflttce, 14^.
Copying objects by photography,
1200.
Cordage, friction of—, 66 ; rigidity
of-, 138.
CouIohA'm electrometer, 664; — elec-
trostatic laws, 673; — ^mode of de-
termining mufnetio force, 639.
Couple, arm ora— , 81; moment of
»-,81.
Couples, theory of—, 81—83.
Crab's daw, joints of a—, 250.
Crane, the, 131 ; the hydraulic—, 469.
Crank, the, 218, u.
CRU
DIA
CnUk$ka»W battery. 7S3.
Cryophorus, WolUuion'9—, 1421.
Cirstals, natural uid artificial — ^SO;
dimorphous — , 29; hemihednl — ,
27 ; twin, or hemitrope^, 28 ; pri-
ms^ forau of—, 25; biaxial—, 1J06,
1207; uniaxial—. 1201—1204.
Crystalline lens of the ejre, 1170.
Crystallisation, 21,22; six systems of
— . 23 ; Cubic system. 2-4, i . ; Pyra-
midal-, 24, n.; Prismatae— . 24,
III. ; Oblique—, 24, iv. ; Hhcunbo-
bedral, 2^ t. ; Anorthic — , 24^ n.
Cubic system of crrstals, 24, i.
Cubical expaosi^ by heat, 1360.
Current, ebotnc — , 766 ; diverging — .
435 ;— excited br magneCiaD, 9ii6 ;
secondarr, or tnonced — , 886 — 889 ;
thermo-electric — , 962; voltaio— ,
766; mutual action of—, 86^
CMhb«H*9 air-pump, 606.
Cyanotfpe, 1316, 1317.
Cycloid, the, 330; time of an oaeiD*-
tion in a— ,331,332.
Cylinders, strength of—. 14.
QrUndrical electric machine, 001.
Daguerreotype, 1298, 1280 ; toaiof of
—plates, 1300.
jyAUmUre$ principle. SSO.
DaUon'i table of gaseous jjicssiues,
619.
Dancers, eleetrie, 711.
DonieU't hygrometer, 1424; — pyro-
meter, 1370;— voltaic arrangement,
774;-battexy. 79^ 795.
Danff Sir A*., experiments in photo-
graphy, 1289.
Dead points of a system, 217.
Declinstion, magnetic, 600, 610.
Declinometer, 622.
Decomposition, prismatic—, of light,
1082;— of heat» 1444,— of water
by heat, 1481 ; electro-chcmieal— ,
821 822.
0« la £<m'« floating battery, 874
Density of matter, 11 ;— of water at
varioos temperature*, 422.
Depolarising axes, 1230.
Descent on a curve^ 828.
Detent, the, various forms of— » 266—
267.
Deviations of the compass, OOS— 607 ;
semicircular — , 004; quadrantal— ,
604 ; sentantal and octantal— , 605.
Dew, 1460;— point, 1483.
Diamagnetic bodies, table of— ^ 648.
Diamagnetism, 641 — 644.
Diamagnetic bodies^ 646 018 y—
fluids, 640.
Diamond jar, 728.
IVDBX.
889
DIA BAB
Diatonio loftto, in iniiBi<^ 668.
Diohzoism, 1280.
Dieleotrie^ 716, 717.
Differeooe engine, 233; prinoiple of
the— .234.
Differeotial thermometer, 1356.
Diffraotion ofUght, 1126—1131.
DiffViaion of gwaee, 47 ;— of UqoidB,
50.
Dimorpboofl crystalB, 29.
Dip, seenlar chuiffe of—, 612.
Dipping needle, 611.
Direct tranaition flrom the solid to4he
^eoQt etatOj 1428.
Disgmaed eleotncitj, 716.
Dioptrics, 1064—1061.
Dipping needle, 611.
Directional relation defined, 169.
Discharge^ electric—, 716 ; lateral — ,
744 ;— of atmoepheric electricity,
766—768 ; ipontaneoua— , 756; toI-
taic, 80fr-d0e.
DtMbarging rod, 722.
Diaobaiger, univerMl, 729.
Diaoora in mnaio, 674.
Discs, revolntion of—, by a current,
870 ; producing a cnrrvnit^ 894.
Di^pused electricity, 716.
Dispersion, chromatio— , lOffl ; inter*
nal— , 1113.
DispersiTe powers, table of—, 1097.
Distillation, 1409.
DiatDrbanoe*Tariations of magnetio
elements, 620.
Diurnal magnetio Tariation, 616—
617 1 — ^barometric-, 491.
Diriaibility of matter, 12 ; finite—, 1.
Dofm^, on the electricity of animal
membranes, 1009 p—of T^etables,
1017.
Doable image prism, 1213; — nose-
piece^ Brookt9—t 1163 1 — refrac-
tion. 1200— 1206 ;— weighing, me-
thod of—, 117; — ^tou<», in mag-
netism, 663.
Doublet, WolUuton*9 1166.
Driver and follower, in mechanism,
164.
Dry piles, 818 ;— steam, 1401.
U% Boi9-B€3fmond'$ galvanometer,
868: — reae«rohes in organic elec-
tricity^, 896.
Dynamic equivalent of heat, 1346;
—theory of electricity, 997, 998;
—heat, 1344 ;— light, 1019—1021.
Dynamios, 270—878.
Barth, the, directive power of—, 693.
figure of— ,324; magnetic poles
o^— , 606;— ourrents of eleetnoity,
626.
BBU
BLL
BbuUiiion of fluids, temperature of — ,
1411 ;— depends on pressure, 1412 ;
rresenoe of matter necessary for —
414.
Bocentric, the—, 147 ;— wheels, 239.
Bcho, 661.
Elastic bodies, collision of—, 293-296.
Blaaticity, 18 ; perfect—, 19 ; modulus
of— , 289;— of air illustrated, 611,
c— e.
Electricity, animal — , sources of,
1007 ; atmoBpherio— ,747— 766; dis-
guised—, 716—719; Franklinio— ,
654—760 ; organic—, 976—1017 ; re-
sinous— , 666; thermic—, 962 — 976 ;
ritreous— , 656 ; voltaic—, 761—849.
Electric alarum, 918 ;— batter/, 724,
725 ;— bella, 711, G ; — bruab, 703;
— charge, 721 ;— clock, 3ain'»-~,
962; Shepherd' § — , 953;— oondenser,
738— 74U ; — current, 766 ; — dis-
charge 715 ; stratifled — , 811 ; — ex-
citation, 654;— fishes, 970—981;—
induction, 076— 678;— jar, 720;—
lamp, Serrin'if 810 ; — machine, cyUn-
drio— , 691 ; plate—, 692 ;— p&tol,
709 ;— regulator, 961 r-*^*r* 703 ;—
telegraph, 915—935 ; Bain'e chemi-
cal—, 982; Bak«itoelV9 copying—,
935 ; magneto—, 920—925 ; Jfor««
printing—, 929: needle— ,916; J2o-
nald$'»-^, 016; Wheuietone* 9 dito^,
919 ;— magneto— , 920 ;— printing
— , 933 ;— private— , 923—926;—
time-baU, 060.
Electrodes, 766.
Electro-dynamics, 860 — 914.
Electro-dynamic induction, 886—914.
Electrolysia, 821—841 ;— by Franklinio
electricity, 842, 843;— of water,
821 ;— of metallic chlorides, 836;
silicon reduced by—, 838; potaa-
ainm, and aodium — , 839; ammo-
nium—, 840, 841.
Electro-magnetiam, 871 — 878.
Electro-magnetic engines, 896—897.
Electrometer, Coulomb' »t—OM ; Heih
ter**— , 710; PeUier^t—, 666;
Thomaon'i^ Sir W^.— , 666.
Electromotive force, S02,
Electromotors, 767.
Electrophorus, 685—688.
Electroscope, pith-ball — , 661 ; gold-
leaf-, 662 ; condensing—, 742 ; dis-
tinguishing—, 747; J2»ic/t/r<'«— .668
Blectro>static laws, 673.
Electro-thermic effects, 976.
Blectrotype, 776.
Elliptic constants, 1279; — mirror,
1050 ;— pohurization of light^ 1276—
1279 ;— wheels, 236, 286.
840
IKDKX.
BND FIE
Sndlemband, 209: mode of shiftiog
— . 218.
EndleM screw, 202 ; diBgnised— , 206.
EndoBCope, 1199.
Sudounose, and ezo9inoee» 44—46; —
influenced bj ft Toltaio oorrent, 46.
Epif^oloidel teeth of wheeb, 183^
Equator, magnetio— , 612.
Equatoiial moanting of telesoopefl,
1145.
Equilibrated arch, 103—106.
Equilibrium of a solid on its base, 93;
stable—, 94, 06; unstable—, 96;
indifferent — , 97; — ^related to path of
centre of gravitV, 96—100 ;— of an
arch, 104 ; — of the same in practice,
106— 108;— of fluids. 884, 886 ;— of
the ocean, 473 ;— of floating bodies,
404, 406 ; stable or unstable—, 407,
406.
EquiTalent, dynamio^, of heat, 1845.
Errors, index-, 1366.
Escapement, 266; verge — , 267; an-
chor or lerer^, 268.
Establishment of a port, 476.
Evaporation of liqmds, 618;— in ani-
mals, 1427.
Exceptions to law of expansion b;
heat, 1353.
Exobangee, theorr of—, 1486.
Excitation, electiie, 654.
Exhausting syringe, 503.
Expanding puller, 246.
Expansion, cubical—, 1350; linear
— of solids, 1850;— of liquids,
1361; relative— of glass, water, and
mercury, 1861 ;— of intses, 1852.
Extension, a property of matter, 8.
Extrados of an arch, 102.
Extraordinary ray, 1200, — ^refraction,
1061.
Eye, the, optical structure of—, 1179
—1188 ; adaptation of —to focal
distances, 1186 ; inversion of images
in—, 1183.
Eye-piece, Suygtn^ at the negative
— , 1169 ; Ram$d«n'$ or the positive
— , 1160.
Fakrtnkeifi helioeUt, 1177; — ther-
mometer-scale, 1868.
Fabe balance, 116.
Faradojf, researohes in diamagnetism,
641 — 648;— on induction, (wl ;— on
magneto-electric indttotioQ, 904; —
on organic electricity, 979; — on the
action of magnets on polarized
light, 1266 ; toUe of electnos, 668.
Ferrotype, 1316.
Field, magnetic, 698.
FIG
Figure of the earth, 324.
Fire-engine, 410.
Fire-damp indicator, 48.
Fishes, electric, 976—081.
FUgrw, ^(/m., forecasts of
626.
FlexibiUty, 13.
Flight of a rocket, 817.
Floatation, plane of—, 406.
Flosting battery, De la Biv0'» — , cnr j
Floating bodiee, fluid diaplaoed bj — ,
404 ; equilibrium of—, 406—407.
FldVentine experiment, 881.
Fluids, oompreseibiHty of — , S81;
elastic ana inelastic— , 880 : expan-
sion of— by heat, 1361 ; mobili^ of
their particles, 379; downward prea-
snre of—, 892; lateral presanre of
— , 808—400; upward pressure of—,
397 ; pressure of—, in conical tabee,
486; resultant of pressure of—,
402, 403; properties of—, 379;
spouting—, velocity of—, 4S6k 487 ;
surface of— hbriaontal, 387; velocitT
of— in tubes, 481 ; unduladoaa of—,
463—471.
Ilnoreseence, 1113, 1114.
Fooal length of lenses, 1072 ;— of mir>
mirrors, 1039— 1041;— of a sphere,
1069. 1070.
Fooal lines, 1048.
Foci, coigngate— defined, 1090.
Focus, neffim?e— defined, 1046 ; prin-
dpsl— defined, lOia
Fin'o«$, researches on the refraetioiB,
and polarisation of beat, 14B3—
1491 ;-thermopae, 1441.
Force, accelerating—, 273; centrifbgal
— , 380— 885 ;— related to time,
space, and velocity, 305$ electro-
motive—, 802.
Forced vibrations, theory of—, 680.
Foroing-pump, 437.
Foreeasts of weather, 626.
Form an attribute of matter, 4.
Forms of cams, 264 ; — of pulleys, 210
—216.
Foucaulft glass speculum, 1068;
mode of silvering—, 1068.
Fountain, artifictu— , 482,611,0.;
FranUinic electricity, 664—700.
JF^utMin'i electric kite, 761 s-thrni-
der house, 768.
Frammk^€7'9 lines, 1099.
Freesing in vacuo, by evaporatioo,
1420, 1421;- in red-hot vsbmIs,
1417.
lVwn«r« experiments on interferenoe
of light, 1123;— laws of poJsrised
light, 1229;— rhomb. 1269.
Friction, 62; a uniform retardiBg
IKDEX.
841
TBO
OSO aRO
force, 64; ooefBdent of—, 66;—'
of cordage, 66 j — of flaida, 434 ;—
rollers, 67.
Frog-bstteiy, 900 ;— rbeoaoope, 991.
IfQlgaritea, 769.
Pusee, the, 246.
Fnaion, temperatare of—, 1407; in-
flaenoeofmiztore of elements on—,
1409 ;— of pressure on — , 1408.
Fose, Armtiron^t ooxtcusaion— , 280.
OaUUtft telescope, 1171.
Qalvam'i discoveries, 989.
GaWanic knife, 816; — cautery, 816.
Galranometer, 856; astatic—, 866;
<7a«^«»'« tangenc— , 861 ; Tkomp^
ton* 9 marine—, 860; reflecting'—,
869.
OalTanoscope, MaiteuceCt, 991.
Oamnt in music, 668.
Oases, condensation of—, 14S9 ; dif-
fusion of—, 47 ; law of pressure of
— , 488— 600;— liquefied by pres-
sure, 10; relatiTe expansion of—,
1362; specific gravity of—, 423;
specific beat of—, 1394; traiMpira-
tion of—, 48.
Oas-batiexy, Ofwo***— , 819, 820.
Gas-engine, Lenoir^B — , 629.
Gaugain's tangent galvanometer, 881.
Gauge, air-pump—, 607; syphon—,
Oauti^ agonic lines, 609.
Gttuing, 178.
Generating plate, 768.
Geneva stop, 248.
Genou, the, 130.
Geometrical focus of reflected rays,
1039 ;— refracted— , 1066.
Oersers explained, 4SB.
Oiilett'i condenser, 1167.
Glass, unannealed, optical properties
of—, 1262 — 1266; photographic
processes on—, 1323— 1330 ;— spe-
culum, JVii«aai//'a — ^,1052; mode of
silvering—, 1063.
Gold, ductiHty of—, 12.
Gold-leaf electroscope, 662.
Oorkam*» colour-top, 1092.
Grave harmonics, MB,
Gravitation, 68; law of— , 69; con-
sequences of—, 60—62; lateral—,
60.
Gravity, centre ot-^, 75, 76 ; aeode>
rating force of—, 296—300,
Gravity, specific— v defined, 11;— of
gases, 423;— of fluids, 416—400;—
ofsolids, 412—416.
Oregoty'i telescope, 1148.
Gridiron pendulum, 854—366.
Grooves in pulleys, forms of—, 210.
HJO
Qrovt^t air-pump, 606;— battery, 796;
—gas-battery, 819, 820.
Gulfstream, 1386.
Gynmotus electrieus, 978; Faradag
on the — , 979.
Gyration, radius of—, 346.
Gyraaoone, the, 866—367; theory of
""", 366.
Gyrometric governor, Sinuiuf — , 461.
Haldatt on fluid pressure, 392.
Hardness, 9.
Harift hydrometer, 410.
Harmomc sounds, 676.
Hartuft unit jar, 737.
HawlctbM*$ air-i)nmp, 604.
Heat and light, identioal in their n^
ture,1437.
Heat, absorption of— by gsses, 1470
—1473;— liquids, 1466 ;- solids.
1461 ; — ^vapours, 1475 ;— perfumea
vapours, 1477; — in liquefaction of
souds, 1400; — formation of vapours,
1406 ; chemical action of—, 1490; cir-
cular polarixation of—, 1483; — by
internal reflection, 1491^ — by re-
fraction, 1486; conduction of—,
1373 ; convection of—, 1382, 1383 1
decomposition of water by — , 1481 1
dynamic theory of—, 1344 ; — equi-
valent of—, 1346 ; evolution of—, by
solidiflcation, 1401: expansion of
'fluids by— 1361 ;— of gases— , 1362 ;
—of solids— ,1360; latent— , 1397 ;
polarixation ^— by mica, 1484 ;— by
reflection, 1491 ; — by tourmalines,
1483 ; proximate causes of—, 1343 1
radiant—, 1433; refraction of —
through lenses, 1443 ; — through
prisms, 1444; relations of — and
cold, 1348; specific—, 1388— 1394 1
terrestrial—, 1432 : theoiies of—,
1342.
Heat-spectrum of electric lamp, 1446.
Heeling error of oompass, 607.
Heliacfu conductors, 876.
Heliostat, FakrenluUt—, 1177; SO-
hermamie»—t 1178.
RelmhoUt^ resonator, 639.
Hemihedral forms of crystals, 27.
Hemispheres, Kacdeburg — , 611, a.
Hemitrope orystus, 28.
SeiUsf'9 electrometer, 710.
Herapathite, 1216.
Heneh4L Sir J., researches in pho-
to^aphy, 1816—1321 ;— table of lu-
mmiferous undulations, 1086;— on
animal electricity, 1016.
Heterogeneous organic snbstanosa
produce a current 1013.
Hiero'a fountain, 448.
EjortVB electro-magnetio engine, 886.
842
INDEX.
HOA
IKD
Hoai^froat, 14S0.
HaU^ indaotion machine, 701, 702.
Hook^9 joint, 247 ; Tolocitj-ratio in
~, MB : ooropoand— , 248 ; itla8tra>
tion oi— , in the aoimal economr,
260.
Horuontal sorfaoe of a fluid at rest,
884,386.
Horseshoe magnets, 635.
Hot-water apparatus, 384.
Homonrt of the ^e, refraction of the
— , 1180.
HwUer'a aoreWf 144
SMggetu^ eje-pieoe, 1 169.
Hydraulio engines, 459 ; — lifting-jaok,
396; — ^panohingmaobine, 395; Bra-
mak'§ — press, §94 ; — ^ram, the, 441.
Hjdro-electrio machine, 698—700.
Hydrometer, the, 418 ; Hart't—, 420;
mekoUm't--, 419 ; Sgke$'$—, 418 ;
Hydrodynamics, 426—476.
HydrosUtics, 379->42o.
HydrosUtio bellows, 393 ;~leTe1 388 ;
-—principle of Arekimede$, 400.
Hygrometer, DaitUU'§, 1424; Afo-
«>»'»-, 1426; SeffnoHlft—, 1426;
P§Uiwr'§ thermo-eleoirio — , 970.
Hyperbolic mirrors, 1030.
Hypospeotral rays not luminous, 1464.
Ice-makinff machines, 1420.
Identity of nature of light and heat,
1487.
Idio-eleetrics, 657.
Idle wheels, 226.
Illumination, law of—, 1082;— of
microscopic objects, 1165; dark-
ground—, 1166 ; oblique — , 1167.
Imsgea, formation of—, by lenses,
1077; — by mirrors, 1044, 1046;
duration of— on the retina, 1189.
Impact, direct — , of elastic bodies,
203—295 ;— of inelastic bodies, 291,
282 ; nature and effects of—, 297.
Impenetrability of matter, 2.
Incidence and reflection of elastic bo-
dies, 296; angles of-of light, 1333.
Incidental colours, 1190—1193.
Inclination, magnetic — , 611.
Inclined plane, 140 ; equilibrium on
the—, l4l, 142; Telocity acquired
on the— J 326.
Indestructibility of matter, 8.
Index of refraction ; 1057 ;— errors of
thermometers, 1866.
Indici^or (electric telegraph), 915.
Induced rotation, 912, 913.
Induction, electric, 676—678 ; electro-
dynamic — , 871—914; — machine,
SoW^—» 701, 702; magnetic—, 696,
696; speciflo— , 680.
Indnctorium, XAiiTe, 90S I M0urd9^»
— , 902 ; ^MJkMiborf 'e— , 901 ; Sle-
«eiM and Kalmh?*—, 906.
Inertia, 278; effects of—, 279; mCK
ment of—, 846.
Inferred nature of solar photocpheret
1111.
Inflection of undulations, 407s^-of
sound, 665 ;— of lieht, 1128.
Insects, electric — ^ 982.
Insulated wire, 856.
Insulation, electric, 659.
Intensity armature, 906 ;— of deetric
charge, 674 ;— of sound, 634— 6M.
Interference of light, 1121—1139 ;—
of sound, 640, 5w ;— of nndnUtioiw,
aeo.
Intervals in music. 674b
Inteados of an aron, 102.
Inversor, electric, 854.
luTisible spectrum, lines in the, 1116.
Involute teeth of wheels, 186w
lodo-quina, sulphate of, 1216.
Iron, passive, 778.
Irrationality of the spectrum, 1098.
Isochroniam of vibrations, 378; — of
the pendulum, 831, 332.
Isoclinal lines, 612.
Isodynamio lines, 627.
Isothermal Unes, 1387.
•ToeoM's electetvmagDetio engine^ 88(;
— method of magnetiiing, 634.
Jacketed pipes, 1403.
Jaequard loom, 950;— telegraph, 984.
Jar. Leyden — , 720 ; diamond—, 728.
JeiUt's saccharometer, U72a.
Johnson' $ pressnre>-gange, 883;— d«ep-
sea thermometer, 1360.
Jointed discharger. 728.
JouU, on the mechanical equivalent
of heat, 1846.
Jupiter's satdUtea, ooenltetkm ol^
1028.
Kaleidoscope, 1088.
Kaisr'i pendulum, 850.
Kopp*9 oalorimeter, 1389.
KfuffPt law of the magaetie dip, 618.
Laeovhtre on fluid-motion, 489.
Lagnxvifa organic battery, 1013.
Lantf* discharger, 706.
Lantern or trundle^ 174.
Laryngoscope, IISO.
Latent heat, 1897—1406 ;— of steam,
1405;— of Tapours, 1406;— of va-
rious bodies, 1899;— of water, 1887.
Lateral accumulation of waves, 4(B;
— explosion, 748 1 — gravitatioo, 68 ;
•^pressure of fluids, 896.
IVDEX.
843
LAV
LIN
Lavender band of specinnii, 1087.
Law of graritation, 69 ;— beneath the
earth's sarfaoe, 64 ;— of sines, 1054 ;
—of srnunetry, in crystals, 26.
Lawsonnotion, JITtfwton'f^, 282—287;
eleotroeutic— » 673;— of interfe-
rence of polarised light, 1229.
Leaning towers, stability of, 93.
Least circle of aberration, 1047.
Zeeouni'i polsriscope, 1261.
'Lt—onf* battery, 781.
Left-handed quarts, 27, 1260;—
screws, 202.
Leichienberfft electric figures of— ,736.
X«ttotr'f gas-engine, 629.
Lenses, varieties of, 1068 ; spherical
— , 1069; focus of—, 1072; coavex — ,
focns of— , 1071 ; conoaTe— , virtual
focus of—, 1073 ; combined — , focus
of—, 1076.
Xie*Ue'§ differential thermometer,
1366; — freezing proeess, 1419; — on
the radiation of heat, 1438.
Level, hydrostatic — , 388; spirit — ,
886 ;— surface of a fluid at rest, 385.
Lever, the straight—, 110, 111 ; the
bent — , 112, 113; different classes
of—, 121 — 126 ; compound — , 126 ;
—escapement, 268 ; the pneumatic
—,627.
Leverage of muscles, 148 — 150.
L^den jar, 720 ;— battery, 724;—
vacuum, 707.
Liebig, researches in organic electri-
city, 1010.
Lifting-jack, hydraulic-, 396.
Liftf the hydraulic — , 440.
Lifting pump, 436.
Light, aberration of—, 1023; — and
heat identical in their nature, 1437 ;
absorption of—, 1093, 1004 ; corpus-
cular theory of—, 1018 ; prismatic
decomposition of—, 1062; diflVac-
tion of—, 1125—1138 ; epipoKo dis-
fersion of—, 1113: fluorescence of
113 ; inflection of—, 1126 ; inter-
ference of—, 1121 — , polarised—,
1200; reflectionof— , 1033; refrac-
tion of—', 1064; undttlatory theory
of—, 1019—1022; velocity of—,
1023 ; relations of heat and—, 1437.
Limiting angle of refraction, 1060;
—of resistance, 56.
Line of action, 165, 168;— centres, 155.
Lines, aclinic—, 612: agonic — , 609,
focal—, 1048 ; isoclinal—, 612 ; iso-
dynamic— , 627 ; isothermal — , 1387 ;
—of magnetic force, 692 ; — i>a«M-
A^/^r's— , 1099;— of invisible spec-
trum, 1116;— of no polarixatioo,
1260.
LIN MAB
Link-work defined, 167: velocity-
ratio in — , 162 : parallel motion by
— , 217, 218^ mode of obtaining a
given velocity-ratio in — , 219, 2^ ;
— of pedal harps, 262 ; oscillations
multiplied by — , 284 ; variable velo-
city oDtained by—, 263.
Liquefaction of gases by pressure, 10,
6U0.
Liquids, compressibilitT of, 382; —
COD vey pressure in all directions,
384 ; diffusion of—, 60 ; f|[enend pro-
perties of—-, 379 ; mobility of the
particles of—, 379 ; spheroidal state
of—, 1416. 1416.
Loadstone, 691.
Lobed wheels, 237, 238.
Longitudinal vibrations, 376; pro-
gressive and stationary — , illns-
trated, 377.
Luiiar-diomal change of declination,
617.
Haded crystals, 28.
Magdeburg hemispheres, 611.
Magic lantern, 1161.
Msgne>ci7Stauio axis, 653.
Magnet, electro-. 806 ; permanent — ,
692, Ac. ; revolution of a— round a
conductor, 866.
Magnetic force, annual change of—,
618 ; horisontal and vertical com-
ponents of—, 619.
Majgnetism, 553; — excited by eleotri-
city, 871, 872; action of— on pola-
rised light, 1265.
Magnetic declination, 609 ;— dip, 611 ;
— e<)uator, 612 ;— field, 698 ;— incli-
nation, 611 ; — induction, 596; — Klines
of force, 592 : — ^metals, &SiO ;^polea
of the earth, 608 ;— registration,
624 ; — variations, annual — , 618 ;
lunar-dinmal — , 617 ; solar^iinmal,
615 ; secular—, 609.
Magneto-electric machine, 906 — 907 ;
-telegraph, 921 ;— Sinstfiu'- , 922 ;
TFAMtofoiM'f— , 920.
Magnetometers, 622.
Magnets, natural—, 691 ; artificial — ,
691.
Magnifying |>ower of lenses, 1060; —
of a refracting telescope, 1160;— of
object-glasses, 1164.
Magnwt* formula and table, 619.
Mahu^ discovery of polarised light,
1217.
Mangle-racks, 263; -wheels, 261, 36S.
Marcet*9 apparatus for artifioial re-
spiration, 613.
Mareut' thermopile, 971.
Maris Davg'i pile, 792.
844
INDEX.
MAB
MOB
Hftriner'i oompMS, 601 ;— card 602.
Mown* 9 hygrometer, 1^.
Mass of a bodj defined, 274.
Matter, finite divisibilirf of—, 1 ; ixn*
peaetrabili^ of— ,2; extension of
— , 3 ; indestructibility of—. 8 ; den-
sity of— , 11 ; diyisibility of—, 12.
Mait4ueeCi researches la organic
electricity, 990, 901.
Maynooik battery, 779.
Mean height of the barometer, 491.
Mechanical powers, 110—144.
Mechanism, animsL 148 — 162; prin-
ciples of— defined, 163—160.
Meffsseope, the, 1148.
If «»/«'« acoustic apparatus, 677.
Melloni't researches on heat, 1461 —
1463.
Melting-point, 1407 ; influence of mix-
ture of elements on — , 1409; — of
presaure on—, 1406.
Meniscus lens, 1068.
Mercurial barometer, 484 ; syphon — ,
486; standard—, 486;— thermo-
meter, 1357 ;— and air — compared,
1863 ; scales of division of—, 1358 ;
maximum—, 1362; — nBinimnm — ^
1363.
Mercury and air, ratio of densities of
— , 616 ; capillary repulsion of—, 41.
Metacentre, the, 406.
Metals, conductivity of, 806 ; elliptic
polarization by reflection from — ,
1276 i magnetic and diamagaetic — ,
648; thermo-electric series of—,
963 ; yoltaic series of—, 764.
Meteors, 760.
Mica, action of—, on polarised light»
1232; oolonred rings in— , 1942;
action of— on heat, 1484, 1486, 1488 ;
mode of preparing — , 1481.
Microscope, achromatic — , 1167; com-
pound— , 1166; lucemal — , 1160;
simple—, 1164; solar—, 1160; illu-
minating apparatus for the—, 1166
—1168.
Mini^ ball, principle of the— , 316.
Mirage, 1061.
Mirrors, refleotion firom — , 1039 —
1061.
Modulus of elasticity, 289.
Moment of a couple, 81 ;— of a pres-
sure, 77 ; — of inertia, 346.
Moments, equilibrium of—, 78 — 80.
Momentum defined, 276 j — ^firequently
confounded with vis viva, 277.
MotUgolJler^i air-balloon, 1382.
JfoHn'ff apparatus, 301.
Mont alphabet, 928; — electric tele-
graph, 927;— key, 926 ;— printing
telegraph.
MOB
OEB
Mortice wheels, 175.
2f&0r-« figures, 1452.
Motion, absolute, rdatiye, anifbrm,
accelerated, retarded, 270; modes
of communicating — , 156 ;— commu-
nicated by rolling contact, 170 ; —
wheels of^a clock, 228 ; — of prcgW*
tiles, 307—314.
Movable diagram of reiraction, 1060.
Multiple inductor, 741.
Multiplier, yoltaic—, 866; nstetaea
856 ; thermo— , 968. 909.
Mnsdes, leverage of, 148— 160l
Musouls^ currents, 930.
Musical intervals, 674 ;— reprcaented
graphically, 674.
Myopso vision, 1186.
Natural cxystals, 20;— magnets, 691.
Neap-tides, 474.
Needle, magnetic — , 60S ;— eleotrie
tele^ph, 916.
Negative electricity, 660 ;— opCio axis,
1203.
Nerye-cnrrents,906.
Nefcton't first law of motion, 282 ;
second law, 283; third law, 286 ; ita
consequences, 287 ; — ^rlngs, 1133; —
table of colours, 1136; — teleaoopSL
1142.
Nichol$om*i hydrometer, 419.
Nickel, magnetism ol^ 630.
NieoPt prism, 1212.
Niipc^i disooTcries in photogrspliy.
Nitre, oolonred rings in—, 1244.
2fob«re§ line^ 1139.
NobilC$ astatic galvanometer, 866.
Nodal lines, 683 ; — points of a chord,
675 ; — points of a vibrating body,S73.
Non-conductors, eLectrio— . 659.
Nose-piece, Brooke't double—, finr
microscopes, 1163.
Notations for dock-trains, 229, 230.
Notes in music, 668 ; relative lengths
of waves in — ^ 670; relatiye num-
bers of waves in — , 670.
Objeot-glass, achromatic, 1156, 1160 ;
angle of aperture of—, 1158 ; ad-
justment of— -for aberration, 1160;
magnifjring powers of—, 116^
Oblique reflection, 1048; — iUmnina-
tion, 1167 ; — system of crjstab^
24, IT. ; — ^hemihedral crystals, 27.
Ocean, equilibrium of the^ 472.
Octave in music, 570.
Odontograph, WiUiiTi, 109.
CB^taajr magnetic process, 633.
OtrtUtu piesometer, 381 ;— elcetro-
djnamic researches^ 860, 851.
INDEX.
846
OHM
PHO PHO
Okm*$ theory, 797, 799.
Opaque bodies defined, 1024.
Ophibalmoacope, 1196.
Optic ftzee of crystals, 1808.
Optical acoustic figures, 6fi3.
Optical instruments, 1141—1199.
Ordinary ray, 1200.
Organic electricity, 976 — 1017 ; struc-
tures, polarising power of—, 1868
—1866.
Oscillation of a body, 389 ;— in a cy-
oloidal arc, 830 ; time of an — in a
cycloid, 331:— in a small circular
arc, 332 ;— of a pendulum, thne of
— , 333 ; centre of—, 347, 349.
Ozida^n, a source of aidmal eleo-
tricity, 1007.
Oxygen, magnetism of—, 616.
Osone, 640, 823;— generator, 824.
Pallets, 288, 867.
Paper, photographic processes on—,
1301—1328.
P^^s digester, 1418.
Parabolic path of a prcgeetile, 806 ;
—reflector, 1060.
Parallel absorption and radiation in
gase^l478.
Parallel hemihedral aystals, 27.
Parallel pressures, resultant of two — ,
76, 76:— of any number of—, 79;
ease of constant equillibrium of—,
80 :— motion bv Unk-work, 817, 818.
Parallelogram of statical pressures,
70 ;— of Tclocities, 284.
Parameters of a crystal, 23.
Paratonnerres, 767, 768.
Path of the centre of graTity, 96—100;
—of a projectile, in practice, 314.
Pedal-hup, link-work m tbe— , 262.
Pendulum, applied to physical geo-
graphy, 366—367 ; conical—, §S3 ;
oorrection of unequal arcs, 332;
gridiron—, 354; Xaier'*—, 360;
len^h of—, 383 ; mercurial compen-
sation of—, 366; rotation of— ,367;
oscillations of—, 838.
FeltUr'i electrometer, 666 ;— thermo-
electric hygrometer, 970.
Penetration of electric churge, 726.
Percussion, centre of—, 851—363;
ener^ of—-, 297.
Periodicity of distnrbaaoe-Tariations,
681.
Persian wheel, 449.
FetgvaVt orthoscopic lens, 1896.
Phases of a wave, 369.
Phonograph, 8eoU and £bm^»— ,
651 ; Linajottt and i^MOtiw— , 668.
Phosphorescence, 1118; — ^from elec-
tric disdiarge, 734,
P08
Photo-galTanography, 1339; -glyphy,
1338; •lithography, 1337; -sotUpture,
1340; •■incograpby, 1337.
Photographic camera, 1293 ; — paper,
1302 ; — ^processes on paper^ ISUl—
1322 ;— registration of the baro-
meter, 496 ;— of magnetometers,
624 ;— of the thermometer, 1367.
Photographs, positiTC, and negatiTO,
1291.
Photography, 1289— 1341; celestial—,
1836.
Photometiry, 1027.
Physiologic effecta of an eleotrio
current, 997.
Picture-firame (photography), 1297.
Piesometer, OtnMTt—, 881; B^g-
nmilf§ — , 382.
Pile, Mari€ Dav^9—, 798; Pafotfr-
maekei'*^, 790 ; FoZ^o't— , 788, 789;
Zamboni'9^, 761.
Pin-and-aHt motion, 241.
Pin-wheels, 188, 1«).
Pinion in clockwork, 176.
Pistol, the electric, 711, o.
PitehAdrcle of wheel*, 173.
Pitch, concert-, in music, 671.
Pith-ball electroscope, 661.
Plane of floaUtion,40tf.
Plane, the inclined, 140; equilibrium
on — , 141; experimentally illus-
trated, 142 ; time of descent on— ,
326.
Plane mirror, reflection flrom ft— ,
1U36.
Planes of polarisation, 1210.
Plate eleotrio machine, 692.
Plates, colours of thin, 1138.
Flweket'M magnetic researches, 662.
Plumb-line, deriation of th»— , 69.
Pneumatics, 477-629.
Pneumatic leTer, the, 627.
points and knobs, discharge by—',
689.
Polar dock, WheaMo»t^$, 1228.
Polarisoope, JSto^r— , 1217 ; BrookTt
—,1219: Leeounf 9— ,1251.
PoUriied light, plane—, 1210; eircu*
larly— , 1256 ; eUiptically- , 1276 j
—heat, 1483-1491;— b^ pUtea of
mica, 1484 ;-^^ retraction through
rock-salt, 14B6; — of solar heat
1486; circular-, 1469, 1490;— by
Frtmtrs rhomb, 1491.
Polarised reUy, Sitmmu^-^, 931.
Poles, magnetic, 693 ;— of the eafth»
606.
Polygon of pressures, 73.
Port, esiabushment of a, 476.
Positire electricity, 666 ;— optic axis,
1803;— photographs, 1891, 1327.
846
IKDEZ.
POT
PUM PUN
Fotesaiam, ndnetion of—, 839.
Potential, elaotric, diflfuaioa of—, 736;
—of the etmosphere, 747 — 763} — of
doada, 764—766.
Power ^xohMieed for time, 109.
Powers, meoDanioil— , 109 ; — of
lenaea, 1079.
Precticu eqailibrimn of en aroh, 106
—106. f
Presbyopic vision, 1187.
Pressore of fluids, 386 ;— on the base
of a Teasel, 392 ; l&teral— , 398—400 ;
upward— , 397 ; centre of— , 401 ; —
proportionid to sorfaoe, 394 ;— on a
plane immersed, 401 ; — of gases^^aw
of— ,*498— 600 ;— of Tapoors, 618 ;—
of aqneons vapour, 619 ;— of gaseous
mixtures. 616 ; — measured bj
*' atmospheres," 620.
Pressure-irame (photography), 1290.
Pressure-gauffe, Joknton »— , 383 ;
AUan'$—, 602.
PreestireSj composition of—, 09:
eqni ibnum of—, 66 ; — ^represented
by lines and numbers, 67 : resolu-
tion of—, 69 ; statical — denned, 66.
PrtUeh't process of photo-gal ?ano-
naphy,lS39.
Pnmary coil, 886 ;r-oo1ours, 1089 ;—
focus, 1048 ;— forms of crystals, 26.
Prime conductor, 691.
Primitive polarisation, plane of—,
1244.
Principal focus of a lens, 1072 ;— of a
{;lass sphere, 1069 j— of a mirror,
039.
Principal section of crystals, 1201.
Principle of virtual velocities, 126;—
of the rifle, 316.
Principles of mechanism deflned, 163.
Prismatic system of crystals, 24, ui. ;
—dispersion of Ught^ 1062.
Projectiles, motion or—, 307; path
of— , a parabola, 306;— in a resist-
ing meaium, 314 ; horiiontal range
of^, 312 ; time of flight of—, 311.
Projection, velocity of, 309.
Propagation of heat, law of—, 1374.
Propeller, the screw, 453.
PropelmeDts,269; Wh0aUlon^a~,2S9.
Pseodoscope, the, 1197.
Psychrometer, 1426.
Pulley, the, 132 ; single moveable—,
133; expanding— , 246.
Pulleys, systems of—, 134—136.
Puhtermacher'g battery, 790.
Pump, the California—, 439; the ceo-
trifugal— . 442; AppoUTM—, 443;
chain and bucket — ,444; forcing-,
437; lifting—, 436; rope— , 416;
stomach-, 438.
BEG
Punching machine, hydraulic — ,
Pyramidal sjstem of crystals, 24, xx.
Pjro-electric minerals, 670.
Pyrometer, DaxM/^f- , 1370;hydxo-
—,1^71; IFm^«woo«I'«—, 1389.
Quadrant electrometer, 710.
Quality, variable—, of heat, 1468 ;
meana of comparing — , 14fi9;— H>f
transealency, 1461.
Quantity armature, 907.
Quarts, crystalline form of^ 27; cir-
cular polarising power of, 1260.
QueteUion mas>netio variations, <14«
Quina, iodo-, sulphate of—, 1216.
Quinine, optical properties of^— , 1114.
Ratchet-wheel and dick, 266.
Rack and pinion, 178 ; mangie— , 263.
Radiant heat, 1433—1401.
Radiation and absorption of heat,
parallel properties, 1464 ;— of hcttt,
1456 — 1472 ;— influenced by diemi-
cal constitution, 1456 ;^>y oom*
minution, 1456 ; — from gase^ I4n t
perceptiOD of—, 1433 ; ibtenial — oc
solids, 1479 ;— from flames, 1482.
Radius of gyration, 349.
Rainbow, 1140.
Kam, the hydraulic—, 441.
Rate of cooling l^ convectioQ, 1438;
—by radiation, 1438.
Ray of light, modiflcationa of a—,
1090; ordinary and extraordinair
—,1200.
Rays, chemical—, action of^ 1281 —
1288; convergent, divergent, and
parallel—, defined, 1036 ;— lost by
reflection, 1034.
Reciprocation of sound, 610.
Reeas ; free reeds, 682.
Reflecting teleecope, 1143.
Reflection of elastic bodiea, 296 1'^ti
heat, 1434, 1436;— of lig-ht, 1083;
— from concave aurfacea, 1099;
from convex—, 1042; fVom plane
— , 1036 ; internal—, 1060 ; obliqne^
— , 1048; total—, 1060 ;— of sound,
660— 662;— of undulations, 464,
466.
Refracting telescope, 1109 ; erecting
glasses of a-, 1170; magnifying
power of a—, 1160.
Refraction of light, 1064; douUe— ^
1200; index oT— , 1067; moveable
diagram of—, 1066; — of cokmred
rays, 1084; prismatic—, 1082; un-
usual-, 1061;— of heat, 1413—1444
— of sound, 663.
R^elation, 1410.
Registration of magnetie Tariati<uw,
IHDBZ.
847
RBO
BOT BUB
824;— of bftrometrio— , 406;— of
thBrmometeic — ^ 1387.
S^gnanlt§ hygrometer. 1424;— pie-
someter, 382 ;— ciilonmeter, 1390.
Begalator, AppoUTt temperature—,
1888.
BeiatiTe motion, 270 ; — ^velocity, 272.
BelAj, 930 ; Siemens polarised — , 931 .
BeptiUion of »tom>, 9; cspillary — ,
41 ; electric—, W5 ;— oi electric
onrrenta, 884.
Betidoal charge of electricity, 728.
BesinooB electricity, 858.
Benatance to oompreuion, 18.
Be8istinfl[ mediam, motion of a pro-
jectile m a — , 314.
Bmolntion of statical preuorea, 89.
Besonator ot StUnkolU^ 639.
Beeultant axis, 1202;— of two pree-
Borea, 70 ;— of any number of prea-
aurea, 71 ;— of two equal parallel
preaaurea, 75 ; — ^unequal — , 78 ;— of
any preasurea in a plane, 76; — of
any parallel—, 79 ;— of fluid prea-
aure. 402.
Betarded motion, 870.
Bheometer, 865.
Rheostat, 804.
Bhombohedral ayatem of cryatals,
24, V.
Bifle, principle of the, 316.
Bight-banded quarta, 27, 1280; —
acrew, 202.
Bigidity of oordage, 139; — of atruo-
torea, 14.
Binga, NnstonX 1136 ;— by polarized
light, 12S8.
Itiiehie'* rotating electro-magnet,879.
Soberts't battery, 780.
Boberval'i balance, 128.
Book-salt, action of—, on heat, 1481 ;
— l4<ut permeable to its own radia-
tiona, 1486.
Boeket, flight of a—, 817.
Bods, vibrationa of—, 374, 376.
Hoemer^t diaoo?ery of aberration,
1023.
Bogefg electrical spiral, 886.
Boiling contact, 168 ; conditiona of— .
163 — 169; motion communicated
by—, 170—172.
BoUing corres, 236—238.
BoHold*' electric telegraph, 916.
Botg' achromatic compound miero-
aoope, 1163.
Botation, principal axea of—, 869; —
from unequal reaiatanoe, 319; — of
two bodiea round their common
centre of grarity, 822; induced—,
912, 913; thermo-electric—, 967:
— of the peadolum, 867 ;— of a rigia
BUB
body, 869 ; equilibrium of—, 382 x
inatantaneoua azia of—, 387:— of
Toltaic carrenta, 867, 882 ;— of oiaca,
884 ; — of the plane of polarisation,
1280—1273.
Rubber of electrical machinea, 891.
Rubidium discovered by apeotnun-
analyaia, 1108.
Saccharometer, JeUetfa, 1272a.
Salta, electrolysis of, 832—848.
Sanetoritu' air thermometer, 1364.
Saturn's ring, formation of—, 326.
Scales and beam, 114;— of the ther-
mometer, 1358 ; fi>rmulas for cob-
Terting- , 1358.
Scape» or awing-wheel, 228.
Sekeibler'i tonometer, 573.
Sehihtbein'i battery, ^8;^on osone^
684 ;— on passive iron, 778.
Scktoeigger'M galvanometer, 866.
Screw, ihe, 143 ;— and nut, 202 ; eod-
lesa- , 203—206 ; Hutder't—^ 144 ,*—
of Arehimedet, 449 ; right- and lelt-
handed—, 202;— propeller, the, 453.
Secondary coil, 887 ;— eurrenta, 886;
— focua, 1048 ;— planea of cryatala.
25.
Section, principal — , of cryatala, 1201.
Secular magnetic raiiationa, 814.
Selenite, action of—, on polariaed
liffht, 1230, 1231.
Sel^luminous bodiea, 10S4.
Self-regiatering baromet«r, 496 j~-
magnetom^tera, — 824; — thexmo-
meters, 1367.
Sensibility of a balance, 116.
Strrin'B electric lamp, 810.
8hadbolf$ parabolic reflector, 1186.
Shadow, acoustic — , 565.
Sheph^d'§ eleetrio clock, 963 ; — aetro-
nomical— , 959.
Shifting endless band, mode of—, SIS*
Shock, electric— , 717.
SUberwutnn' 9 heU<MUt, 1178.
Siement' gyrometric governor, 401 j
magneto-electric machine, 906 ;->
telegraph, 922 ;— polariaed relay,
Ml, — submarine cabla^ 808.
Silent click, the, 268.
Silicon, redaction of—, 833.
Silurus eleotricua, (M31.
Simple machinea, 109 ;— microaoopa.
1164
Since, Uw of, 1054—1058.
Single-iniige priam, 1218;— tonoh,
magnetisation by — , 632.
Siriua, the light of—, 1027.
Sliding contact, 156 ; motion oomma-
nicated bijr— , 180.
Smeatou'i air-pump, 606.
848
INDEX.
8MB
BTA
Boap-bnbblep ooloun of i^— , 1132.
Somain, redaction of—, 839.
80fkD6S8, 9.
Bolar-dinnuJ Tarutionof deoliiiation,
615, 616.
Solar photoepbere, its nature in-
ferred, 1111.
SoIm speotram, 1063; fixed lines
in — ,1099,1107: innriUerajein — ,
1107; irraUonelitT of—, 1098.
Solids, expansion of—hj bsat, 1350.
Sotfsi«imii/« mirror, 1163.
Sondhaut^ aoonstic lens, 664.
Sonoroos ribrations, 681; interfe-
renoe of<— , 649.
Sound, the production of—, 590; —
requires a conducting medium, 638 ;
causes affecting its transmission,
636 ; direction of the propagation
of-- 1 668; inflection or— ,666; in-
tensitT of—, 634-^636 : interference
of— ,549, 650; reflection of— . 560 ;
reoiproeation of—, 540; Tcilocitj
of—, in air, 648 ;— in Tsrious media,
646—648.
Bounds, barmonio—, 575 ; toosJ— ,688.
Space, time, yelocitj, and force re-
lated, 305.
Sparks, electric—, 686.
Speoiflc capacity of gases, 1396.
Specific gravitj, 412 ;— of fluids, 418—
480;— of gases, 483;— of solids,
412— il5 ; table of—. 485.
Specific heat, 1388— 1394 ;— of Tari-
ous bodies, 1392;— of gsses, 1394;
—of water at vanons temperatures,
1393; influence of temperature
on—, 1393.
Spectral colours, 1190—1198.
Spectroscope, 1173; rigid—, 1174;
straight—, 1175.
Speotro-microsoope, 1176.
Spectrum, solar—, 1082;— snaljsis,
1106— 1112;— of a temporary star,
1112.
Speculum defined, 1051; Foueaulfi — ,
1062 ; mode of silvenns — , 1063.
Spherical aberration of mirrors, 1046 ;
—of lenses, 1081.
Spheroida] sUte of fluids, 1416, 1416;
—figure of the earth, 324.
Spiral; SofMfi eleotrioal— , 666.
Spirit-level, 386.
Spouting finids, Telodtyof-^, 426, 427.
Spring-tides, 474.
Spur-wheels, 176.
Stability of a supported body, 98,
99, 100:— of a suspended body, 98;
—of a floating boojr, 407.
Stable equilibrium deflnsd, 94.
8TA TEE
StoHhope lens, 1164.
Star, electrio— , 703.
Starch, action of — , on
li^t, 1255.
Statical couples, 81 — 63 ;—
Suilibrium of—, 66 ; comparison
— , 67 ; composition of—, 09 :
resolution of— ,69 ;— problemSt im-
portance of, 84.
Stationary ribrations, 372.
Steam-engine, atmospheric , 465;
high-pressure—, 457; Wait 9^,
466 ; — Shammer, 460.
Steel, magnetasm of—, 501 ; tenacity
of—, 16.
Steelyard, the, 120.
Stereometer, Uie, 421.
Stereoscope, the, 1196; theory of— ^
1195.
Sioke», Prof., on flnoreserace, 1113,
1114 ;— on astigmatism, 1188.
Stop, the Oenera— , 218.
Straight lerer, the, 110, 111.
Stratified discharge, 811.
8irMk^» experiments, 664.
SMi^aitm't battery, 791.
Structure influencing condnctirity,
1375.
Submarine telegraphy, 837—047.
Suction pump, 486.
Sugar, action of— , on polsrixed light,
1269.
Superficial distribution of electricity,
Superpoaition of small moti<ms, 376.
Supra-m>ectral rays, chemical action
Surface of a fluid at rest, horisontal,
884.
Surface radiation, 1448.
Sutton' 9 panoramic lens, 1296,
Swash plate, 255.
•Vyitef' hydrometer, 418.
Syphon, the, 446;— barometer, 466;
—gauge, 607; the Wirtembung— ,
417.
Syringe, condensing, 609; exhnist*
ing— , 608.
System, rotation of a—
Tabasheer, refraction of, 1070.
Talboe9,Ur. 7.,calotype, 1806;— pbo-
toglyphy, 1838,
TalbotjFpe, 1306. i
TofUMut* cup, 447.
Tappets, 254
Teeth and staTss of wheds, forms of
— , 188; necessary number of—,
180. I
Teeth of wheels, uniform action of,
180— 188| episydoidal and hypo-
INDEX.
849
TBL Till
qjoknaal— , 18S; radial—, 184;—
aod pina, 188; mvolate — , 186;
forms of— in praotioe, 197 — dOl.
Telejpi^h, sAeotric,Bain*tciundoal — ,
992; 3akew«lff$ oopTing^, 035;
magiteto-elcotrio— y MO; Mont^§
printing— , 920 ; needle—, 016; in-
dioator of—, 017 ; Bonald/'-^. 016;
Sitm€uf magneto—, 922; Wh^od'
ttone't diao— y 010 ;— magneto-
eleotrio— , 921 ;— rainttng— , 838»
OM ;— priTate— , 02S.
Telegnpny* electro-, 015—061; mb-
marlne — ^,937.
Teleioope, aatronomieal— , 1160;
GMM^ram**— , UM; OnUUift^,
1171; Qfgarftr^, 1143; HmoUm'a
—,1142.
Temponuj star, speotmm of a—,
Tenaoitj, 16.
Teireetnal heat, 1432; relation of
tnnioalenaj to—, 1482 r— radiation,
1460 ; means of oomparinfl^- 1461.
Testa, eleotrioal— , 041—040 ;— of in-
solatioD, 044; — of indnotion, 046:
—for £Milto. 046, 047;— in land
Hnes, 048 ;- for the resistaooe of a
Utterj, 040.
Thallinm disoovered by qpeotnun-
analysifl, 1108.
Theory of oonples, 81—83 ;— of ex-
changes, 1436;— of foroed Tibn-
tions, 600 .—of tides, 47^ 478.
Thermies, 1342—1432.
Thermic rajrs of the spedamm, 1107.
Thermic unit, 1872.
Thermopile, 068, 060 ; JOSsMMrcTr—,
071; IfaraM'- , 071; Jbrfrw*— , 1441.
Thermo-electrio hygrometer, 070; —
rotation, 066,067.
Thermo-eleetrioity, 062—076.
Thermography, 1468.
Thermometer, air — , 1364; BngtttCt
— , 1360 ; differential—, 1866 ; Jdkw
9on*9 deep-sea—, 1360; mazimnm
and mini mnm — , ButkemnreT*, 1361 ;
meronrial — . 1367 ; graduation of—,
1368: self-registering—. 1367; al-
oohol— , 1861; NeorUtpB meronrial
maximnm— , and rhiiUpt*—, 1362;
Cauelc^t mercurial minimum — ,
1868.
Thermophone, 687.
Three-throw crank, principle of the
—,80.
Thunder-house, the, 768.
Tides, theoiyof— ,472; BerfumiUCt—,
473 ; Xop/otf/t— ,478 1 interference
of—, 476 ; spring and ne^H-, 474.
Timbre of soondSk 667.
3
TIM UNI
Time and power exchanged, 100.
Time, space, reloeity, and force re-
nted, 806.
Tints in a soap-bnbble, 1138.
Tonometer. JfeUibUr^w^, 673.
Toothed wheels, action of—, 178 j—
with obBqne teeth, 170.
Topaa, coloured rings' in, 1241.
Torpedo, eleotrio organs of
TorrieMFt theorem, 426;— Taoaom,
484.
Torsion eIeotrometer,Gn(fesi6's— ,664.
Total reileotion, 1060.
Tourmaline, crystalline form of—,
27 ; optical ^perties of—, 1216 ;
pyro-eleoMoity of—, 670.
Tous-lea-mois, optical properties of— ,
1266.
Trade-winds^ 1886.
Trains of wheels, 224—231; forelocks,
228—280; notation for—, 280.
Transcalency and transparency con-
trasted, 1446.
Transcalenoy of gases, 1460 ;— at dif-
ferent densities, 1470;— of liquids.
1406 ,— of solids, 1461.
Translucent bodies defined, 1024.
Transparent bodies defined, 1024.
TransTcrseTibrations, 876.
2WMlvaa'f experiments, 687.
Triangle of preasureL 72.
Trundle or umtem, 174.
Tubes, capillary—. 88—40; friction
of fluids in—, 434 ; Tslooi^ of fluids
in — ,-431; vibrations or air in — ,
670—681.
Tuning*fbr^ experiments with a^^,
664,666.
Turbine, the, 461.
Twin crystals, 28.
Tfmpannm, effect of ribrations on
the— ,641.
J)fndaie§ magnetic researches, 658 ;
—on transcalency of gases, 1468.
Unannealed glass, optical properties
of—, 1208, 1247—1260.
Undulatory theory of light, 1019—
1023 r-hypotheees, 1 138.
Undulations, phases of—, 360 ; pro-
gresaiT»— , 871 ; stationary— ,872;
—of air, 480-471;— of iluids,
462-471 ; reflection of— , 464, 486 ;
interference of—, 466; liberal-
accumulation of—, 468.
Uniaxial crystals, 1208—1206.
Uniform accelerating forces, 296—
305 ;— motion defined, 270.
Unipolar bodies, 746.
UniflOQ in music, 674. '
850
iin>Bx«
UBTI VOL
Unit-Jir, BarrVt, 797.
Unit of work, 349 )— of beat, or tho^
mio unit, 187S.
UniTeraal diachanar, 729.
Unstable aqniUbrnim. 98— lOL .
Upward preasnre of noida, 397.
Ursninm-glaaa. optioal propertiea
of-iluSr^
Tacnmn, luminoaa diaobarga In •-*-,
7067L«jdaii— , 707 ; Torrio^Wi j
484.
VaUiTi deotrio reaMrahea, 960.
yaporia»tionofflaida,lil8; abaoip-
tionofheatin— , 1419.
Taponr at oommon tempflratoraa,
1422.
Tariation of themagnetio needle^ 608;
— of pitoh, in mono, 671.
Farle]i*$ mmtiple indnotor, 741.
Taimah for ooUodion pictnrea,
1826.
Vtdj/'i aaaroid barometer, 496L
Yegetablea, eleotrioit j of— ^ 1017.
Yfllooitiea, paraUelogram of—, 284}
Tirtoal— ,126.
Yelooi^ defined, 271 ; relative—, 272 :
relation of— to SfMoe, time^ ana
force. 806;— of li^i, 1023 r-of
Bonna in air, 642—644 s^in Tariona
media. 645—648; —of apontinff
floida, 426 ;— of wmda, 628.
Velodty-ratio defined, IM;— in oon-
taot motiona^ 163;— in linkworl^
162.
Vena oontraota of floida, 488.
Terge eaoaiMment, 266.
Tibrationa,iaoohrooooa— , 878 : longi-
tudinal—, 876 ;— of a row ok parti-
dea, 877 ; profpieaaiTe— , 371 ; aono-
roua— , Hal; stationary — , 872;
tnuarene— , 876 ; — of air in tobea,
679 — 681; modes of exdting^,
682 ;— of ehord^ 371—873 ;— or
elaattc platea, 683; Jbivulinr** ez-
perimenta on—, SM; — of a neated
[Mur, 687;— of membranes. 666;—
of roda, 870; theory of Ibroed— ,
600 ;— of oonduotora, 800.
Tlrtnal yelodtiea, prindpie of— k 126 ;
— iUustrated, 127:
Vis Tira oonfonnded with momentom,
277; conservation of—, 840; when
a in^TOWiWHT or wiwliwmw^ 841 i-^KD'
pHed to meehanism, 846.
Yuion,binooular— , 1196; oonditiooa
of diatinot— , 1186 ; myopio— « 1186;
preabyopio— . 1187.
Yitreona deotndty, 666.
Yooal aonnd^ 688, 688.
Yo]tai«leolK)m«ter»828. '
YOL
wnr
768;— diacharge^ 806r-deetricity,
761.
Fojla'adeetrophorus, 686 ;— pila, 788,
789f— rpaearehea in argaoio electri*
oily, 964.
Yolume of a body defined, 275l
Youawin of an axoh, 102: vaifimn
—,106.
Yowd Boonda, 669.
Water, eompreaaibiHty of — ^, 381 ;
greatest density of — , 1361 ; deeoss-
posttton of — by heat, 14S1 ; dee-
trolysis of—, 821;— ^wheels, 460.
Ware-i^paratos, 877, 1266.
Wave, phases of a—, 869;— fJraat,
1209.
Waves of light vaiiooaly modified,
1030; mDllant— ^1267, 1268; taUe
of lengths of—, 1066.
Waxed paper photooapUo prooesa,
1818.
B^cAer^creseanhes on vibnting oord^
676L
Wedge, the^ 146 ; power gained by the
— »148.
WedgwootFi experiments in l^do*
graphy, 1289 ;— pyrometer, 1909.
Weigniag marhineu thsw 129 ; meAod
oTdooble-, 117.
Weight, 63, 64.
Wdght lost by a body immeraed iaa
flmd, 410.
WdU, on dew, 1460.
W^iUuun*i panbolio refieetor, 1166 ;
— binoenlar miorosoope^ 1162.
WheatMion^t bridge, 800^ 801; —
obronoeoope^ 967;— disotdegraph,
919 ;— magneto- eleetiio teleiraph,
920, 021;— photometer, 1087;—
polar dodc, 1228;— printiaf dec-
trio tdegrai^, OU ;— prapdmeBt,
269;— pseudoeoope, 1197;— dieo-
stat. 804;— stereoeoopeb 1196.
Wheel and axle^ the^ 181.
Wheels, mortice—, 176; mr— , pin—,
erown— , annular—, 179; bevlDed
-, 177 : breast-, 478 ; eeoentno-,
289; elliptio— , 286, 236 ; lobed-^
237, 238; forms of teeth of—-,
189—201; idle—, 226; mangle ,
261,262; overshot— , 4m, c ; paddle
—,462; tzainsof— k228— 281| nn-
dernhot— , 460.
WiU^M msgneto-deotno machine
911.
WitkbuoH^a dectciod msearches,
990.
WUliiTi odontogn^ 199.
Winoh, the, 181.
L
IXDEZ.
851
wnr woL
Wmd]M8,the,181.
WindmiU mOs, 464.
Windi, Telooitj of—, 6SS.
Wire, insulated— ,866; Tibnlang^-^
oOV.
WoUaaton't battory, 793;— oamtra
looida, 1162 ;— oryoplimu, 1481 ;—
dcmblet, 1166.
WOB
ZIN
Work, unit of—, Stf ; aooumulited— t
348 i— related to vis viva, 344.
Wn^ping oonneoton, 167 ; motion by
— , i06— 816 ) yariaUe— , a44>-a4B.
Zantboni^M pilee, 818.
Zero of thermometer toalea, 1368.
I Zinoy amalgamated—, 766.
THE END.
LOVIMV:
•▲TILL, IDWASOI JOnt CO., PBIVTSBS, OKAVDOI 8TBVXT,
OOTXlfT ai.u>sv.
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